Auto merge of #114811 - estebank:impl-ambiguity, r=wesleywiser

Show more information when multiple `impl`s apply

- When there are `impl`s without type params, show only those (to avoid showing overly generic `impl`s).
```
error[E0283]: type annotations needed
  --> $DIR/multiple-impl-apply.rs:34:9
   |
LL |     let y = x.into();
   |         ^     ---- type must be known at this point
   |
note: multiple `impl`s satisfying `_: From<Baz>` found
  --> $DIR/multiple-impl-apply.rs:14:1
   |
LL | impl From<Baz> for Bar {
   | ^^^^^^^^^^^^^^^^^^^^^^
...
LL | impl From<Baz> for Foo {
   | ^^^^^^^^^^^^^^^^^^^^^^
   = note: required for `Baz` to implement `Into<_>`
help: consider giving `y` an explicit type
   |
LL |     let y: /* Type */ = x.into();
   |          ++++++++++++
```

- Lower the importance of `T: Sized`, `T: WellFormed` and coercion errors, to prioritize more relevant errors. The pre-existing deduplication logic deals with hiding redundant errors better that way, and we show errors with more metadata that is useful to the user.

- Show `<SelfTy as Trait>::assoc_fn` suggestion in more cases.
```
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
  --> $DIR/cross-return-site-inference.rs:38:16
   |
LL |     return Err(From::from("foo"));
   |                ^^^^^^^^^^ cannot call associated function of trait
   |
help: use a fully-qualified path to a specific available implementation
   |
LL |     return Err(</* self type */ as From>::from("foo"));
   |                +++++++++++++++++++     +
```

Fix #88284.

compiler/rustc_error_codes/src/error_codes/E0282.md

@@ -3,7 +3,7 @@ The compiler could not infer a type and asked for a type annotation.
 Erroneous code example:
 
 ```compile_fail,E0282
-let x = "hello".chars().rev().collect();
+let x = Vec::new();
 ```
 
 This error indicates that type inference did not result in one unique possible
@@ -11,21 +11,24 @@ type, and extra information is required. In most cases this can be provided
 by adding a type annotation. Sometimes you need to specify a generic type
 parameter manually.
 
-A common example is the `collect` method on `Iterator`. It has a generic type
-parameter with a `FromIterator` bound, which for a `char` iterator is
-implemented by `Vec` and `String` among others. Consider the following snippet
-that reverses the characters of a string:
+In the example above, type `Vec` has a type parameter `T`. When calling
+`Vec::new`, barring any other later usage of the variable `x` that allows the
+compiler to infer what type `T` is, the compiler needs to be told what it is.
 
-In the first code example, the compiler cannot infer what the type of `x` should
-be: `Vec<char>` and `String` are both suitable candidates. To specify which type
-to use, you can use a type annotation on `x`:
+The type can be specified on the variable:
 
 ```
-let x: Vec<char> = "hello".chars().rev().collect();
+let x: Vec<i32> = Vec::new();
 ```
 
-It is not necessary to annotate the full type. Once the ambiguity is resolved,
-the compiler can infer the rest:
+The type can also be specified in the path of the expression:
+
+```
+let x = Vec::<i32>::new();
+```
+
+In cases with more complex types, it is not necessary to annotate the full
+type. Once the ambiguity is resolved, the compiler can infer the rest:
 
 ```
 let x: Vec<_> = "hello".chars().rev().collect();

compiler/rustc_error_codes/src/error_codes/E0283.md

@@ -1,7 +1,51 @@
-An implementation cannot be chosen unambiguously because of lack of information.
+The compiler could not infer a type and asked for a type annotation.
 
 Erroneous code example:
 
+```compile_fail,E0283
+let x = "hello".chars().rev().collect();
+```
+
+This error indicates that type inference did not result in one unique possible
+type, and extra information is required. In most cases this can be provided
+by adding a type annotation. Sometimes you need to specify a generic type
+parameter manually.
+
+A common example is the `collect` method on `Iterator`. It has a generic type
+parameter with a `FromIterator` bound, which for a `char` iterator is
+implemented by `Vec` and `String` among others. Consider the following snippet
+that reverses the characters of a string:
+
+In the first code example, the compiler cannot infer what the type of `x` should
+be: `Vec<char>` and `String` are both suitable candidates. To specify which type
+to use, you can use a type annotation on `x`:
+
+```
+let x: Vec<char> = "hello".chars().rev().collect();
+```
+
+It is not necessary to annotate the full type. Once the ambiguity is resolved,
+the compiler can infer the rest:
+
+```
+let x: Vec<_> = "hello".chars().rev().collect();
+```
+
+Another way to provide the compiler with enough information, is to specify the
+generic type parameter:
+
+```
+let x = "hello".chars().rev().collect::<Vec<char>>();
+```
+
+Again, you need not specify the full type if the compiler can infer it:
+
+```
+let x = "hello".chars().rev().collect::<Vec<_>>();
+```
+
+We can see a self-contained example below:
+
 ```compile_fail,E0283
 struct Foo;