Use root obligation on E0277 for some cases
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).
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error[E0277]: the trait bound `Box<{integer}>: Foo` is not satisfied
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--> $DIR/kindck-inherited-copy-bound.rs:21:16
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|
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LL | take_param(&x);
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| ---------- ^^ the trait `Copy` is not implemented for `Box<{integer}>`, which is required by `Box<{integer}>: Foo`
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| |
| required by a bound introduced by this call
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|
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note: required for `Box<{integer}>` to implement `Foo`
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--> $DIR/kindck-inherited-copy-bound.rs:14:14
|
LL | impl<T:Copy> Foo for T {
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| ---- ^^^ ^
| |
| unsatisfied trait bound introduced here
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note: required by a bound in `take_param`
--> $DIR/kindck-inherited-copy-bound.rs:17:17
|
LL | fn take_param<T:Foo>(foo: &T) { }
| ^^^ required by this bound in `take_param`
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error[E0038]: the trait `Foo` cannot be made into an object
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--> $DIR/kindck-inherited-copy-bound.rs:28:13
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|
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LL | let z = &x as &dyn Foo;
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| ^^ `Foo` cannot be made into an object
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|
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note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> $DIR/kindck-inherited-copy-bound.rs:10:13
|
LL | trait Foo : Copy {
| --- ^^^^ ...because it requires `Self: Sized`
| |
| this trait cannot be made into an object...
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= note: required for the cast from `&Box<i32>` to `&dyn Foo`
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error: aborting due to 2 previous errors
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Some errors have detailed explanations: E0038, E0277.
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For more information about an error, try `rustc --explain E0038`.
