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Suggest fn ptr rather than fn item and suggest to use Fn trait bounds rather than the unique closure type in E0121

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This is a squash of the titular commit along with these minor commits:
- Improve note
- Improve note pt2
This commit is contained in:
ThePuzzlemaker 2020-12-21 20:23:21 -06:00
parent 353f3a3846
commit 4f6be466fd
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GPG Key ID: 968CD9D71C9FBB6C
1 changed files with 21 additions and 6 deletions
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27
compiler/rustc_typeck/src/collect.rs
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@ -1544,12 +1544,27 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
let mut diag = bad_placeholder_type(tcx, visitor.0);
let ret_ty = fn_sig.output();
if ret_ty != tcx.ty_error() {
diag.span_suggestion(
ty.span,
"replace with the correct return type",
ret_ty.to_string(),
Applicability::MaybeIncorrect,
);
if !ret_ty.is_closure() {
let ret_ty_str = match ret_ty.kind() {
// Suggest a function pointer return type instead of a unique function definition
// (e.g. `fn() -> i32` instead of `fn() -> i32 { f }`, the latter of which is invalid
// syntax)
ty::FnDef(..) => ret_ty.fn_sig(tcx).to_string(),
_ => ret_ty.to_string(),
};
diag.span_suggestion(
ty.span,
"replace with the correct return type",
ret_ty_str,
Applicability::MaybeIncorrect,
);
} else {
// We're dealing with a closure, so we should suggest using `impl Fn` or trait bounds
// to prevent the user from getting a papercut while trying to use the unique closure
// syntax (e.g. `[closure@src/lib.rs:2:5: 2:9]`).
diag.help("consider using an `Fn`, `FnMut`, or `FnOnce` trait bound");
diag.note("for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html");
}
}
diag.emit();
ty::Binder::bind(fn_sig)