diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ea4f6232..dd01b50a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -70,7 +70,7 @@ When the tests run, `cargo test` runs a test suite binary from `target`, which t
 
 As a result, attaching a debugger to the test binary will let you see the code that launches the subprocess and that inspects the output, but it won't let you step through cargo-mutants itself, which is probably the most interesting part.
 
-Probably the easiest path is to just make note of the command run by the test, and then run that command yourself, under a debugger, outside of the test suite. For example, `./target/debug/cargo-mutants -d ./testdata/tree/factorial --list`.
+Probably the easiest path is to just make note of the command run by the test, and then run that command yourself, under a debugger, outside of the test suite. For example, `./target/debug/cargo-mutants -d ./testdata/factorial --list`.
 
 You may wish to turn off the timeouts with `-t 0`.
 
diff --git a/Cargo.toml b/Cargo.toml
index 8535bed8..6f575ede 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -98,35 +98,35 @@ resolver = "2"
 # so that support for testing workspaces does not try to test the whole
 # cargo-mutants tree.
 exclude = [
-    "testdata/tree/already_failing_tests",
-    "testdata/tree/already_hangs",
-    "testdata/tree/cdylib",
-    "testdata/tree/cfg_attr_mutants_skip",
-    "testdata/tree/cfg_attr_test_skip",
-    "testdata/tree/dependency",
-    "testdata/tree/diff0",
-    "testdata/tree/diff1",
-    "testdata/tree/error_value",
-    "testdata/tree/everything_skipped",
-    "testdata/tree/factorial",
-    "testdata/tree/fails_without_feature",
-    "testdata/tree/hang_avoided_by_attr/",
-    "testdata/tree/hang_when_mutated",
-    "testdata/tree/insta",
-    "testdata/tree/integration_tests",
-    "testdata/tree/missing_test",
-    "testdata/tree/mut_ref",
-    "testdata/tree/never_type",
-    "testdata/tree/override_dependency",
-    "testdata/tree/package-fails/",
-    "testdata/tree/patch_dependency",
-    "testdata/tree/relative_dependency",
-    "testdata/tree/replace_dependency",
-    "testdata/tree/small_well_tested",
-    "testdata/tree/strict_warnings",
-    "testdata/tree/struct_with_no_default",
-    "testdata/tree/unapply",
-    "testdata/tree/unsafe",
-    "testdata/tree/well_tested",
-    "testdata/tree/with_child_directories",
+    "testdata/already_failing_tests",
+    "testdata/already_hangs",
+    "testdata/cdylib",
+    "testdata/cfg_attr_mutants_skip",
+    "testdata/cfg_attr_test_skip",
+    "testdata/dependency",
+    "testdata/diff0",
+    "testdata/diff1",
+    "testdata/error_value",
+    "testdata/everything_skipped",
+    "testdata/factorial",
+    "testdata/fails_without_feature",
+    "testdata/hang_avoided_by_attr/",
+    "testdata/hang_when_mutated",
+    "testdata/insta",
+    "testdata/integration_tests",
+    "testdata/missing_test",
+    "testdata/mut_ref",
+    "testdata/never_type",
+    "testdata/override_dependency",
+    "testdata/package-fails/",
+    "testdata/patch_dependency",
+    "testdata/relative_dependency",
+    "testdata/replace_dependency",
+    "testdata/small_well_tested",
+    "testdata/strict_warnings",
+    "testdata/struct_with_no_default",
+    "testdata/unapply",
+    "testdata/unsafe",
+    "testdata/well_tested",
+    "testdata/with_child_directories",
 ]
diff --git a/DESIGN.md b/DESIGN.md
index 7367d6e4..3ccd3b59 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -188,7 +188,7 @@ Cargo-mutants is primarily tested on its public interface, which is the command
 
 ### `testdata` trees
 
-The primary means of testing is Rust source trees under `testdata/tree`: you can copy an existing tree and modify it to show the new behavior that you want to test.
+The primary means of testing is Rust source trees under `testdata`: you can copy an existing tree and modify it to show the new behavior that you want to test.
 
 A selection of test trees are available for testing different scenarios. If there is an existing suitable tree, please use it. If you need to test a situation that is not covered yet, please add a new tree.
 
diff --git a/src/build_dir.rs b/src/build_dir.rs
index 76573aca..f53c016b 100644
--- a/src/build_dir.rs
+++ b/src/build_dir.rs
@@ -152,7 +152,7 @@ mod test {
     #[test]
     fn build_dir_debug_form() {
         let options = Options::default();
-        let workspace = Workspace::open("testdata/tree/factorial").unwrap();
+        let workspace = Workspace::open("testdata/factorial").unwrap();
         let build_dir = BuildDir::new(&workspace.dir, &options, &Console::new()).unwrap();
         let debug_form = format!("{build_dir:?}");
         assert!(
diff --git a/src/in_diff.rs b/src/in_diff.rs
index 87c1e17e..f90c40f2 100644
--- a/src/in_diff.rs
+++ b/src/in_diff.rs
@@ -335,8 +335,8 @@ index eb42779..a0091b7 100644
 
     #[test]
     fn reconstruct_partial_new_file() {
-        let old = read_to_string("testdata/tree/diff0/src/lib.rs").unwrap();
-        let new = read_to_string("testdata/tree/diff1/src/lib.rs").unwrap();
+        let old = read_to_string("testdata/diff0/src/lib.rs").unwrap();
+        let new = read_to_string("testdata/diff1/src/lib.rs").unwrap();
         let diff = make_diff(&old, &new);
         let patch = Patch::from_single(&diff).unwrap();
         let reconstructed = partial_new_file(&patch);
diff --git a/src/manifest.rs b/src/manifest.rs
index 15b8db8c..15152aba 100644
--- a/src/manifest.rs
+++ b/src/manifest.rs
@@ -165,7 +165,7 @@ mod test {
 
     #[test]
     fn fix_path_absolute_unchanged() {
-        let dependency_abspath = Utf8Path::new("testdata/tree/dependency")
+        let dependency_abspath = Utf8Path::new("testdata/dependency")
             .canonicalize_utf8()
             .unwrap();
         assert_eq!(
@@ -181,13 +181,13 @@ mod test {
     fn fix_path_relative() {
         let fixed_path: Utf8PathBuf = super::fix_path(
             "../dependency",
-            Utf8Path::new("testdata/tree/relative_dependency"),
+            Utf8Path::new("testdata/relative_dependency"),
         )
         .expect("path was adjusted")
         .into();
         assert_eq!(
             &fixed_path,
-            Utf8Path::new("testdata/tree/relative_dependency/../dependency"),
+            Utf8Path::new("testdata/relative_dependency/../dependency"),
         );
     }
 
diff --git a/src/mutate.rs b/src/mutate.rs
index 4b125f09..2cb2737e 100644
--- a/src/mutate.rs
+++ b/src/mutate.rs
@@ -208,7 +208,7 @@ mod test {
 
     #[test]
     fn discover_factorial_mutants() {
-        let tree_path = Utf8Path::new("testdata/tree/factorial");
+        let tree_path = Utf8Path::new("testdata/factorial");
         let workspace = Workspace::open(tree_path).unwrap();
         let options = Options::default();
         let mutants = workspace
@@ -262,7 +262,7 @@ mod test {
 
     #[test]
     fn filter_by_attributes() {
-        let mutants = Workspace::open(Utf8Path::new("testdata/tree/hang_avoided_by_attr"))
+        let mutants = Workspace::open(Utf8Path::new("testdata/hang_avoided_by_attr"))
             .unwrap()
             .mutants(&PackageFilter::All, &Options::default(), &Console::new())
             .unwrap();
@@ -275,7 +275,7 @@ mod test {
 
     #[test]
     fn mutate_factorial() -> Result<()> {
-        let tree_path = Utf8Path::new("testdata/tree/factorial");
+        let tree_path = Utf8Path::new("testdata/factorial");
         let mutants = Workspace::open(tree_path)?.mutants(
             &PackageFilter::All,
             &Options::default(),
diff --git a/src/workspace.rs b/src/workspace.rs
index e0a4a5e1..955fa287 100644
--- a/src/workspace.rs
+++ b/src/workspace.rs
@@ -317,8 +317,8 @@ mod test {
 
     #[test]
     fn open_subdirectory_of_crate_opens_the_crate() {
-        let workspace = Workspace::open("testdata/tree/factorial/src")
-            .expect("open source tree from subdirectory");
+        let workspace =
+            Workspace::open("testdata/factorial/src").expect("open source tree from subdirectory");
         let root = &workspace.dir;
         assert!(root.is_dir());
         assert!(root.join("Cargo.toml").is_file());
@@ -328,7 +328,7 @@ mod test {
 
