Make unnecessary-lambda an always-unsafe fix

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

@@ -1,6 +1,5 @@
 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;
@@ -27,15 +26,23 @@ use crate::checkers::ast::Checker;
 /// ```
 ///
 /// ## Fix safety
-/// This rule's fix is marked as unsafe in cases in which the lambda body itself
-/// contains an effect.
+/// This rule's fix is marked as unsafe for two primary reasons.
+///
+/// First, the lambda body itself could contain 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.
+/// However, even when the lambda body itself is pure, the lambda may
+/// change the argument names, which can lead to a change in behavior when
+/// callers pass arguments by name.
+///
+/// For example, replacing `foo = lambda x, y: func(x, y)` with `foo = func`,
+/// where `func` is defined as `def func(a, b): return a + b`, would be a
+/// breaking change for callers that execute the lambda by passing arguments by
+/// name, as in: `foo(x=1, y=2)`. Since `func` does not define the arguments
+/// `x` and `y`, unlike the lambda, the call would raise a `TypeError`.
 #[violation]
 pub struct UnnecessaryLambda;
 
@@ -206,11 +213,7 @@ pub(crate) fn unnecessary_lambda(checker: &mut Checker, lambda: &ExprLambda) {
             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
-        },
+        Applicability::Unsafe,
     ));
     checker.diagnostics.push(diagnostic);
 }

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

@@ -9,7 +9,7 @@ unnecessary_lambda.py:1:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe fix
 1   |-_ = lambda: print()  # [unnecessary-lambda]
   1 |+_ = print  # [unnecessary-lambda]
 2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
@@ -26,7 +26,7 @@ unnecessary_lambda.py:2:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe fix
 1 1 | _ = lambda: print()  # [unnecessary-lambda]
 2   |-_ = lambda x, y: min(x, y) # [unnecessary-lambda]
   2 |+_ = min # [unnecessary-lambda]
@@ -45,7 +45,7 @@ unnecessary_lambda.py:4:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe fix
 1 1 | _ = lambda: print()  # [unnecessary-lambda]
 2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
 3 3 | 
@@ -65,7 +65,7 @@ unnecessary_lambda.py:5:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe fix
 2 2 | _ = lambda x, y: min(x, y) # [unnecessary-lambda]
 3 3 | 
 4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
@@ -85,7 +85,7 @@ unnecessary_lambda.py:6:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe fix
 3 3 | 
 4 4 | _ = lambda *args: f(*args) # [unnecessary-lambda]
 5 5 | _ = lambda **kwargs: f(**kwargs) # [unnecessary-lambda]
@@ -106,7 +106,7 @@ unnecessary_lambda.py:7:5: PLW0108 [*] Lambda may be unnecessary; consider inlin
   |
   = help: Inline function call
 
-ℹ Safe fix
+ℹ Unsafe 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]
@@ -155,5 +155,3 @@ unnecessary_lambda.py:10:5: PLW0108 [*] Lambda may be unnecessary; consider inli
 11 11 | 
 12 12 | # default value in lambda parameters
 13 13 | _ = lambda x=42: print(x)
-
-