Account for ty::Error when suggesting impl Trait or Box<dyn Trait>

src/librustc/traits/error_reporting/suggestions.rs

@@ -606,11 +606,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let tables = self.in_progress_tables.map(|t| t.borrow()).unwrap();
 
         let mut ret_types = visitor.0.iter().filter_map(|expr| tables.node_type_opt(expr.hir_id));
-        let (last_ty, all_returns_have_same_type) =
+        let (last_ty, all_returns_have_same_type) = ret_types.clone().fold(
-            ret_types.clone().fold((None, true), |(last_ty, mut same), returned_ty| {
+            (None, true),
-                same &= last_ty.map_or(true, |ty| ty == returned_ty);
+            |(last_ty, mut same): (std::option::Option<Ty<'_>>, bool), ty| {
-                (Some(returned_ty), same)
+                same &= last_ty.map_or(true, |last_ty| last_ty == ty) && ty.kind != ty::Error;
-            });
+                (Some(ty), same)
+            },
+        );
         let all_returns_conform_to_trait =
             if let Some(ty_ret_ty) = tables.node_type_opt(ret_ty.hir_id) {
                 match ty_ret_ty.kind {
@@ -625,7 +627,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             })
                         })
                     }
-                    _ => true,
+                    _ => false,
                 }
             } else {
                 true

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs

@@ -22,6 +22,32 @@ fn bal() -> dyn Trait { //~ ERROR E0746
     }
     42
 }
+fn bam() -> Box<dyn Trait> {
+    if true {
+        return Struct; //~ ERROR mismatched types
+    }
+    42 //~ ERROR mismatched types
+}
+fn baq() -> Box<dyn Trait> {
+    if true {
+        return 0; //~ ERROR mismatched types
+    }
+    42 //~ ERROR mismatched types
+}
+fn baz() -> Box<dyn Trait> {
+    if true {
+        Struct //~ ERROR mismatched types
+    } else {
+        42 //~ ERROR mismatched types
+    }
+}
+fn baw() -> Box<dyn Trait> {
+    if true {
+        0 //~ ERROR mismatched types
+    } else {
+        42 //~ ERROR mismatched types
+    }
+}
 
 // Suggest using `impl Trait`
 fn bat() -> dyn Trait { //~ ERROR E0746
@@ -30,5 +56,12 @@ fn bat() -> dyn Trait { //~ ERROR E0746
     }
     42
 }
+fn bay() -> dyn Trait { //~ ERROR E0746
+    if true {
+        0u32
+    } else {
+        42u32
+    }
+}
 
 fn main() {}

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -95,8 +95,136 @@ LL |     }
 LL |     Box::new(42)
    |
 
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:27:16
+   |
+LL | fn bam() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+LL |     if true {
+LL |         return Struct;
+   |                ^^^^^^
+   |                |
+   |                expected struct `std::boxed::Box`, found struct `Struct`
+   |                help: store this in the heap by calling `Box::new`: `Box::new(Struct)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+              found struct `Struct`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:5
+   |
+LL | fn bam() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+...
+LL |     42
+   |     ^^
+   |     |
+   |     expected struct `std::boxed::Box`, found integer
+   |     help: store this in the heap by calling `Box::new`: `Box::new(42)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:16
+   |
+LL | fn baq() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+LL |     if true {
+LL |         return 0;
+   |                ^
+   |                |
+   |                expected struct `std::boxed::Box`, found integer
+   |                help: store this in the heap by calling `Box::new`: `Box::new(0)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:35:5
+   |
+LL | fn baq() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+...
+LL |     42
+   |     ^^
+   |     |
+   |     expected struct `std::boxed::Box`, found integer
+   |     help: store this in the heap by calling `Box::new`: `Box::new(42)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:39:9
+   |
+LL | fn baz() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+LL |     if true {
+LL |         Struct
+   |         ^^^^^^
+   |         |
+   |         expected struct `std::boxed::Box`, found struct `Struct`
+   |         help: store this in the heap by calling `Box::new`: `Box::new(Struct)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+              found struct `Struct`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:41:9
+   |
+LL | fn baz() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+...
+LL |         42
+   |         ^^
+   |         |
+   |         expected struct `std::boxed::Box`, found integer
+   |         help: store this in the heap by calling `Box::new`: `Box::new(42)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:46:9
+   |
+LL | fn baw() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+LL |     if true {
+LL |         0
+   |         ^
+   |         |
+   |         expected struct `std::boxed::Box`, found integer
+   |         help: store this in the heap by calling `Box::new`: `Box::new(0)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:48:9
+   |
+LL | fn baw() -> Box<dyn Trait> {
+   |             -------------- expected `std::boxed::Box<(dyn Trait + 'static)>` because of return type
+...
+LL |         42
+   |         ^^
+   |         |
+   |         expected struct `std::boxed::Box`, found integer
+   |         help: store this in the heap by calling `Box::new`: `Box::new(42)`
+   |
+   = note: expected struct `std::boxed::Box<(dyn Trait + 'static)>`
+                found type `{integer}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
 error[E0746]: return type cannot have an unboxed trait object
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:27:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:53:13
    |
 LL | fn bat() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -107,7 +235,27 @@ help: return `impl Trait` instead, as all return paths are of type `{integer}`,
 LL | fn bat() -> impl Trait {
    |             ^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error[E0746]: return type cannot have an unboxed trait object
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:59:13
+   |
+LL | fn bay() -> dyn Trait {
+   |             ^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = 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>
+   = note: if all the returned values were of the same type you could use `impl Trait` as the return type
+   = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
+   = note: you can create a new `enum` with a variant for each returned type
+help: return a boxed trait object instead
+   |
+LL | fn bay() -> Box<dyn Trait> {
+LL |     Box::new(if true {
+LL |         0u32
+LL |     } else {
+LL |         42u32
+LL |     })
+   |
+
+error: aborting due to 18 previous errors
 
 Some errors have detailed explanations: E0277, E0308, E0746.
 For more information about an error, try `rustc --explain E0277`.