     #[test]
     fn find_root_from_subdirectory_of_workspace_finds_the_workspace_root() {
-        let root = Workspace::open("testdata/tree/workspace/main")
+        let root = Workspace::open("testdata/workspace/main")
             .expect("Find root from within workspace/main")
             .dir;
         assert_eq!(root.file_name(), Some("workspace"), "Wrong root: {root:?}");
@@ -336,8 +336,7 @@ mod test {
 
     #[test]
     fn find_top_source_files_from_subdirectory_of_workspace() {
-        let workspace =
-            Workspace::open("testdata/tree/workspace/main").expect("Find workspace root");
+        let workspace = Workspace::open("testdata/workspace/main").expect("Find workspace root");
         assert_eq!(
             workspace
                 .packages(&PackageFilter::All)
@@ -361,7 +360,7 @@ mod test {
 
     #[test]
     fn package_filter_all_from_subdir_gets_everything() {
-        let subdir_path = Utf8Path::new("testdata/tree/workspace/main");
+        let subdir_path = Utf8Path::new("testdata/workspace/main");
         let workspace = Workspace::open(subdir_path).expect("Find workspace root");
         let packages = workspace.packages(&PackageFilter::All).unwrap();
         assert_eq!(
@@ -372,7 +371,7 @@ mod test {
 
     #[test]
     fn auto_packages_in_workspace_subdir_finds_single_package() {
-        let subdir_path = Utf8Path::new("testdata/tree/workspace/main");
+        let subdir_path = Utf8Path::new("testdata/workspace/main");
         let workspace = Workspace::open(subdir_path).expect("Find workspace root");
         let packages = workspace
             .packages(&PackageFilter::Auto(subdir_path.to_owned()))
@@ -382,7 +381,7 @@ mod test {
 
     #[test]
     fn auto_packages_in_virtual_workspace_gets_everything() {
-        let path = Utf8Path::new("testdata/tree/workspace");
+        let path = Utf8Path::new("testdata/workspace");
         let workspace = Workspace::open(path).expect("Find workspace root");
         let packages = workspace
             .packages(&PackageFilter::Auto(path.to_owned()))
@@ -395,8 +394,7 @@ mod test {
 
     #[test]
     fn filter_by_single_package() {
-        let workspace =
-            Workspace::open("testdata/tree/workspace/main").expect("Find workspace root");
+        let workspace = Workspace::open("testdata/workspace/main").expect("Find workspace root");
         let root_dir = &workspace.dir;
         assert_eq!(
             root_dir.file_name(),
@@ -426,7 +424,7 @@ mod test {
 
