path, not name, in sole-argument variant type mismatch suggestion

We want the suggested replacement (which IDE tooling and such might offer to
automatically swap in) to, like, actually be correct: suggesting `MyVariant(x)`
when the actual fix is `MyEnum::MyVariant(x)` might be better than nothing, but
Rust is supposed to be the future of computing: we're better than better than
nothing.

As an exceptional case, we excise the prelude path, preferring to suggest
`Some` or `Ok` rather than `std::prelude::v1::Some` and
`std::prelude::v2::Ok`. (It's not worth the effort to future-proof against
hypothetical preludes v2, v3, &c.: we trust our successors to grep—excuse me,
ripgrep—for that.)

Also, don't make this preëmpt the existing probe-for-return-type suggestions,
despite their being looked unfavorably upon, at least in this situation
(#42764 (comment)): Cody
Schafer pointed out that that's a separate issue
(#43178 (comment)).

This is in the matter of #42764.

src/librustc_typeck/check/demand.rs

@@ -106,7 +106,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  let sole_field = &variant.fields[0];
  let sole_field_ty = sole_field.ty(self.tcx, substs);
  if self.can_coerce(expr_ty, sole_field_ty) {
- compatible_variants.push(variant.name);
+ let mut variant_path = self.tcx.item_path_str(variant.did);
+ variant_path = variant_path.trim_left_matches("std::prelude::v1::")
+ .to_string();
+ compatible_variants.push(variant_path);
  }
  }
  }
@@ -117,7 +120,6 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  err.span_suggestions(expr.span,
  "perhaps you meant to use a variant of the expected type",
  suggestions);
- return Some(err);
  }
  }
 

src/test/ui/did_you_mean/issue-42764.rs

@@ -11,7 +11,7 @@
 enum DoubleOption<T> {
  FirstSome(T),
  AlternativeSome(T),
- None,
+ Nothing,
 }
 
 fn this_function_expects_a_double_option<T>(d: DoubleOption<T>) {}

src/test/ui/did_you_mean/issue-42764.stderr

@@ -8,10 +8,10 @@ error[E0308]: mismatched types
  found type `usize`
 help: perhaps you meant to use a variant of the expected type
  |
-21 | this_function_expects_a_double_option(FirstSome(n));
- | ^^^^^^^^^^^^
-21 | this_function_expects_a_double_option(AlternativeSome(n));
- | ^^^^^^^^^^^^^^^^^^
+21 | this_function_expects_a_double_option(DoubleOption::FirstSome(n));
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+21 | this_function_expects_a_double_option(DoubleOption::AlternativeSome(n));
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error