Suggest replacing Self with the right type on type error

When encountering a type error caused by the use of `Self`, suggest
using the actual type name instead.

```
error[E0308]: mismatched types
  --> $DIR/struct-path-self-type-mismatch.rs:13:9
   |
LL |   impl<T> Foo<T> {
   |        -  ------ this is the type of the `Self` literal
   |        |
   |        found type parameter
LL |       fn new<U>(u: U) -> Foo<U> {
   |              -           ------ expected `Foo<U>` because of return type
   |              |
   |              expected type parameter
LL | /         Self {
LL | |
LL | |             inner: u
LL | |
LL | |         }
   | |_________^ expected `Foo<U>`, found `Foo<T>`
   |
   = note: expected struct `Foo<U>`
              found struct `Foo<T>`
   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
help: use the type name directly
   |
LL |         Foo::<U> {
   |         ~~~~~~~~
```

Fix #76086.

Author: estebank

compiler/rustc_hir_typeck/src/coercion.rs

@@ -36,7 +36,9 @@
 //! ```
 
 use crate::FnCtxt;
-use rustc_errors::{struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
+use rustc_errors::{
+    struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
+};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{self, Visitor};
@@ -1738,7 +1740,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         // label pointing out the cause for the type coercion will be wrong
         // as prior return coercions would not be relevant (#57664).
         let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
-            self.explain_self_literal(fcx, &mut err, expr);
+            self.explain_self_literal(fcx, &mut err, expr, expected, found);
             let pointing_at_return_type =
                 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
             if let (Some(cond_expr), true, false) = (
@@ -1816,12 +1818,14 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         fcx: &FnCtxt<'_, 'tcx>,
         err: &mut Diagnostic,
         expr: &'tcx hir::Expr<'tcx>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
     ) {
         match expr.peel_drop_temps().kind {
             hir::ExprKind::Struct(
                 hir::QPath::Resolved(
                     None,
-                    hir::Path { res: hir::def::Res::SelfTyAlias { alias_to, .. }, .. },
+                    hir::Path { res: hir::def::Res::SelfTyAlias { alias_to, .. }, span, .. },
                 ),
                 ..,
             )
@@ -1830,7 +1834,11 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                     kind:
                         hir::ExprKind::Path(hir::QPath::Resolved(
                             None,
-                            hir::Path { res: hir::def::Res::SelfTyAlias { alias_to, .. }, .. },
+                            hir::Path {
+                                res: hir::def::Res::SelfTyAlias { alias_to, .. },
+                                span,
+                                ..
+                            },
                         )),
                     ..
                 },
@@ -1843,6 +1851,17 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 {
                     err.span_label(self_ty.span, "this is the type of the `Self` literal");
                 }
+                if let ty::Adt(e_def, e_args) = expected.kind()
+                    && ty::Adt(f_def, _f_args) = found.kind()
+                    && e_def == f_def
+                {
+                    err.span_suggestion_verbose(
+                        *span,
+                        "use the type name directly",
+                        fcx.tcx.value_path_str_with_args(*alias_to, e_args),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
             }
             _ => {}
         }

tests/ui/structs/struct-path-self-type-mismatch.stderr

@@ -42,6 +42,10 @@ LL | |         }
               found struct `Foo<T>`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+help: use the type name directly
+   |
+LL |         Foo::<U> {
+   |         ~~~~~~~~
 
 error: aborting due to 3 previous errors