     #[test]
     fn filter_by_multiple_packages() {
-        let workspace = Workspace::open("testdata/tree/workspace/main").unwrap();
+        let workspace = Workspace::open("testdata/workspace/main").unwrap();
         assert_eq!(
             workspace.dir.file_name(),
             Some("workspace"),
diff --git a/testdata/tree/.gitignore b/testdata/.gitignore
similarity index 100%
rename from testdata/tree/.gitignore
rename to testdata/.gitignore
diff --git a/testdata/tree/already_failing_doctests/Cargo.toml b/testdata/already_failing_doctests/Cargo.toml
similarity index 100%
rename from testdata/tree/already_failing_doctests/Cargo.toml
rename to testdata/already_failing_doctests/Cargo.toml
diff --git a/testdata/tree/already_failing_doctests/src/lib.rs b/testdata/already_failing_doctests/src/lib.rs
similarity index 100%
rename from testdata/tree/already_failing_doctests/src/lib.rs
rename to testdata/already_failing_doctests/src/lib.rs
diff --git a/testdata/tree/already_failing_tests/Cargo.toml b/testdata/already_failing_tests/Cargo.toml
similarity index 100%
rename from testdata/tree/already_failing_tests/Cargo.toml
rename to testdata/already_failing_tests/Cargo.toml
diff --git a/testdata/tree/already_failing_tests/src/lib.rs b/testdata/already_failing_tests/src/lib.rs
similarity index 100%
rename from testdata/tree/already_failing_tests/src/lib.rs
rename to testdata/already_failing_tests/src/lib.rs
diff --git a/testdata/tree/already_hangs/Cargo.toml b/testdata/already_hangs/Cargo.toml
similarity index 100%
rename from testdata/tree/already_hangs/Cargo.toml
rename to testdata/already_hangs/Cargo.toml
diff --git a/testdata/tree/already_hangs/src/lib.rs b/testdata/already_hangs/src/lib.rs
similarity index 100%
rename from testdata/tree/already_hangs/src/lib.rs
rename to testdata/already_hangs/src/lib.rs
diff --git a/testdata/tree/cdylib/Cargo.toml b/testdata/cdylib/Cargo.toml
similarity index 100%
rename from testdata/tree/cdylib/Cargo.toml
rename to testdata/cdylib/Cargo.toml
diff --git a/testdata/tree/cdylib/README.md b/testdata/cdylib/README.md
similarity index 100%
rename from testdata/tree/cdylib/README.md
rename to testdata/cdylib/README.md
diff --git a/testdata/tree/cdylib/src/entry.rs b/testdata/cdylib/src/entry.rs
similarity index 100%
rename from testdata/tree/cdylib/src/entry.rs
rename to testdata/cdylib/src/entry.rs
diff --git a/testdata/tree/cfg_attr_mutants_skip/Cargo.toml b/testdata/cfg_attr_mutants_skip/Cargo.toml
similarity index 100%
rename from testdata/tree/cfg_attr_mutants_skip/Cargo.toml
rename to testdata/cfg_attr_mutants_skip/Cargo.toml
diff --git a/testdata/tree/cfg_attr_mutants_skip/src/lib.rs b/testdata/cfg_attr_mutants_skip/src/lib.rs
similarity index 100%
rename from testdata/tree/cfg_attr_mutants_skip/src/lib.rs
rename to testdata/cfg_attr_mutants_skip/src/lib.rs
diff --git a/testdata/tree/cfg_attr_test_skip/Cargo.toml b/testdata/cfg_attr_test_skip/Cargo.toml
similarity index 100%
rename from testdata/tree/cfg_attr_test_skip/Cargo.toml
rename to testdata/cfg_attr_test_skip/Cargo.toml
diff --git a/testdata/tree/cfg_attr_test_skip/src/lib.rs b/testdata/cfg_attr_test_skip/src/lib.rs
similarity index 100%
rename from testdata/tree/cfg_attr_test_skip/src/lib.rs
rename to testdata/cfg_attr_test_skip/src/lib.rs
diff --git a/testdata/tree/dependency/Cargo.toml b/testdata/dependency/Cargo.toml
similarity index 100%
rename from testdata/tree/dependency/Cargo.toml
rename to testdata/dependency/Cargo.toml
diff --git a/testdata/tree/dependency/src/lib.rs b/testdata/dependency/src/lib.rs
similarity index 100%
rename from testdata/tree/dependency/src/lib.rs
rename to testdata/dependency/src/lib.rs
diff --git a/testdata/tree/diff0/Cargo.toml b/testdata/diff0/Cargo.toml
similarity index 100%
rename from testdata/tree/diff0/Cargo.toml
rename to testdata/diff0/Cargo.toml
diff --git a/testdata/tree/diff0/src/lib.rs b/testdata/diff0/src/lib.rs
similarity index 100%
rename from testdata/tree/diff0/src/lib.rs
rename to testdata/diff0/src/lib.rs
diff --git a/testdata/tree/diff1/Cargo.toml b/testdata/diff1/Cargo.toml
similarity index 100%
rename from testdata/tree/diff1/Cargo.toml
rename to testdata/diff1/Cargo.toml
diff --git a/testdata/tree/diff1/src/lib.rs b/testdata/diff1/src/lib.rs
similarity index 100%
rename from testdata/tree/diff1/src/lib.rs
rename to testdata/diff1/src/lib.rs
diff --git a/testdata/tree/error_value/.cargo/mutants.toml b/testdata/error_value/.cargo/mutants.toml
similarity index 100%
rename from testdata/tree/error_value/.cargo/mutants.toml
rename to testdata/error_value/.cargo/mutants.toml
diff --git a/testdata/tree/error_value/Cargo.toml b/testdata/error_value/Cargo.toml
similarity index 100%
rename from testdata/tree/error_value/Cargo.toml
rename to testdata/error_value/Cargo.toml
diff --git a/testdata/tree/error_value/README.md b/testdata/error_value/README.md
similarity index 100%
rename from testdata/tree/error_value/README.md
rename to testdata/error_value/README.md
diff --git a/testdata/tree/error_value/src/lib.rs b/testdata/error_value/src/lib.rs
similarity index 100%
rename from testdata/tree/error_value/src/lib.rs
rename to testdata/error_value/src/lib.rs
diff --git a/testdata/tree/everything_skipped/Cargo.toml b/testdata/everything_skipped/Cargo.toml
similarity index 100%
rename from testdata/tree/everything_skipped/Cargo.toml
rename to testdata/everything_skipped/Cargo.toml
diff --git a/testdata/tree/everything_skipped/src/bin/everything-skipped.rs b/testdata/everything_skipped/src/bin/everything-skipped.rs
similarity index 100%
rename from testdata/tree/everything_skipped/src/bin/everything-skipped.rs
rename to testdata/everything_skipped/src/bin/everything-skipped.rs
diff --git a/testdata/tree/factorial/Cargo.toml b/testdata/factorial/Cargo.toml
similarity index 100%
rename from testdata/tree/factorial/Cargo.toml
rename to testdata/factorial/Cargo.toml
diff --git a/testdata/tree/factorial/src/bin/factorial.rs b/testdata/factorial/src/bin/factorial.rs
similarity index 100%
rename from testdata/tree/factorial/src/bin/factorial.rs
rename to testdata/factorial/src/bin/factorial.rs
diff --git a/testdata/tree/fails_without_feature/Cargo.toml b/testdata/fails_without_feature/Cargo.toml
similarity index 100%
rename from testdata/tree/fails_without_feature/Cargo.toml
rename to testdata/fails_without_feature/Cargo.toml
diff --git a/testdata/tree/fails_without_feature/README.md b/testdata/fails_without_feature/README.md
similarity index 100%
rename from testdata/tree/fails_without_feature/README.md
rename to testdata/fails_without_feature/README.md
diff --git a/testdata/tree/fails_without_feature/src/bin/factorial.rs b/testdata/fails_without_feature/src/bin/factorial.rs
similarity index 100%
rename from testdata/tree/fails_without_feature/src/bin/factorial.rs
rename to testdata/fails_without_feature/src/bin/factorial.rs
diff --git a/testdata/tree/hang_avoided_by_attr/Cargo.toml b/testdata/hang_avoided_by_attr/Cargo.toml
similarity index 100%
rename from testdata/tree/hang_avoided_by_attr/Cargo.toml
rename to testdata/hang_avoided_by_attr/Cargo.toml
diff --git a/testdata/tree/hang_avoided_by_attr/src/lib.rs b/testdata/hang_avoided_by_attr/src/lib.rs
similarity index 100%
rename from testdata/tree/hang_avoided_by_attr/src/lib.rs
rename to testdata/hang_avoided_by_attr/src/lib.rs
diff --git a/testdata/tree/hang_when_mutated/Cargo.toml b/testdata/hang_when_mutated/Cargo.toml
similarity index 100%
rename from testdata/tree/hang_when_mutated/Cargo.toml
rename to testdata/hang_when_mutated/Cargo.toml
diff --git a/testdata/tree/hang_when_mutated/src/lib.rs b/testdata/hang_when_mutated/src/lib.rs
similarity index 100%
rename from testdata/tree/hang_when_mutated/src/lib.rs
rename to testdata/hang_when_mutated/src/lib.rs
diff --git a/testdata/tree/insta/Cargo.toml b/testdata/insta/Cargo.toml
similarity index 100%
rename from testdata/tree/insta/Cargo.toml
rename to testdata/insta/Cargo.toml
diff --git a/testdata/tree/insta/README.md b/testdata/insta/README.md
similarity index 100%
rename from testdata/tree/insta/README.md
rename to testdata/insta/README.md
diff --git a/testdata/tree/insta/src/lib.rs b/testdata/insta/src/lib.rs
similarity index 100%
rename from testdata/tree/insta/src/lib.rs
rename to testdata/insta/src/lib.rs
diff --git a/testdata/tree/insta/tests/insta.rs b/testdata/insta/tests/insta.rs
similarity index 100%
rename from testdata/tree/insta/tests/insta.rs
rename to testdata/insta/tests/insta.rs
diff --git a/testdata/tree/insta/tests/snapshots/insta__say_hello_vs_insta_snapshot.snap b/testdata/insta/tests/snapshots/insta__say_hello_vs_insta_snapshot.snap
similarity index 100%
rename from testdata/tree/insta/tests/snapshots/insta__say_hello_vs_insta_snapshot.snap
rename to testdata/insta/tests/snapshots/insta__say_hello_vs_insta_snapshot.snap
diff --git a/testdata/tree/integration_tests/Cargo.toml b/testdata/integration_tests/Cargo.toml
similarity index 100%
rename from testdata/tree/integration_tests/Cargo.toml
rename to testdata/integration_tests/Cargo.toml
diff --git a/testdata/tree/integration_tests/README.md b/testdata/integration_tests/README.md
similarity index 100%
rename from testdata/tree/integration_tests/README.md
rename to testdata/integration_tests/README.md
diff --git a/testdata/tree/integration_tests/src/lib.rs b/testdata/integration_tests/src/lib.rs
similarity index 100%
rename from testdata/tree/integration_tests/src/lib.rs
rename to testdata/integration_tests/src/lib.rs
diff --git a/testdata/tree/integration_tests/tests/api.rs b/testdata/integration_tests/tests/api.rs
similarity index 100%
rename from testdata/tree/integration_tests/tests/api.rs
rename to testdata/integration_tests/tests/api.rs
diff --git a/testdata/tree/missing_test/Cargo.toml b/testdata/missing_test/Cargo.toml
similarity index 100%
rename from testdata/tree/missing_test/Cargo.toml
rename to testdata/missing_test/Cargo.toml
diff --git a/testdata/tree/missing_test/src/lib.rs b/testdata/missing_test/src/lib.rs
similarity index 100%
rename from testdata/tree/missing_test/src/lib.rs
rename to testdata/missing_test/src/lib.rs
diff --git a/testdata/tree/mut_ref/Cargo.toml b/testdata/mut_ref/Cargo.toml
similarity index 100%
rename from testdata/tree/mut_ref/Cargo.toml
rename to testdata/mut_ref/Cargo.toml
diff --git a/testdata/tree/mut_ref/README.md b/testdata/mut_ref/README.md
similarity index 100%
rename from testdata/tree/mut_ref/README.md
rename to testdata/mut_ref/README.md
diff --git a/testdata/tree/mut_ref/src/lib.rs b/testdata/mut_ref/src/lib.rs
similarity index 100%
rename from testdata/tree/mut_ref/src/lib.rs
rename to testdata/mut_ref/src/lib.rs
diff --git a/testdata/tree/never_type/Cargo.toml b/testdata/never_type/Cargo.toml
similarity index 100%
rename from testdata/tree/never_type/Cargo.toml
rename to testdata/never_type/Cargo.toml
diff --git a/testdata/tree/never_type/src/lib.rs b/testdata/never_type/src/lib.rs
similarity index 100%
rename from testdata/tree/never_type/src/lib.rs
rename to testdata/never_type/src/lib.rs
diff --git a/testdata/tree/nightly_only/Cargo.toml b/testdata/nightly_only/Cargo.toml
similarity index 100%
rename from testdata/tree/nightly_only/Cargo.toml
rename to testdata/nightly_only/Cargo.toml
diff --git a/testdata/tree/nightly_only/README.md b/testdata/nightly_only/README.md
similarity index 74%
rename from testdata/tree/nightly_only/README.md
rename to testdata/nightly_only/README.md
index e52136d4..d1665098 100644
--- a/testdata/tree/nightly_only/README.md
+++ b/testdata/nightly_only/README.md
@@ -5,11 +5,11 @@ mutants` uses the corresponding `cargo` and `rustc` when building candidates.
 
