Show trait method signature when impl differs

[estebank]

When the trait's span is available, it is already being used, add a
note for the cases where the span isn't available:

error[E0053]: method `fmt` has an incompatible type for trait
  --> $DIR/trait_type.rs:17:4
   |
17 |    fn fmt(&self, x: &str) -> () { }
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected type `fn(&MyType, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`
              found type `fn(&MyType, &str)`

error[E0050]: method `fmt` has 1 parameter but the declaration in trait `std::fmt::Display::fmt` has 2
  --> $DIR/trait_type.rs:21:11
   |
21 |    fn fmt(&self) -> () { }
   |           ^^^^^ expected 2 parameters, found 1
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
  --> $DIR/trait_type.rs:25:4
   |
25 |    fn fmt() -> () { }
   |    ^^^^^^^^^^^^^^^^^^ expected `&self` in impl
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`

error[E0046]: not all trait items implemented, missing: `fmt`
  --> $DIR/trait_type.rs:28:1
   |
28 | impl std::fmt::Display for MyType4 {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `fmt` in implementation
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`

Fix #28011.

[rust-highfive]

r? @arielb1

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[shepmaster]

Thanks @estebank ! We'll get a wonderful reviewer like @arielb1 or someone equally awesome on the case.

[arielb1]

why the .0? It looks like a skip_binder that makes functions show as fn(&Self) rather than for<'a> fn(&'a Self). Could it be changed to skip_binder?

[arielb1]

Nice! r+ with skip_binder and comment instead of .0 (yes I know the .0 is in the original).

[Mark-Simulacrum]

@bors r=arielb1

[bors]

📌 Commit 98885c5 has been approved by arielb1

[arielb1]

@bors r-

with skip_binder and comment instead of .0

There should be comments for all skip_binder calls. For example, here we would want

    pub fn signature<'a, 'tcx>(&self, tcx: &TyCtxt<'a, 'tcx, 'tcx>) -> String {
        match self.kind {
            ty::AssociatedKind::Method => {
                format!("{}",
                    // We skip the binder here because the binder would deanonymize all
                    // late-bound regions, and we don't want method signatures to show up
                    // `as for<'r> fn(&'r MyType)`. Pretty-printing handles late-bound
                    // regions just fine, showing `fn(&MyType)`.
                    tcx.type_of(self.def_id).fn_sig().skip_binder())
            }
            ty::AssociatedKind::Type => format!("type {};", self.name.to_string()),
            ty::AssociatedKind::Const => {
                format!("const {}: {:?};", self.name.to_string(), tcx.type_of(self.def_id))
            }
        }
+    }

[estebank]

@arielb1 forgot about the comment. Added the one proposed.

@bors r=arielb1

[bors]

📌 Commit e324919 has been approved by arielb1

[bors]

⌛ Testing commit e324919 with merge 0418fa9...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing 0418fa9 to master...