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[feat]: make unnecessary_mut_passed lint auto applicable #14626

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[feat]: make unnecessary_mut_passed lint auto applicable #14626

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d5a4c52
something nice
MCJOHN974 Apr 16, 2025
6ab0b14
feat: make unnecessary mut autofixable
MCJOHN974 Apr 16, 2025
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11 changes: 9 additions & 2 deletions clippy_lints/src/mut_reference.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,6 @@
use clippy_utils::diagnostics::span_lint;
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet;
use rustc_errors::Applicability;
use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
Expand Down Expand Up @@ -86,11 +88,16 @@ fn check_arguments<'tcx>(
match parameter.kind() {
ty::Ref(_, _, Mutability::Not) | ty::RawPtr(_, Mutability::Not) => {
if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _) = argument.kind {
span_lint(
let snippet = snippet(cx, argument.span, "..");
let trimmed_snippet = &snippet[5..];
span_lint_and_sugg(
Comment on lines +91 to +92
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@samueltardieu samueltardieu Apr 16, 2025 •
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Cursory review:

It would probably be best if you use span_lint_and_then(), with .span_suggestion() (or .span_suggestion_verbose(), whether this gives a prettier result or not), and propose to rewrite &mut as &, instead of reconstructing the snippet. You can proceed as follows:

  • The span to replace is from the start of argument to the start of the AddrOf argument.
  • You should also ensure that the context of argument is the same one as the context of the AddrOf argument, to ensure that the &mut has not been built by a macro.
  • Add tests where &mut comes from a macro for example (build a macro refmut and call refmut!(variable)), and where the expression comes from a macro (build a macro identity and call &mut identity!(var)).

Also, in the PR message (changelog) and title, you should use the lint name in lower case.

cx,
UNNECESSARY_MUT_PASSED,
argument.span,
format!("the {fn_kind} `{name}` doesn't need a mutable reference"),
"try",
format!("&{trimmed_snippet}"),
Applicability::MachineApplicable,
);
}
},
Expand Down
6 changes: 3 additions & 3 deletions tests/ui/mut_reference.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@ error: the function `takes_an_immutable_reference` doesn't need a mutable refere
--> tests/ui/mut_reference.rs:30:34
|
LL | takes_an_immutable_reference(&mut 42);
| ^^^^^^^
| ^^^^^^^ help: try: `&42`
|
= note: `-D clippy::unnecessary-mut-passed` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::unnecessary_mut_passed)]`
Expand All @@ -11,13 +11,13 @@ error: the function `as_ptr` doesn't need a mutable reference
--> tests/ui/mut_reference.rs:34:12
|
LL | as_ptr(&mut 42);
| ^^^^^^^
| ^^^^^^^ help: try: `&42`

error: the method `takes_an_immutable_reference` doesn't need a mutable reference
--> tests/ui/mut_reference.rs:39:44
|
LL | my_struct.takes_an_immutable_reference(&mut 42);
| ^^^^^^^
| ^^^^^^^ help: try: `&42`

error: aborting due to 3 previous errors

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