 For example this should fail:
 
-    cargo +stable mutants -d ./testdata/tree/nightly_only/
+    cargo +stable mutants -d ./testdata/nightly_only/
 
 and this should succeed:
 
-    cargo +nightly mutants -d ./testdata/tree/nightly_only/
+    cargo +nightly mutants -d ./testdata/nightly_only/
 
 This isn't covered by an integration test because there's no guarantee the user
 has both toolchains installed...
diff --git a/testdata/tree/nightly_only/src/lib.rs b/testdata/nightly_only/src/lib.rs
similarity index 100%
rename from testdata/tree/nightly_only/src/lib.rs
rename to testdata/nightly_only/src/lib.rs
diff --git a/testdata/override_dependency/.cargo/config.toml b/testdata/override_dependency/.cargo/config.toml
new file mode 100644
index 00000000..b1e59646
--- /dev/null
+++ b/testdata/override_dependency/.cargo/config.toml
@@ -0,0 +1 @@
+paths = ["../../mutants_attrs"]
diff --git a/testdata/tree/override_dependency/Cargo.toml b/testdata/override_dependency/Cargo.toml
similarity index 100%
rename from testdata/tree/override_dependency/Cargo.toml
rename to testdata/override_dependency/Cargo.toml
diff --git a/testdata/tree/override_dependency/src/lib.rs b/testdata/override_dependency/src/lib.rs
similarity index 100%
rename from testdata/tree/override_dependency/src/lib.rs
rename to testdata/override_dependency/src/lib.rs
diff --git a/testdata/tree/override_dependency/tests/api.rs b/testdata/override_dependency/tests/api.rs
similarity index 100%
rename from testdata/tree/override_dependency/tests/api.rs
rename to testdata/override_dependency/tests/api.rs
diff --git a/testdata/tree/package_fails/Cargo.toml b/testdata/package_fails/Cargo.toml
similarity index 100%
rename from testdata/tree/package_fails/Cargo.toml
rename to testdata/package_fails/Cargo.toml
diff --git a/testdata/tree/package_fails/README.md b/testdata/package_fails/README.md
similarity index 100%
rename from testdata/tree/package_fails/README.md
rename to testdata/package_fails/README.md
diff --git a/testdata/tree/package_fails/failing/Cargo.toml b/testdata/package_fails/failing/Cargo.toml
similarity index 100%
rename from testdata/tree/package_fails/failing/Cargo.toml
rename to testdata/package_fails/failing/Cargo.toml
diff --git a/testdata/tree/package_fails/failing/src/lib.rs b/testdata/package_fails/failing/src/lib.rs
similarity index 100%
rename from testdata/tree/package_fails/failing/src/lib.rs
rename to testdata/package_fails/failing/src/lib.rs
diff --git a/testdata/tree/package_fails/passing/Cargo.toml b/testdata/package_fails/passing/Cargo.toml
similarity index 100%
rename from testdata/tree/package_fails/passing/Cargo.toml
rename to testdata/package_fails/passing/Cargo.toml
diff --git a/testdata/tree/package_fails/passing/src/lib.rs b/testdata/package_fails/passing/src/lib.rs
similarity index 100%
rename from testdata/tree/package_fails/passing/src/lib.rs
rename to testdata/package_fails/passing/src/lib.rs
diff --git a/testdata/tree/parse_fails/Cargo.toml b/testdata/parse_fails/Cargo.toml
similarity index 100%
rename from testdata/tree/parse_fails/Cargo.toml
rename to testdata/parse_fails/Cargo.toml
diff --git a/testdata/tree/parse_fails/src/lib.rs b/testdata/parse_fails/src/lib.rs
similarity index 100%
rename from testdata/tree/parse_fails/src/lib.rs
rename to testdata/parse_fails/src/lib.rs
diff --git a/testdata/tree/patch_dependency/Cargo.toml b/testdata/patch_dependency/Cargo.toml
similarity index 91%
rename from testdata/tree/patch_dependency/Cargo.toml
rename to testdata/patch_dependency/Cargo.toml
index ac8fe47e..2ec603f3 100644
--- a/testdata/tree/patch_dependency/Cargo.toml
+++ b/testdata/patch_dependency/Cargo.toml
@@ -17,4 +17,4 @@ publish = false
 mutants = { version = "0" }
 
 [patch.crates-io]
-"mutants" = { path = "../../../mutants_attrs" }
+"mutants" = { path = "../../mutants_attrs" }
diff --git a/testdata/tree/patch_dependency/src/lib.rs b/testdata/patch_dependency/src/lib.rs
similarity index 100%
rename from testdata/tree/patch_dependency/src/lib.rs
rename to testdata/patch_dependency/src/lib.rs
diff --git a/testdata/tree/patch_dependency/tests/api.rs b/testdata/patch_dependency/tests/api.rs
similarity index 100%
rename from testdata/tree/patch_dependency/tests/api.rs
rename to testdata/patch_dependency/tests/api.rs
diff --git a/testdata/tree/relative_dependency/Cargo.toml b/testdata/relative_dependency/Cargo.toml
similarity index 100%
rename from testdata/tree/relative_dependency/Cargo.toml
rename to testdata/relative_dependency/Cargo.toml
diff --git a/testdata/tree/relative_dependency/src/lib.rs b/testdata/relative_dependency/src/lib.rs
similarity index 100%
rename from testdata/tree/relative_dependency/src/lib.rs
rename to testdata/relative_dependency/src/lib.rs
diff --git a/testdata/tree/relative_dependency/tests/api.rs b/testdata/relative_dependency/tests/api.rs
similarity index 100%
rename from testdata/tree/relative_dependency/tests/api.rs
rename to testdata/relative_dependency/tests/api.rs
diff --git a/testdata/tree/replace_dependency/Cargo.toml b/testdata/replace_dependency/Cargo.toml
similarity index 90%
rename from testdata/tree/replace_dependency/Cargo.toml
rename to testdata/replace_dependency/Cargo.toml
index a5e6d18d..4c75e615 100644
--- a/testdata/tree/replace_dependency/Cargo.toml
+++ b/testdata/replace_dependency/Cargo.toml
@@ -17,4 +17,4 @@ publish = false
 mutants = { version = "0" }
 
