Implement autofix for unnecessary-lambda (PLW0108) (#8621)

Closes https://github.com/astral-sh/ruff/issues/8618.

crates/ruff_linter/src/rules/pylint/rules/unnecessary_lambda.rs

@@ -1,5 +1,6 @@
-use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_diagnostics::{AlwaysFixableViolation, Applicability, Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::helpers::contains_effect;
 use ruff_python_ast::visitor::Visitor;
 use ruff_python_ast::{self as ast, visitor, Expr, ExprLambda, Parameter, ParameterWithDefault};
 use ruff_text_size::Ranged;
@@ -24,14 +25,29 @@ use crate::checkers::ast::Checker;
 /// ```python
 /// df.apply(str)
 /// ```
+///
+/// ## Fix safety
+/// This rule's fix is marked as unsafe in cases in which the lambda body itself
+/// contains an effect.
+///
+/// For example, replacing `lambda x, y: (func()(x, y))` with `func()` would
+/// lead to a change in behavior, as `func()` would be evaluated eagerly when
+/// defining the lambda, rather than when the lambda is called.
+///
+/// When the lambda body contains no visible effects, the fix is considered
+/// safe.
 #[violation]
 pub struct UnnecessaryLambda;
 
-impl Violation for UnnecessaryLambda {
+impl AlwaysFixableViolation for UnnecessaryLambda {
     #[derive_message_formats]
     fn message(&self) -> String {
         format!("Lambda may be unnecessary; consider inlining inner function")
     }
+
+    fn fix_title(&self) -> String {
+        "Inline function call".to_string()
+    }
 }
 
 /// PLW0108
@@ -184,9 +200,19 @@ pub(crate) fn unnecessary_lambda(checker: &mut Checker, lambda: &ExprLambda) {
         }
     }
 
-    checker
+    let mut diagnostic = Diagnostic::new(UnnecessaryLambda, lambda.range());
-        .diagnostics
+    diagnostic.set_fix(Fix::applicable_edit(
-        .push(Diagnostic::new(UnnecessaryLambda, lambda.range()));
+        Edit::range_replacement(
+            checker.locator().slice(func.as_ref()).to_string(),
+            lambda.range(),
+        ),
+        if contains_effect(func.as_ref(), |id| checker.semantic().is_builtin(id)) {
+            Applicability::Unsafe
+        } else {
+            Applicability::Safe
+        },
+    ));
+    checker.diagnostics.push(diagnostic);
 }
 
 /// Identify all `Expr::Name` nodes in an AST.

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLW0108_unnecessary_lambda.py.snap

@@ -1,14 +1,22 @@
 ---
 source: crates/ruff_linter/src/rules/pylint/mod.rs
 ---
-unnecessary_lambda.py:1:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+unnecessary_lambda.py:1:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 1 | _ = lambda: print()  # [unnecessary-lambda]
   |     ^^^^^^^^^^^^^^^ PLW0108
 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:2:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+1   |-_ = lambda: print()  # [unnecessary-lambda]
+  1 |+_ = print  # [unnecessary-lambda]
+2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
+3 3 | 
+4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
+
+unnecessary_lambda.py:2:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 1 | _ = lambda: print()  # [unnecessary-lambda]
 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
@@ -16,8 +24,17 @@ unnecessary_lambda.py:2:5: PLW0108 Lambda may be unnecessary; consider inlining
 3 | 
 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:4:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+1 1 | _ = lambda: print()  # [unnecessary-lambda]
+2   |-_ = lambda x, y: min(x, y) # [unnecessary-lambda]
+  2 |+_ = min # [unnecessary-lambda]
+3 3 | 
+4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
+5 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
+
+unnecessary_lambda.py:4:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
 3 | 
@@ -26,8 +43,19 @@ unnecessary_lambda.py:4:5: PLW0108 Lambda may be unnecessary; consider inlining
 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:5:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+1 1 | _ = lambda: print()  # [unnecessary-lambda]
+2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
+3 3 | 
+4   |-_ = lambda *args: f(*args) # [unnecessary-lambda]
+  4 |+_ = f # [unnecessary-lambda]
+5 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
+6 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
+7 7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+
+unnecessary_lambda.py:5:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
@@ -35,8 +63,19 @@ unnecessary_lambda.py:5:5: PLW0108 Lambda may be unnecessary; consider inlining
 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
 7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:6:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
+3 3 | 
+4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
+5   |-_ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
+  5 |+_ = f # [unnecessary-lambda]
+6 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
+7 7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+8 8 | 
+
+unnecessary_lambda.py:6:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
@@ -44,8 +83,19 @@ unnecessary_lambda.py:6:5: PLW0108 Lambda may be unnecessary; consider inlining
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLW0108
 7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:7:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+3 3 | 
+4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
+5 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
+6   |-_ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
+  6 |+_ = f # [unnecessary-lambda]
+7 7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+8 8 | 
+9 9 | _ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
+
+unnecessary_lambda.py:7:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
   |
 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
@@ -54,8 +104,19 @@ unnecessary_lambda.py:7:5: PLW0108 Lambda may be unnecessary; consider inlining
 8 | 
 9 | _ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
   |
+  = help: Inline function call
 
-unnecessary_lambda.py:9:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Safe fix
+4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
+5 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
+6 6 | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
+7   |-_ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+  7 |+_ = f # [unnecessary-lambda]
+8 8 | 
+9 9 | _ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
+10 10 | _ = lambda x, y: f(lambda x, y: x + y)(x, y) # [unnecessary-lambda]
+
+unnecessary_lambda.py:9:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
    |
  7 | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
  8 | 
@@ -63,8 +124,19 @@ unnecessary_lambda.py:9:5: PLW0108 Lambda may be unnecessary; consider inlining
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLW0108
 10 | _ = lambda x, y: f(lambda x, y: x + y)(x, y) # [unnecessary-lambda]
    |
+   = help: Inline function call
 
-unnecessary_lambda.py:10:5: PLW0108 Lambda may be unnecessary; consider inlining inner function
+ℹ Unsafe fix
+6  6  | _ = lambda *args, **kwargs: f(*args, **kwargs) # [unnecessary-lambda]
+7  7  | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+8  8  | 
+9     |-_ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
+   9  |+_ = f(lambda x: x) # [unnecessary-lambda]
+10 10 | _ = lambda x, y: f(lambda x, y: x + y)(x, y) # [unnecessary-lambda]
+11 11 | 
+12 12 | # default value in lambda parameters
+
+unnecessary_lambda.py:10:5: PLW0108 [*] Lambda may be unnecessary; consider inlining inner function
    |
  9 | _ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
 10 | _ = lambda x, y: f(lambda x, y: x + y)(x, y) # [unnecessary-lambda]
@@ -72,5 +144,16 @@ unnecessary_lambda.py:10:5: PLW0108 Lambda may be unnecessary; consider inlining
 11 | 
 12 | # default value in lambda parameters
    |
+   = help: Inline function call
+
+ℹ Unsafe fix
+7  7  | _ = lambda x, y, z, *args, **kwargs: f(x, y, z, *args, **kwargs) # [unnecessary-lambda]
+8  8  | 
+9  9  | _ = lambda x: f(lambda x: x)(x) # [unnecessary-lambda]
+10    |-_ = lambda x, y: f(lambda x, y: x + y)(x, y) # [unnecessary-lambda]
+   10 |+_ = f(lambda x, y: x + y) # [unnecessary-lambda]
+11 11 | 
+12 12 | # default value in lambda parameters
+13 13 | _ = lambda x=42: print(x)