Rollup merge of #90627 - camelid:suggest-box-deref, r=davidtwco

Suggest dereference of `Box` when inner type is expected

For example:

    enum Ty {
        Unit,
        List(Box<Ty>),
    }

    fn foo(x: Ty) -> Ty {
        match x {
            Ty::Unit => Ty::Unit,
            Ty::List(elem) => foo(elem),
        }
    }

Before, the only suggestion was to rewrap `inner` with `Ty::Wrapper`,
which is unhelpful and confusing:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^
      |                               |
      |                               expected enum `Ty`, found struct `Box`
      |                               help: try using a variant of the expected enum: `Ty::List(elem)`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`

Now, rustc will first suggest dereferencing the `Box`, which is most
likely what the user intended:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^ expected enum `Ty`, found struct `Box`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`
    help: try dereferencing the `Box`
      |
    9 |         Ty::List(elem) => foo(*elem),
      |                               +
    help: try using a variant of the expected enum
      |
    9 |         Ty::List(elem) => foo(Ty::List(elem)),
      |                               ~~~~~~~~~~~~~~

r? ``@davidtwco``

src/test/ui/suggestions/boxed-variant-field.rs

@@ -0,0 +1,16 @@
+enum Ty {
+    Unit,
+    List(Box<Ty>),
+}
+
+fn foo(x: Ty) -> Ty {
+    match x {
+        Ty::Unit => Ty::Unit,
+        Ty::List(elem) => foo(elem),
+        //~^ ERROR mismatched types
+        //~| HELP try dereferencing the `Box`
+        //~| HELP try using a variant of the expected enum
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/boxed-variant-field.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/boxed-variant-field.rs:9:31
+   |
+LL |         Ty::List(elem) => foo(elem),
+   |                               ^^^^ expected enum `Ty`, found struct `Box`
+   |
+   = note: expected enum `Ty`
+            found struct `Box<Ty>`
+help: try dereferencing the `Box`
+   |
+LL |         Ty::List(elem) => foo(*elem),
+   |                               +
+help: try using a variant of the expected enum
+   |
+LL |         Ty::List(elem) => foo(Ty::List(elem)),
+   |                               ~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/terr-sorts.stderr

@@ -6,6 +6,10 @@ LL |     want_foo(b);
    |
    = note: expected struct `Foo`
               found struct `Box<Foo>`
+help: try dereferencing the `Box`
+   |
+LL |     want_foo(*b);
+   |              +
 
 error: aborting due to previous error