 [replace]
-"mutants:0.0.3" = { path = "../../../mutants_attrs" }
+"mutants:0.0.3" = { path = "../../mutants_attrs" }
diff --git a/testdata/tree/replace_dependency/src/lib.rs b/testdata/replace_dependency/src/lib.rs
similarity index 100%
rename from testdata/tree/replace_dependency/src/lib.rs
rename to testdata/replace_dependency/src/lib.rs
diff --git a/testdata/tree/replace_dependency/tests/api.rs b/testdata/replace_dependency/tests/api.rs
similarity index 100%
rename from testdata/tree/replace_dependency/tests/api.rs
rename to testdata/replace_dependency/tests/api.rs
diff --git a/testdata/tree/small_well_tested/Cargo.toml b/testdata/small_well_tested/Cargo.toml
similarity index 100%
rename from testdata/tree/small_well_tested/Cargo.toml
rename to testdata/small_well_tested/Cargo.toml
diff --git a/testdata/tree/small_well_tested/src/lib.rs b/testdata/small_well_tested/src/lib.rs
similarity index 100%
rename from testdata/tree/small_well_tested/src/lib.rs
rename to testdata/small_well_tested/src/lib.rs
diff --git a/testdata/tree/strict_warnings/Cargo.toml b/testdata/strict_warnings/Cargo.toml
similarity index 100%
rename from testdata/tree/strict_warnings/Cargo.toml
rename to testdata/strict_warnings/Cargo.toml
diff --git a/testdata/tree/strict_warnings/src/lib.rs b/testdata/strict_warnings/src/lib.rs
similarity index 100%
rename from testdata/tree/strict_warnings/src/lib.rs
rename to testdata/strict_warnings/src/lib.rs
diff --git a/testdata/tree/struct_with_no_default/Cargo.toml b/testdata/struct_with_no_default/Cargo.toml
similarity index 100%
rename from testdata/tree/struct_with_no_default/Cargo.toml
rename to testdata/struct_with_no_default/Cargo.toml
diff --git a/testdata/tree/struct_with_no_default/src/lib.rs b/testdata/struct_with_no_default/src/lib.rs
similarity index 100%
rename from testdata/tree/struct_with_no_default/src/lib.rs
rename to testdata/struct_with_no_default/src/lib.rs
diff --git a/testdata/tree/override_dependency/.cargo/config.toml b/testdata/tree/override_dependency/.cargo/config.toml
deleted file mode 100644
index e9521b28..00000000
--- a/testdata/tree/override_dependency/.cargo/config.toml
+++ /dev/null
@@ -1 +0,0 @@
-paths = ["../../../mutants_attrs"]
diff --git a/testdata/tree/typecheck_fails/Cargo.toml b/testdata/typecheck_fails/Cargo.toml
similarity index 100%
rename from testdata/tree/typecheck_fails/Cargo.toml
rename to testdata/typecheck_fails/Cargo.toml
diff --git a/testdata/tree/typecheck_fails/src/lib.rs b/testdata/typecheck_fails/src/lib.rs
similarity index 100%
rename from testdata/tree/typecheck_fails/src/lib.rs
rename to testdata/typecheck_fails/src/lib.rs
diff --git a/testdata/tree/unapply/Cargo.toml b/testdata/unapply/Cargo.toml
similarity index 100%
rename from testdata/tree/unapply/Cargo.toml
rename to testdata/unapply/Cargo.toml
diff --git a/testdata/tree/unapply/README.md b/testdata/unapply/README.md
similarity index 100%
rename from testdata/tree/unapply/README.md
rename to testdata/unapply/README.md
diff --git a/testdata/tree/unapply/src/a.rs b/testdata/unapply/src/a.rs
similarity index 100%
rename from testdata/tree/unapply/src/a.rs
rename to testdata/unapply/src/a.rs
diff --git a/testdata/tree/unapply/src/b.rs b/testdata/unapply/src/b.rs
similarity index 100%
rename from testdata/tree/unapply/src/b.rs
rename to testdata/unapply/src/b.rs
diff --git a/testdata/tree/unapply/src/c.rs b/testdata/unapply/src/c.rs
similarity index 100%
rename from testdata/tree/unapply/src/c.rs
rename to testdata/unapply/src/c.rs
diff --git a/testdata/tree/unapply/src/lib.rs b/testdata/unapply/src/lib.rs
similarity index 100%
rename from testdata/tree/unapply/src/lib.rs
rename to testdata/unapply/src/lib.rs
diff --git a/testdata/tree/unsafe/Cargo.toml b/testdata/unsafe/Cargo.toml
similarity index 100%
rename from testdata/tree/unsafe/Cargo.toml
rename to testdata/unsafe/Cargo.toml
diff --git a/testdata/tree/unsafe/src/lib.rs b/testdata/unsafe/src/lib.rs
similarity index 100%
rename from testdata/tree/unsafe/src/lib.rs
rename to testdata/unsafe/src/lib.rs
diff --git a/testdata/tree/well_tested/Cargo.toml b/testdata/well_tested/Cargo.toml
similarity index 100%
rename from testdata/tree/well_tested/Cargo.toml
rename to testdata/well_tested/Cargo.toml
diff --git a/testdata/tree/well_tested/README.md b/testdata/well_tested/README.md
similarity index 100%
rename from testdata/tree/well_tested/README.md
rename to testdata/well_tested/README.md
diff --git a/testdata/tree/well_tested/src/arc.rs b/testdata/well_tested/src/arc.rs
similarity index 100%
rename from testdata/tree/well_tested/src/arc.rs
rename to testdata/well_tested/src/arc.rs
diff --git a/testdata/tree/well_tested/src/empty_fns.rs b/testdata/well_tested/src/empty_fns.rs
similarity index 100%
rename from testdata/tree/well_tested/src/empty_fns.rs
rename to testdata/well_tested/src/empty_fns.rs
diff --git a/testdata/tree/well_tested/src/inside_mod.rs b/testdata/well_tested/src/inside_mod.rs
similarity index 100%
rename from testdata/tree/well_tested/src/inside_mod.rs
rename to testdata/well_tested/src/inside_mod.rs
diff --git a/testdata/tree/well_tested/src/item_mod.rs b/testdata/well_tested/src/item_mod.rs
similarity index 100%
rename from testdata/tree/well_tested/src/item_mod.rs
rename to testdata/well_tested/src/item_mod.rs
diff --git a/testdata/tree/well_tested/src/lib.rs b/testdata/well_tested/src/lib.rs
similarity index 100%
rename from testdata/tree/well_tested/src/lib.rs
rename to testdata/well_tested/src/lib.rs
diff --git a/testdata/tree/well_tested/src/methods.rs b/testdata/well_tested/src/methods.rs
similarity index 100%
rename from testdata/tree/well_tested/src/methods.rs
rename to testdata/well_tested/src/methods.rs
diff --git a/testdata/tree/well_tested/src/nested_function.rs b/testdata/well_tested/src/nested_function.rs
similarity index 100%
rename from testdata/tree/well_tested/src/nested_function.rs
rename to testdata/well_tested/src/nested_function.rs
diff --git a/testdata/tree/well_tested/src/numbers.rs b/testdata/well_tested/src/numbers.rs
similarity index 100%
rename from testdata/tree/well_tested/src/numbers.rs
rename to testdata/well_tested/src/numbers.rs
diff --git a/testdata/tree/well_tested/src/result.rs b/testdata/well_tested/src/result.rs
similarity index 100%
rename from testdata/tree/well_tested/src/result.rs
rename to testdata/well_tested/src/result.rs
diff --git a/testdata/tree/well_tested/src/sets.rs b/testdata/well_tested/src/sets.rs
similarity index 100%
rename from testdata/tree/well_tested/src/sets.rs
rename to testdata/well_tested/src/sets.rs
diff --git a/testdata/tree/well_tested/src/simple_fns.rs b/testdata/well_tested/src/simple_fns.rs
similarity index 100%
rename from testdata/tree/well_tested/src/simple_fns.rs
rename to testdata/well_tested/src/simple_fns.rs
diff --git a/testdata/tree/well_tested/src/slices.rs b/testdata/well_tested/src/slices.rs
similarity index 100%
rename from testdata/tree/well_tested/src/slices.rs
rename to testdata/well_tested/src/slices.rs
diff --git a/testdata/tree/well_tested/src/struct_with_lifetime.rs b/testdata/well_tested/src/struct_with_lifetime.rs
similarity index 100%
rename from testdata/tree/well_tested/src/struct_with_lifetime.rs
rename to testdata/well_tested/src/struct_with_lifetime.rs
diff --git a/testdata/tree/with_child_directories/Cargo.toml b/testdata/with_child_directories/Cargo.toml
similarity index 100%
rename from testdata/tree/with_child_directories/Cargo.toml
rename to testdata/with_child_directories/Cargo.toml
diff --git a/testdata/tree/with_child_directories/README.md b/testdata/with_child_directories/README.md
similarity index 100%
rename from testdata/tree/with_child_directories/README.md
rename to testdata/with_child_directories/README.md
diff --git a/testdata/tree/with_child_directories/src/lib.rs b/testdata/with_child_directories/src/lib.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/lib.rs
rename to testdata/with_child_directories/src/lib.rs
diff --git a/testdata/tree/with_child_directories/src/methods.rs b/testdata/with_child_directories/src/methods.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/methods.rs
rename to testdata/with_child_directories/src/methods.rs
diff --git a/testdata/tree/with_child_directories/src/module.rs b/testdata/with_child_directories/src/module.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module.rs
rename to testdata/with_child_directories/src/module.rs
diff --git a/testdata/tree/with_child_directories/src/module/module_methods.rs b/testdata/with_child_directories/src/module/module_methods.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/module_methods.rs
rename to testdata/with_child_directories/src/module/module_methods.rs
diff --git a/testdata/tree/with_child_directories/src/module/unreferenced_mod.rs b/testdata/with_child_directories/src/module/unreferenced_mod.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/unreferenced_mod.rs
rename to testdata/with_child_directories/src/module/unreferenced_mod.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/inside_mod.rs b/testdata/with_child_directories/src/module/utils/inside_mod.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/inside_mod.rs
rename to testdata/with_child_directories/src/module/utils/inside_mod.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/inside_utils_empty_fns.rs b/testdata/with_child_directories/src/module/utils/inside_utils_empty_fns.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/inside_utils_empty_fns.rs
rename to testdata/with_child_directories/src/module/utils/inside_utils_empty_fns.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/mod.rs b/testdata/with_child_directories/src/module/utils/mod.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/mod.rs
rename to testdata/with_child_directories/src/module/utils/mod.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/nested_function.rs b/testdata/with_child_directories/src/module/utils/nested_function.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/nested_function.rs
rename to testdata/with_child_directories/src/module/utils/nested_function.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/sub_utils/mod.rs b/testdata/with_child_directories/src/module/utils/sub_utils/mod.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/sub_utils/mod.rs
rename to testdata/with_child_directories/src/module/utils/sub_utils/mod.rs
diff --git a/testdata/tree/with_child_directories/src/module/utils/sub_utils/subutils_nested_function.rs b/testdata/with_child_directories/src/module/utils/sub_utils/subutils_nested_function.rs
similarity index 100%
rename from testdata/tree/with_child_directories/src/module/utils/sub_utils/subutils_nested_function.rs
rename to testdata/with_child_directories/src/module/utils/sub_utils/subutils_nested_function.rs
diff --git a/testdata/tree/workspace/Cargo.toml b/testdata/workspace/Cargo.toml
similarity index 100%
rename from testdata/tree/workspace/Cargo.toml
rename to testdata/workspace/Cargo.toml
diff --git a/testdata/tree/workspace/README.md b/testdata/workspace/README.md
similarity index 100%
rename from testdata/tree/workspace/README.md
rename to testdata/workspace/README.md
diff --git a/testdata/tree/workspace/main/Cargo.toml b/testdata/workspace/main/Cargo.toml
similarity index 100%
rename from testdata/tree/workspace/main/Cargo.toml
rename to testdata/workspace/main/Cargo.toml
diff --git a/testdata/tree/workspace/main/src/main.rs b/testdata/workspace/main/src/main.rs
similarity index 100%
rename from testdata/tree/workspace/main/src/main.rs
rename to testdata/workspace/main/src/main.rs
diff --git a/testdata/tree/workspace/main2/Cargo.toml b/testdata/workspace/main2/Cargo.toml
similarity index 100%
rename from testdata/tree/workspace/main2/Cargo.toml
rename to testdata/workspace/main2/Cargo.toml
diff --git a/testdata/tree/workspace/main2/src/main.rs b/testdata/workspace/main2/src/main.rs
similarity index 100%
rename from testdata/tree/workspace/main2/src/main.rs
rename to testdata/workspace/main2/src/main.rs
diff --git a/testdata/tree/workspace/utils/Cargo.toml b/testdata/workspace/utils/Cargo.toml
similarity index 100%
rename from testdata/tree/workspace/utils/Cargo.toml
rename to testdata/workspace/utils/Cargo.toml
diff --git a/testdata/tree/workspace/utils/src/lib.rs b/testdata/workspace/utils/src/lib.rs
similarity index 100%
rename from testdata/tree/workspace/utils/src/lib.rs
rename to testdata/workspace/utils/src/lib.rs
diff --git a/tests/cli/in_diff.rs b/tests/cli/in_diff.rs
index bd165c3c..731b5ed6 100644
--- a/tests/cli/in_diff.rs
+++ b/tests/cli/in_diff.rs
@@ -23,8 +23,8 @@ fn diff_trees_well_tested() {
 
