[red-knot] Fallback to requires-python if no python-version is specified

crates/red_knot_project/src/metadata.rs

@@ -874,7 +874,7 @@ expected `.`, `]`
             return Err(anyhow!("Expected project discovery to fail because the `requires-python` doesn't specify a lower bound (it only specifies an upper bound)."));
         };
 
-        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `<3.12` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`).");
+        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `<3.12` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.");
 
         Ok(())
     }
@@ -899,7 +899,7 @@ expected `.`, `]`
             return Err(anyhow!("Expected project discovery to fail because the `requires-python` specifiers are empty and don't define a lower bound."));
         };
 
-        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`).");
+        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.");
 
         Ok(())
     }

crates/red_knot_project/src/metadata/pyproject.rs

@@ -105,13 +105,11 @@ impl Project {
         let minor =
             u8::try_from(minor).map_err(|_| ResolveRequiresPythonError::TooLargeMajor(minor))?;
 
-        let python_version = PythonVersion::from((major, minor));
-
-        Ok(Some(if let Some(range) = requires_python.range() {
-            RangedValue::with_range(python_version, requires_python.source().clone(), range)
-        } else {
-            RangedValue::new(python_version, requires_python.source().clone())
-        }))
+        Ok(Some(
+            requires_python
+                .clone()
+                .map_value(|_| PythonVersion::from((major, minor))),
+        ))
     }
 }
 
@@ -121,7 +119,7 @@ pub enum ResolveRequiresPythonError {
     TooLargeMajor(u64),
     #[error("The minor version `{0}` is larger than the maximum supported value 255")]
     TooLargeMinor(u64),
-    #[error("value `{0}` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`).")]
+    #[error("value `{0}` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.")]
     NoLowerBound(String),
 }
 

crates/red_knot_project/src/metadata/value.rs

@@ -118,6 +118,15 @@ impl<T> RangedValue<T> {
         self
     }
 
+    #[must_use]
+    pub fn map_value<R>(self, f: impl FnOnce(T) -> R) -> RangedValue<R> {
+        RangedValue {
+            value: f(self.value),
+            source: self.source,
+            range: self.range,
+        }
+    }
+
     pub fn into_inner(self) -> T {
         self.value
     }