Suggest dereferencing when Deref is implemented.

This commit suggests dereferencing a type when it implements `Deref`
with the correct `Output` associated type.

Author: davidtwco

src/librustc_typeck/check/demand.rs

@@ -1,6 +1,6 @@
 use crate::check::FnCtxt;
 use rustc::infer::InferOk;
-use rustc::traits::{ObligationCause, ObligationCauseCode};
+use rustc::traits::{self, ObligationCause, ObligationCauseCode};
 
 use syntax::util::parser::PREC_POSTFIX;
 use syntax_pos::Span;
@@ -463,7 +463,37 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     }
                 }
             }
-            _ => {}
+            _ => {
+                // If neither type is a reference, then check for `Deref` implementations by
+                // constructing a predicate to prove: `<T as Deref>::Output == U`
+                let deref_trait = self.tcx.lang_items().deref_trait().unwrap();
+                let item_def_id = self.tcx.associated_items(deref_trait).next().unwrap().def_id;
+                let predicate = ty::Predicate::Projection(ty::Binder::bind(ty::ProjectionPredicate {
+                    // `<T as Deref>::Output`
+                    projection_ty: ty::ProjectionTy {
+                        // `T`
+                        substs: self.tcx.mk_substs_trait(
+                            checked_ty,
+                            self.fresh_substs_for_item(sp, item_def_id),
+                        ),
+                        // `Deref::Output`
+                        item_def_id,
+                    },
+                    // `U`
+                    ty: expected,
+                }));
+                let obligation = traits::Obligation::new(self.misc(sp), self.param_env, predicate);
+                if self.infcx.predicate_may_hold(&obligation) {
+                    if let (Ok(code), true) = (cm.span_to_snippet(sp), sp == expr.span) {
+                        let msg = if is_struct_pat_shorthand_field {
+                            format!("{}: *{}", code, code)
+                        } else {
+                            format!("*{}", code)
+                        };
+                        return Some((sp, "consider dereferencing the type", msg));
+                    }
+                }
+            }
         }
         None
     }

src/test/ui/infinite/infinite-autoderef.stderr

@@ -5,7 +5,7 @@ LL |         x = box x;
    |             ^^^^^
    |             |
    |             cyclic type of infinite size
-   |             help: try using a conversion method: `box x.to_string()`
+   |             help: consider dereferencing the type: `*box x`
 
 error[E0055]: reached the recursion limit while auto-dereferencing `Foo`
   --> $DIR/infinite-autoderef.rs:25:5

src/test/ui/occurs-check-2.stderr

@@ -5,7 +5,7 @@ LL |     f = box g;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box g.to_string()`
+   |         help: consider dereferencing the type: `*box g`
 
 error: aborting due to previous error
 

src/test/ui/occurs-check.stderr

@@ -5,7 +5,7 @@ LL |     f = box f;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box f.to_string()`
+   |         help: consider dereferencing the type: `*box f`
 
 error: aborting due to previous error
 

src/test/ui/span/coerce-suggestions.stderr

@@ -44,7 +44,7 @@ LL |     f = box f;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box f.to_string()`
+   |         help: consider dereferencing the type: `*box f`
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:21:9

src/test/ui/suggestions/issue-59819.fixed

@@ -0,0 +1,22 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+// Test that suggestion to add `*` characters applies to implementations of `Deref` as well as
+// references.
+
+struct Foo(i32);
+
+impl std::ops::Deref for Foo {
+    type Target = i32;
+    fn deref(&self) -> &i32 {
+        &self.0
+    }
+}
+
+fn main() {
+    let x = Foo(42);
+    let y: i32 = *x; //~ ERROR mismatched types
+    let a = &42;
+    let b: i32 = *a; //~ ERROR mismatched types
+}

src/test/ui/suggestions/issue-59819.rs

@@ -1,3 +1,5 @@
+// run-rustfix
+
 #![allow(warnings)]
 
 // Test that suggestion to add `*` characters applies to implementations of `Deref` as well as

src/test/ui/suggestions/issue-59819.stderr

@@ -1,14 +1,17 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-59819.rs:17:18
+  --> $DIR/issue-59819.rs:19:18
    |
 LL |     let y: i32 = x;
-   |                  ^ expected i32, found struct `Foo`
+   |                  ^
+   |                  |
+   |                  expected i32, found struct `Foo`
+   |                  help: consider dereferencing the type: `*x`
    |
    = note: expected type `i32`
               found type `Foo`
 
 error[E0308]: mismatched types
-  --> $DIR/issue-59819.rs:19:18
+  --> $DIR/issue-59819.rs:21:18
    |
 LL |     let b: i32 = a;
    |                  ^

src/test/ui/terr-sorts.stderr

@@ -2,7 +2,10 @@ error[E0308]: mismatched types
   --> $DIR/terr-sorts.rs:10:14
    |
 LL |     want_foo(b);
-   |              ^ expected struct `Foo`, found struct `std::boxed::Box`
+   |              ^
+   |              |
+   |              expected struct `Foo`, found struct `std::boxed::Box`
+   |              help: consider dereferencing the type: `*b`
    |
    = note: expected type `Foo`
               found type `std::boxed::Box<Foo>`