 #[test]
 fn list_mutants_changed_in_diff1() {
-    let src0 = read_to_string("testdata/tree/diff0/src/lib.rs").unwrap();
-    let src1 = read_to_string("testdata/tree/diff1/src/lib.rs").unwrap();
+    let src0 = read_to_string("testdata/diff0/src/lib.rs").unwrap();
+    let src1 = read_to_string("testdata/diff1/src/lib.rs").unwrap();
     let diff = TextDiff::from_lines(&src0, &src1)
         .unified_diff()
         .context_radius(2)
@@ -70,8 +70,8 @@ fn list_mutants_changed_in_diff1() {
 /// If the text in the diff doesn't look like the tree then error out.
 #[test]
 fn mismatched_diff_causes_error() {
-    let src0 = read_to_string("testdata/tree/diff0/src/lib.rs").unwrap();
-    let src1 = read_to_string("testdata/tree/diff1/src/lib.rs").unwrap();
+    let src0 = read_to_string("testdata/diff0/src/lib.rs").unwrap();
+    let src1 = read_to_string("testdata/diff1/src/lib.rs").unwrap();
     let diff = TextDiff::from_lines(&src0, &src1)
         .unified_diff()
         .context_radius(2)
diff --git a/tests/cli/main.rs b/tests/cli/main.rs
index 3b3bacf9..aa62890c 100644
--- a/tests/cli/main.rs
+++ b/tests/cli/main.rs
@@ -87,7 +87,7 @@ fn copy_of_testdata(tree_name: &str) -> TempDir {
                 .iter()
                 .all(|p| !path.starts_with(p)))
         })
-        .copy_tree(Path::new("testdata/tree").join(tree_name), &tmp_src_dir)
+        .copy_tree(Path::new("testdata").join(tree_name), &tmp_src_dir)
         .unwrap();
     tmp_src_dir
 }
@@ -168,7 +168,7 @@ fn list_diff_json_contains_diffs() {
             "--json",
             "--diff",
             "-d",
-            "testdata/tree/factorial",
+            "testdata/factorial",
         ])
         .assert()
         .success(); // needed for lifetime
@@ -187,7 +187,7 @@ fn list_diff_json_contains_diffs() {
 /// Return paths to all testdata trees, in order, excluding leftover git
 /// detritus with no Cargo.toml.
 fn all_testdata_tree_paths() -> Vec<PathBuf> {
-    let mut paths: Vec<PathBuf> = fs::read_dir("testdata/tree")
+    let mut paths: Vec<PathBuf> = fs::read_dir("testdata")
         .unwrap()
         .map(|r| r.unwrap())
         .filter(|dir_entry| dir_entry.file_type().unwrap().is_dir())
@@ -245,7 +245,7 @@ fn list_mutants_in_factorial() {
     run()
         .arg("mutants")
         .arg("--list")
-        .current_dir("testdata/tree/factorial")
+        .current_dir("testdata/factorial")
         .assert_insta("list_mutants_in_factorial");
 }
 
@@ -255,7 +255,7 @@ fn list_mutants_in_factorial_json() {
         .arg("mutants")
         .arg("--list")
         .arg("--json")
-        .current_dir("testdata/tree/factorial")
+        .current_dir("testdata/factorial")
         .assert_insta("list_mutants_in_factorial_json");
 }
 
@@ -307,7 +307,7 @@ fn list_mutants_with_dir_option() {
         .arg("mutants")
         .arg("--list")
         .arg("--dir")
-        .arg("testdata/tree/factorial")
+        .arg("testdata/factorial")
         .assert_insta("list_mutants_with_dir_option");
 }
 
@@ -317,7 +317,7 @@ fn list_mutants_with_diffs_in_factorial() {
         .arg("mutants")
         .arg("--list")
         .arg("--diff")
-        .current_dir("testdata/tree/factorial")
+        .current_dir("testdata/factorial")
         .assert_insta("list_mutants_with_diffs_in_factorial");
 }
 
@@ -326,7 +326,7 @@ fn list_mutants_well_tested() {
     run()
         .arg("mutants")
         .arg("--list")
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_mutants_well_tested");
 }
 
@@ -335,7 +335,7 @@ fn list_mutants_well_tested_examine_name_filter() {
     run()
         .arg("mutants")
         .args(["--list", "--file", "nested_function.rs"])
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_mutants_well_tested_examine_name_filter");
 }
 
@@ -344,7 +344,7 @@ fn list_mutants_well_tested_exclude_name_filter() {
     run()
         .arg("mutants")
         .args(["--list", "--exclude", "simple_fns.rs"])
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_mutants_well_tested_exclude_name_filter");
 }
 
@@ -353,7 +353,7 @@ fn list_mutants_well_tested_exclude_folder_filter() {
     run()
         .arg("mutants")
         .args(["--list", "--exclude", "*/module/*"])
-        .current_dir("testdata/tree/with_child_directories")
+        .current_dir("testdata/with_child_directories")
         .assert_insta("list_mutants_well_tested_exclude_folder_filter");
 }
 
@@ -368,7 +368,7 @@ fn list_mutants_well_tested_examine_and_exclude_name_filter_combined() {
             "--exclude",
             "nested_function.rs",
         ])
-        .current_dir("testdata/tree/with_child_directories")
+        .current_dir("testdata/with_child_directories")
         .assert_insta("list_mutants_well_tested_examine_and_exclude_name_filter_combined");
 }
 
@@ -378,7 +378,7 @@ fn list_mutants_regex_filters() {
         .arg("mutants")
         .args(["--list", "--re", "divisible"])
         .arg("-d")
-        .arg("testdata/tree/well_tested")
+        .arg("testdata/well_tested")
         .assert_insta("list_mutants_regex_filters");
 }
 
@@ -392,7 +392,7 @@ fn list_mutants_regex_anchored_matches_full_line() {
             r"^src/simple_fns.rs:\d+: replace returns_unit with \(\)$",
         ])
         .arg("-d")
-        .arg("testdata/tree/well_tested")
+        .arg("testdata/well_tested")
         .assert_insta("list_mutants_regex_anchored_matches_full_line");
 }
 
@@ -409,7 +409,7 @@ fn list_mutants_regex_filters_json() {
             "--json",
         ])
         .arg("-d")
-        .arg("testdata/tree/well_tested")
+        .arg("testdata/well_tested")
         .assert_insta("list_mutants_regex_filters_json");
 }
 
@@ -429,7 +429,7 @@ fn list_mutants_well_tested_multiple_examine_and_exclude_name_filter_with_files_
     run()
         .arg("mutants")
         .args(["--list", "--file", "module_methods.rs", "--file", "*/utils/*", "--exclude", "*/sub_utils/*", "--exclude", "nested_function.rs"])
-        .current_dir("testdata/tree/with_child_directories")
+        .current_dir("testdata/with_child_directories")
         .assert_insta("list_mutants_well_tested_multiple_examine_and_exclude_name_filter_with_files_and_folders");
 }
 
@@ -439,7 +439,7 @@ fn list_mutants_json_well_tested() {
         .arg("mutants")
         .arg("--list")
         .arg("--json")
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_mutants_json_well_tested");
 }
 
