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Auto merge of #118054 - max-niederman:pinned-must-use, r=Nilstrieb

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Lint pinned `#[must_use]` pointers (in particular, `Box<T>` where `T` is `#[must_use]`) in `unused_must_use`.

Fixes: #111458

This is motivated by a common async/await pattern:

```rs
fn foo() -> Pin<Box<dyn Future<Output = i32>>> {
    Box::pin(async { 42 })
}

// call `foo`, but forget to await the result
foo();
```

Unlike with `async fn` or return position `impl Future`, this does not currently warn the user that the `Future` is unused.

To fix this, I've extended the `unused_must_use` lint to catch `Pin<P>`, where `P` must be used. In particular, this applies to `Pin<Box<T>>`, where `T` must be used. I'm not sure if there are other pointers where this applies, but I can't think of any situation the user wouldn't want to be warned.
This commit is contained in:
bors 2023-11-19 12:23:59 +00:00
commit 097261f241
3 changed files with 83 additions and 0 deletions
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18
compiler/rustc_lint/src/unused.rs
View file

@ -251,6 +251,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
/// The root of the normal must_use lint with an optional message.
Def(Span, DefId, Option<Symbol>),
Boxed(Box<Self>),
Pinned(Box<Self>),
Opaque(Box<Self>),
TraitObject(Box<Self>),
TupleElement(Vec<(usize, Self)>),
@ -284,6 +285,11 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
is_ty_must_use(cx, boxed_ty, expr, span)
.map(|inner| MustUsePath::Boxed(Box::new(inner)))
}
ty::Adt(def, args) if cx.tcx.lang_items().pin_type() == Some(def.did()) => {
let pinned_ty = args.type_at(0);
is_ty_must_use(cx, pinned_ty, expr, span)
.map(|inner| MustUsePath::Pinned(Box::new(inner)))
}
ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
ty::Alias(ty::Opaque, ty::AliasTy { def_id: def, .. }) => {
elaborate(
@ -425,6 +431,18 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
expr_is_from_block,
);
}
MustUsePath::Pinned(path) => {
let descr_pre = &format!("{descr_pre}pinned ");
emit_must_use_untranslated(
cx,
path,
descr_pre,
descr_post,
plural_len,
true,
expr_is_from_block,
);
}
MustUsePath::Opaque(path) => {
let descr_pre = &format!("{descr_pre}implementer{plural_suffix} of ");
emit_must_use_untranslated(