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Use root obligation on E0277 for some cases

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When encountering trait bound errors that satisfy some heuristics that
tell us that the relevant trait for the user comes from the root
obligation and not the current obligation, we use the root predicate for
the main message.

This allows to talk about "X doesn't implement Pattern<'_>" over the
most specific case that just happened to fail, like  "char doesn't
implement Fn(&mut char)" in
`tests/ui/traits/suggest-dereferences/root-obligation.rs`

The heuristics are:

 - the type of the leaf predicate is (roughly) the same as the type
   from the root predicate, as a proxy for "we care about the root"
 - the leaf trait and the root trait are different, so as to avoid
   talking about `&mut T: Trait` and instead remain talking about
   `T: Trait` instead
 - the root trait is not `Unsize`, as to avoid talking about it in
   `tests/ui/coercion/coerce-issue-49593-box-never.rs`.

```
error[E0277]: the trait bound `&char: Pattern<'_>` is not satisfied
  --> $DIR/root-obligation.rs:6:38
   |
LL |         .filter(|c| "aeiou".contains(c))
   |                             -------- ^ the trait `Fn<(char,)>` is not implemented for `&char`, which is required by `&char: Pattern<'_>`
   |                             |
   |                             required by a bound introduced by this call
   |
   = note: required for `&char` to implement `FnOnce<(char,)>`
   = note: required for `&char` to implement `Pattern<'_>`
note: required by a bound in `core::str::<impl str>::contains`
  --> $SRC_DIR/core/src/str/mod.rs:LL:COL
help: consider dereferencing here
   |
LL |         .filter(|c| "aeiou".contains(*c))
   |                                      +
```

Fix #79359, fix #119983, fix #118779, cc #118415 (the suggestion needs
to change).
This commit is contained in:
Esteban Küber 2024-02-29 00:35:59 +00:00
parent 26907374b9
commit f0c93117ed
49 changed files with 166 additions and 120 deletions
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51
compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
View file

@ -416,9 +416,51 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
let trait_predicate = bound_predicate.rebind(trait_predicate);
let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
let trait_ref = trait_predicate.to_poly_trait_ref();
if let Some(guar) = self.emit_specialized_closure_kind_error(&obligation, trait_ref) {
// Let's use the root obligation as the main message, when we care about the
// most general case ("X doesn't implement Pattern<'_>") over the case that
// happened to fail ("char doesn't implement Fn(&mut char)").
//
// We rely on a few heuristics to identify cases where this root
// obligation is more important than the leaf obligation:
let (main_trait_predicate, o) = if let ty::PredicateKind::Clause(
ty::ClauseKind::Trait(root_pred)
) = root_obligation.predicate.kind().skip_binder()
// The type of the leaf predicate is (roughly) the same as the type
// from the root predicate, as a proxy for "we care about the root"
// FIXME: this doesn't account for trivial derefs, but works as a first
// approximation.
&& (
// `T: Trait` && `&&T: OtherTrait`, we want `OtherTrait`
trait_predicate.self_ty().skip_binder()
== root_pred.self_ty().peel_refs()
// `&str: Iterator` && `&str: IntoIterator`, we want `IntoIterator`
|| trait_predicate.self_ty().skip_binder()
== root_pred.self_ty()
)
// The leaf trait and the root trait are different, so as to avoid
// talking about `&mut T: Trait` and instead remain talking about
// `T: Trait` instead
&& trait_predicate.def_id() != root_pred.def_id()
// The root trait is not `Unsize`, as to avoid talking about it in
// `tests/ui/coercion/coerce-issue-49593-box-never.rs`.
&& Some(root_pred.def_id()) != self.tcx.lang_items().unsize_trait()
{
(
self.resolve_vars_if_possible(
root_obligation.predicate.kind().rebind(root_pred),
),
root_obligation,
)
} else {
(trait_predicate, &obligation)
};
let trait_ref = main_trait_predicate.to_poly_trait_ref();
if let Some(guar) = self.emit_specialized_closure_kind_error(
&obligation,
trait_ref,
) {
return guar;
}
@ -459,8 +501,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
notes,
parent_label,
append_const_msg,
} = self.on_unimplemented_note(trait_ref, &obligation, &mut long_ty_file);
} = self.on_unimplemented_note(trait_ref, o, &mut long_ty_file);
let have_alt_message = message.is_some() || label.is_some();
let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
let is_unsize =
@ -483,7 +524,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
};
let err_msg = self.get_standard_error_message(
&trait_predicate,
&main_trait_predicate,
message,
predicate_is_const,
append_const_msg,