@@ -448,7 +448,7 @@ fn list_files_text_well_tested() {
     run()
         .arg("mutants")
         .arg("--list-files")
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_files_text_well_tested");
 }
 
@@ -459,7 +459,7 @@ fn list_files_respects_file_filters() {
     run()
         .arg("mutants")
         .args(["--list-files", "--exclude", "lib.rs"])
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert()
         .success()
         .stdout(predicate::str::contains("methods.rs"))
@@ -472,7 +472,7 @@ fn list_files_json_well_tested() {
         .arg("mutants")
         .arg("--list-files")
         .arg("--json")
-        .current_dir("testdata/tree/well_tested")
+        .current_dir("testdata/well_tested")
         .assert_insta("list_files_json_well_tested");
 }
 #[test]
@@ -1164,7 +1164,7 @@ fn cargo_mutants_in_override_dependency_tree_passes() {
         .arg("--no-times")
         .arg("--no-shuffle")
         .arg("-d")
-        .arg("testdata/tree/override_dependency")
+        .arg("testdata/override_dependency")
         .assert()
         .success()
         .stdout(predicate::function(|stdout| {
@@ -1182,7 +1182,7 @@ fn cargo_mutants_in_relative_dependency_tree_passes() {
         .arg("--no-times")
         .arg("--no-shuffle")
         .arg("-d")
-        .arg("testdata/tree/relative_dependency")
+        .arg("testdata/relative_dependency")
         .assert()
         .success()
         .stdout(predicate::function(|stdout| {
@@ -1200,7 +1200,7 @@ fn cargo_mutants_in_replace_dependency_tree_passes() {
         .arg("--no-times")
         .arg("--no-shuffle")
         .arg("-d")
-        .arg("testdata/tree/replace_dependency")
+        .arg("testdata/replace_dependency")
         .assert()
         .success()
         .stdout(predicate::function(|stdout| {
@@ -1218,7 +1218,7 @@ fn cargo_mutants_in_patch_dependency_tree_passes() {
         .arg("--no-times")
         .arg("--no-shuffle")
         .arg("-d")
-        .arg("testdata/tree/patch_dependency")
+        .arg("testdata/patch_dependency")
         .assert()
         .success()
         .stdout(predicate::function(|stdout| {
diff --git a/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_json.snap b/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_json.snap
index 493298b9..faaff435 100644
--- a/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_json.snap
+++ b/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_json.snap
@@ -2,7 +2,7 @@
 source: tests/cli/main.rs
 expression: buf
 ---
-## testdata/tree/already_failing_doctests
+## testdata/already_failing_doctests
 
 ```json
 [
@@ -27,7 +27,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/already_failing_tests
+## testdata/already_failing_tests
 
 ```json
 [
@@ -52,7 +52,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/already_hangs
+## testdata/already_hangs
 
 ```json
 [
@@ -68,7 +68,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/cdylib
+## testdata/cdylib
 
 ```json
 [
@@ -93,13 +93,13 @@ expression: buf
 ]
 ```
 
-## testdata/tree/cfg_attr_mutants_skip
+## testdata/cfg_attr_mutants_skip
 
 ```json
 []
 ```
 
-## testdata/tree/cfg_attr_test_skip
+## testdata/cfg_attr_test_skip
 
 ```json
 [
@@ -124,7 +124,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/dependency
+## testdata/dependency
 
 ```json
 [
@@ -149,7 +149,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/diff0
+## testdata/diff0
 
 ```json
 [
@@ -174,7 +174,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/diff1
+## testdata/diff1
 
 ```json
 [
@@ -217,7 +217,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/error_value
+## testdata/error_value
 
 ```json
 [
@@ -251,13 +251,13 @@ expression: buf
 ]
 ```
 
-## testdata/tree/everything_skipped
+## testdata/everything_skipped
 
 ```json
 []
 ```
 
-## testdata/tree/factorial
+## testdata/factorial
 
 ```json
 [
@@ -291,7 +291,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/fails_without_feature
+## testdata/fails_without_feature
 
 ```json
 [
@@ -316,7 +316,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/hang_avoided_by_attr
+## testdata/hang_avoided_by_attr
 
 ```json
 [
@@ -332,7 +332,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/hang_when_mutated
+## testdata/hang_when_mutated
 
 ```json
 [
@@ -375,7 +375,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/insta
+## testdata/insta
 
 ```json
 [
@@ -400,7 +400,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/integration_tests
+## testdata/integration_tests
 
 ```json
 [
@@ -425,7 +425,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/missing_test
+## testdata/missing_test
 
 ```json
 [
@@ -450,7 +450,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/mut_ref
+## testdata/mut_ref
 
 ```json
 [
@@ -475,13 +475,13 @@ expression: buf
 ]
 ```
 
-## testdata/tree/never_type
+## testdata/never_type
 
 ```json
 []
 ```
 
-## testdata/tree/nightly_only
+## testdata/nightly_only
 
 ```json
 [
@@ -515,7 +515,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/override_dependency
+## testdata/override_dependency
 
 ```json
 [
@@ -540,7 +540,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/package_fails
+## testdata/package_fails
 
 ```json
 [
@@ -583,7 +583,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/patch_dependency
+## testdata/patch_dependency
 
 ```json
 [
@@ -608,7 +608,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/relative_dependency
+## testdata/relative_dependency
 
 ```json
 [
@@ -633,7 +633,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/replace_dependency
+## testdata/replace_dependency
 
 ```json
 [
@@ -658,7 +658,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/small_well_tested
+## testdata/small_well_tested
 
 ```json
 [
@@ -683,7 +683,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/strict_warnings
+## testdata/strict_warnings
 
 ```json
 [
@@ -708,7 +708,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/struct_with_no_default
+## testdata/struct_with_no_default
 
 ```json
 [
@@ -724,7 +724,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/typecheck_fails
+## testdata/typecheck_fails
 
 ```json
 [
@@ -749,7 +749,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/unapply
+## testdata/unapply
 
 ```json
 [
@@ -837,13 +837,13 @@ expression: buf
 ]
 ```
 
-## testdata/tree/unsafe
+## testdata/unsafe
 
 ```json
 []
 ```
 
-## testdata/tree/well_tested
+## testdata/well_tested
 
 ```json
 [
@@ -1192,7 +1192,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/with_child_directories
+## testdata/with_child_directories
 
 ```json
 [
@@ -1325,7 +1325,7 @@ expression: buf
 ]
 ```
 
-## testdata/tree/workspace
+## testdata/workspace
 
 ```json
 [
diff --git a/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_text.snap b/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_text.snap
index 9570825e..b1bb0065 100644
--- a/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_text.snap
+++ b/tests/cli/snapshots/cli__list_mutants_in_all_trees_as_text.snap
@@ -2,60 +2,60 @@
 source: tests/cli/main.rs
 expression: buf
 ---
-## testdata/tree/already_failing_doctests
+## testdata/already_failing_doctests
 
 ```
 src/lib.rs:9: replace takes_one_arg -> usize with 0
 src/lib.rs:9: replace takes_one_arg -> usize with 1
 ```
 
-## testdata/tree/already_failing_tests
+## testdata/already_failing_tests
 
 ```
 src/lib.rs:1: replace factorial -> u32 with 0
 src/lib.rs:1: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/already_hangs
+## testdata/already_hangs
 
 ```
 src/lib.rs:8: replace infinite_loop with ()
 ```
 
-## testdata/tree/cdylib
+## testdata/cdylib
 
 ```
 src/entry.rs:1: replace factorial -> u32 with 0
 src/entry.rs:1: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/cfg_attr_mutants_skip
+## testdata/cfg_attr_mutants_skip
 
 ```
 ```
 
-## testdata/tree/cfg_attr_test_skip
+## testdata/cfg_attr_test_skip
 
 ```
 src/lib.rs:17: replace double -> usize with 0
 src/lib.rs:17: replace double -> usize with 1
 ```
 
-## testdata/tree/dependency
+## testdata/dependency
 
 ```
 src/lib.rs:1: replace factorial -> u32 with 0
 src/lib.rs:1: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/diff0
+## testdata/diff0
 
 ```
 src/lib.rs:1: replace one -> String with String::new()
 src/lib.rs:1: replace one -> String with "xyzzy".into()
 ```
 
-## testdata/tree/diff1
+## testdata/diff1
 
 ```
 src/lib.rs:1: replace one -> String with String::new()
@@ -64,7 +64,7 @@ src/lib.rs:5: replace two -> String with String::new()
 src/lib.rs:5: replace two -> String with "xyzzy".into()
 ```
 
-## testdata/tree/error_value
+## testdata/error_value
 
