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fix(match_as_ref): also suggest downcasting references #15935

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fix(match_as_ref): also suggest downcasting references #15935

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b14eba0
misc: move the infallible stuff out of the way
ada4a Oct 22, 2025
789d36f
fix(match_as_ref): also suggest downcasting references
ada4a Oct 22, 2025
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31 changes: 23 additions & 8 deletions clippy_lints/src/matches/match_as_ref.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,20 +21,35 @@ pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>], expr:
} else {
None
}
&& let output_ty = cx.typeck_results().expr_ty(expr)
&& let input_ty = cx.typeck_results().expr_ty(ex)
&& let Some(input_ty) = option_arg_ty(cx, input_ty)
&& let Some(output_ty) = option_arg_ty(cx, output_ty)
&& let ty::Ref(_, output_ty, output_mutbl) = *output_ty.kind()
{
let method = match arm_ref_mutbl {
Mutability::Not => "as_ref",
Mutability::Mut => "as_mut",
};

let output_ty = cx.typeck_results().expr_ty(expr);
let input_ty = cx.typeck_results().expr_ty(ex);

let cast = if let Some(input_ty) = option_arg_ty(cx, input_ty)
&& let Some(output_ty) = option_arg_ty(cx, output_ty)
&& let ty::Ref(_, output_ty, _) = *output_ty.kind()
&& input_ty != output_ty
{
let cast = if arm_ref_mutbl != output_mutbl || input_ty != output_ty {
// We'll need to downcast either the type (`&i32 as &dyn std::fmt::Debug`):
// ```
// let _: Option<&dyn std::fmt::Debug> = match 0i32 {
// Some(ref t) => Some(t),
// None => None,
// };
// ```
// or the reference (`&mut i32 as &i32`)
// ```
// let _: Option<&i32> = match 0i32 {
// Some(ref mut t) => Some(t),
// None => None,
// };
// ```
// or both.
//
// `.map` will take care of all of it.
".map(|x| x as _)"
} else {
""
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6 changes: 6 additions & 0 deletions tests/ui/match_as_ref.fixed
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Expand Up @@ -84,3 +84,9 @@ fn recv_requiring_parens() {

let _ = (!S).as_ref();
}

fn issue15932() {
let _: Option<&i32> = Some(0).as_mut().map(|x| x as _);
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Why not Some(0).as_ref()?

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See #15934 (comment)

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Just to be clear, I'm not opposed to that solution (though I did end up liking this one more) -- I explicitly opened two PRs so that you could compare the outputs and see which one you prefer

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Yeah, that we not clear to me, I get a lot of notifications from GitHub.


let _: Option<&dyn std::fmt::Debug> = Some(0).as_mut().map(|x| x as _);
}
14 changes: 14 additions & 0 deletions tests/ui/match_as_ref.rs
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Expand Up @@ -100,3 +100,17 @@ fn recv_requiring_parens() {
Some(ref v) => Some(v),
};
}

fn issue15932() {
let _: Option<&i32> = match Some(0) {
//~^ match_as_ref
None => None,
Some(ref mut v) => Some(v),
};

let _: Option<&dyn std::fmt::Debug> = match Some(0) {
//~^ match_as_ref
None => None,
Some(ref mut v) => Some(v),
};
}
44 changes: 43 additions & 1 deletion tests/ui/match_as_ref.stderr
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Expand Up @@ -83,5 +83,47 @@ LL - };
LL + let _ = (!S).as_ref();
|

error: aborting due to 4 previous errors
error: manual implementation of `Option::as_mut`
--> tests/ui/match_as_ref.rs:105:27
|
LL | let _: Option<&i32> = match Some(0) {
| ___________________________^
LL | |
LL | | None => None,
LL | | Some(ref mut v) => Some(v),
LL | | };
| |_____^
|
help: use `Option::as_mut()` directly
|
LL - let _: Option<&i32> = match Some(0) {
LL -
LL - None => None,
LL - Some(ref mut v) => Some(v),
LL - };
LL + let _: Option<&i32> = Some(0).as_mut().map(|x| x as _);
|

error: manual implementation of `Option::as_mut`
--> tests/ui/match_as_ref.rs:111:43
|
LL | let _: Option<&dyn std::fmt::Debug> = match Some(0) {
| ___________________________________________^
LL | |
LL | | None => None,
LL | | Some(ref mut v) => Some(v),
LL | | };
| |_____^
|
help: use `Option::as_mut()` directly
|
LL - let _: Option<&dyn std::fmt::Debug> = match Some(0) {
LL -
LL - None => None,
LL - Some(ref mut v) => Some(v),
LL - };
LL + let _: Option<&dyn std::fmt::Debug> = Some(0).as_mut().map(|x| x as _);
|

error: aborting due to 6 previous errors