Account for diverging types in return `impl Trait`

src/librustc_typeck/check/coercion.rs

@@ -1348,6 +1348,34 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 "...is found to be `{}` here",
                 fcx.resolve_vars_with_obligations(expected),
             ));
+            err.note(
+                "`impl Trait` as a return type requires that all the returned values must have \
+                 the same type",
+            );
+            let snippet = fcx
+                .tcx
+                .sess
+                .source_map()
+                .span_to_snippet(return_sp)
+                .unwrap_or_else(|_| "dyn Trait".to_string());
+            let mut snippet_iter = snippet.split_whitespace();
+            let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
+            if has_impl {
+                err.help(&format!(
+                    "you can instead return a trait object using `Box<dyn {}>`",
+                    &snippet[5..]
+                ));
+            }
+            let impl_trait_msg = "for information on `impl Trait`, see \
+                <https://doc.rust-lang.org/book/ch10-02-traits.html\
+                #returning-types-that-implement-traits>";
+            let trait_obj_msg = "for information on trait objects, see \
+                <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
+                #using-trait-objects-that-allow-for-values-of-different-types>";
+            err.note(impl_trait_msg);
+            if has_impl {
+                err.note(trait_obj_msg);
+            }
         }
         err
     }

src/test/ui/impl-trait/equality.stderr

@@ -9,6 +9,11 @@ LL |         return 1_i32;
 LL |     }
 LL |     0_u32
    |     ^^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn Foo>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0277]: cannot add `impl Foo` to `u32`
   --> $DIR/equality.rs:24:11

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -9,6 +9,11 @@ LL |         return 0i32;
 LL |     }
 LL |     1u32
    |     ^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:13:16
@@ -21,6 +26,11 @@ LL |         return 0i32;
 LL |     } else {
 LL |         return 1u32;
    |                ^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:22:9
@@ -33,6 +43,11 @@ LL |         return 0i32;
 LL |     } else {
 LL |         1u32
    |         ^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0308]: `if` and `else` have incompatible types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:31:9
@@ -57,6 +72,11 @@ LL |         0 => return 0i32,
    |                     ---- ...is found to be `i32` here
 LL |         _ => 1u32,
    |              ^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:45:5
@@ -71,6 +91,11 @@ LL | |         1 => 1u32,
 LL | |         _ => 2u32,
 LL | |     }
    | |_____^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:59:13
@@ -83,6 +108,11 @@ LL |             return 0i32;
 ...
 LL |             1u32
    |             ^^^^ expected `i32`, found `u32`
+   |
+   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
 
 error: aborting due to 7 previous errors