 ```
 src/lib.rs:3: replace even_is_ok -> Result<u32, &'static str> with Ok(0)
@@ -72,12 +72,12 @@ src/lib.rs:3: replace even_is_ok -> Result<u32, &'static str> with Ok(1)
 src/lib.rs:3: replace even_is_ok -> Result<u32, &'static str> with Err("injected")
 ```
 
-## testdata/tree/everything_skipped
+## testdata/everything_skipped
 
 ```
 ```
 
-## testdata/tree/factorial
+## testdata/factorial
 
 ```
 src/bin/factorial.rs:1: replace main with ()
@@ -85,20 +85,20 @@ src/bin/factorial.rs:7: replace factorial -> u32 with 0
 src/bin/factorial.rs:7: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/fails_without_feature
+## testdata/fails_without_feature
 
 ```
 src/bin/factorial.rs:9: replace factorial -> u32 with 0
 src/bin/factorial.rs:9: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/hang_avoided_by_attr
+## testdata/hang_avoided_by_attr
 
 ```
 src/lib.rs:14: replace controlled_loop with ()
 ```
 
-## testdata/tree/hang_when_mutated
+## testdata/hang_when_mutated
 
 ```
 src/lib.rs:12: replace should_stop -> bool with true
@@ -107,40 +107,40 @@ src/lib.rs:25: replace controlled_loop -> usize with 0
 src/lib.rs:25: replace controlled_loop -> usize with 1
 ```
 
-## testdata/tree/insta
+## testdata/insta
 
 ```
 src/lib.rs:1: replace say_hello -> String with String::new()
 src/lib.rs:1: replace say_hello -> String with "xyzzy".into()
 ```
 
-## testdata/tree/integration_tests
+## testdata/integration_tests
 
 ```
 src/lib.rs:1: replace double -> u32 with 0
 src/lib.rs:1: replace double -> u32 with 1
 ```
 
-## testdata/tree/missing_test
+## testdata/missing_test
 
 ```
 src/lib.rs:1: replace is_symlink -> bool with true
 src/lib.rs:1: replace is_symlink -> bool with false
 ```
 
-## testdata/tree/mut_ref
+## testdata/mut_ref
 
 ```
 src/lib.rs:1: replace returns_mut_ref -> &mut u32 with Box::leak(Box::new(0))
 src/lib.rs:1: replace returns_mut_ref -> &mut u32 with Box::leak(Box::new(1))
 ```
 
-## testdata/tree/never_type
+## testdata/never_type
 
 ```
 ```
 
-## testdata/tree/nightly_only
+## testdata/nightly_only
 
 ```
 src/lib.rs:2: replace box_an_int -> Box<i32> with Box::new(0)
@@ -148,14 +148,14 @@ src/lib.rs:2: replace box_an_int -> Box<i32> with Box::new(1)
 src/lib.rs:2: replace box_an_int -> Box<i32> with Box::new(-1)
 ```
 
-## testdata/tree/override_dependency
+## testdata/override_dependency
 
 ```
 src/lib.rs:6: replace is_even -> bool with true
 src/lib.rs:6: replace is_even -> bool with false
 ```
 
-## testdata/tree/package_fails
+## testdata/package_fails
 
 ```
 failing/src/lib.rs:1: replace triple -> usize with 0
@@ -164,55 +164,55 @@ passing/src/lib.rs:1: replace triple -> usize with 0
 passing/src/lib.rs:1: replace triple -> usize with 1
 ```
 
-## testdata/tree/patch_dependency
+## testdata/patch_dependency
 
 ```
 src/lib.rs:6: replace is_even -> bool with true
 src/lib.rs:6: replace is_even -> bool with false
 ```
 
-## testdata/tree/relative_dependency
+## testdata/relative_dependency
 
 ```
 src/lib.rs:5: replace double_factorial -> u32 with 0
 src/lib.rs:5: replace double_factorial -> u32 with 1
 ```
 
-## testdata/tree/replace_dependency
+## testdata/replace_dependency
 
 ```
 src/lib.rs:6: replace is_even -> bool with true
 src/lib.rs:6: replace is_even -> bool with false
 ```
 
-## testdata/tree/small_well_tested
+## testdata/small_well_tested
 
 ```
 src/lib.rs:4: replace factorial -> u32 with 0
 src/lib.rs:4: replace factorial -> u32 with 1
 ```
 
-## testdata/tree/strict_warnings
+## testdata/strict_warnings
 
 ```
 src/lib.rs:5: replace some_fn -> usize with 0
 src/lib.rs:5: replace some_fn -> usize with 1
 ```
 
-## testdata/tree/struct_with_no_default
+## testdata/struct_with_no_default
 
 ```
 src/lib.rs:11: replace make_an_s -> S with Default::default()
 ```
 
-## testdata/tree/typecheck_fails
+## testdata/typecheck_fails
 
 ```
 src/lib.rs:5: replace try_value_coercion -> String with String::new()
 src/lib.rs:5: replace try_value_coercion -> String with "xyzzy".into()
 ```
 
-## testdata/tree/unapply
+## testdata/unapply
 
 ```
 src/a.rs:1: replace one -> i32 with 0
@@ -226,12 +226,12 @@ src/c.rs:1: replace one -> i32 with 1
 src/c.rs:1: replace one -> i32 with -1
 ```
 
-## testdata/tree/unsafe
+## testdata/unsafe
 
 ```
 ```
 
-## testdata/tree/well_tested
+## testdata/well_tested
 
 ```
 src/arc.rs:3: replace return_arc -> Arc<String> with Arc::new(String::new())
@@ -274,7 +274,7 @@ src/struct_with_lifetime.rs:14: replace Lex<'buf>::buf_len -> usize with 0
 src/struct_with_lifetime.rs:14: replace Lex<'buf>::buf_len -> usize with 1
 ```
 
-## testdata/tree/with_child_directories
+## testdata/with_child_directories
 
 ```
 src/methods.rs:1: replace double -> usize with 0
@@ -293,7 +293,7 @@ src/module/utils/sub_utils/subutils_nested_function.rs:2: replace has_nested::in
 src/module/utils/sub_utils/subutils_nested_function.rs:2: replace has_nested::inner -> u32 with 1
 ```
 
-## testdata/tree/workspace
+## testdata/workspace
 
 ```
 utils/src/lib.rs:1: replace triple -> i32 with 0
diff --git a/tests/cli/trace.rs b/tests/cli/trace.rs
index 8ac69bb6..05d56815 100644
--- a/tests/cli/trace.rs
+++ b/tests/cli/trace.rs
@@ -12,7 +12,7 @@ fn env_var_controls_trace() {
         .env("CARGO_MUTANTS_TRACE_LEVEL", "trace")
         .args(["mutants", "--list"])
         .arg("-d")
-        .arg("testdata/tree/never_type")
+        .arg("testdata/never_type")
         .assert()
         // This is a debug!() message; it should only be seen if the trace var
         // was wired correctly to stderr.
diff --git a/tests/cli/windows.rs b/tests/cli/windows.rs
index 7cda14db..26defb37 100644
--- a/tests/cli/windows.rs
+++ b/tests/cli/windows.rs
@@ -9,6 +9,6 @@ fn list_mutants_well_tested_exclude_folder_containing_backslash_on_windows() {
     run()
         .arg("mutants")
         .args(["--list", "--exclude", "*\\module\\*"])
-        .current_dir("testdata/tree/with_child_directories")
+        .current_dir("testdata/with_child_directories")
         .assert_insta("list_mutants_well_tested_exclude_folder_filter");
 }
diff --git a/tests/cli/workspace.rs b/tests/cli/workspace.rs
index 79b4e3d5..1fb12ea1 100644
--- a/tests/cli/workspace.rs
+++ b/tests/cli/workspace.rs
@@ -17,7 +17,7 @@ fn list_warns_about_unmatched_packages() {
             "mutants",
             "--list",
             "-d",
-            "testdata/tree/workspace",
+            "testdata/workspace",
             "-p",
             "notapackage",
         ])
@@ -33,7 +33,7 @@ fn list_files_json_workspace() {
     // Demonstrates that we get package names in the json listing.
     let cmd = run()
         .args(["mutants", "--list-files", "--json"])
-        .current_dir("testdata/tree/workspace")
+        .current_dir("testdata/workspace")
         .assert()
         .success();
     assert_bytes_eq_json(
@@ -61,7 +61,7 @@ fn list_files_json_workspace() {
 fn list_files_as_json_in_workspace_subdir() {
     let cmd = run()
         .args(["mutants", "--list-files", "--json", "--workspace"])
-        .current_dir("testdata/tree/workspace/main2")
+        .current_dir("testdata/workspace/main2")
         .assert()
         .success();
     assert_bytes_eq_json(