Elide E0308 errors in favor of E0746

When a type error involves a `dyn Trait` as the return type, do not emit
the type error, as the "return type is not `Sized`" error will provide
enough information to the user.

6 files changed, 44 insertions, 124 deletions

diff --git a/src/librustc_typeck/check/coercion.rs b/src/librustc_typeck/check/coercion.rs
index 698fdfa3897..77f16fb7914 100644
--- a/src/librustc_typeck/check/coercion.rs
+++ b/src/librustc_typeck/check/coercion.rs
@@ -1222,6 +1222,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 };
 
                 let mut err;
+                let mut unsized_return = false;
                 match cause.code {
                     ObligationCauseCode::ReturnNoExpression => {
                         err = struct_span_err!(
@@ -1243,6 +1244,9 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                             parent_id,
                             expression.map(|expr| (expr, blk_id)),
                         );
+                        if !fcx.tcx.features().unsized_locals {
+                            unsized_return = fcx.is_unsized_return(blk_id);
+                        }
                     }
                     ObligationCauseCode::ReturnValue(id) => {
                         err = self.report_return_mismatched_types(
@@ -1254,6 +1258,10 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                             id,
                             None,
                         );
+                        if !fcx.tcx.features().unsized_locals {
+                            let id = fcx.tcx.hir().get_parent_node(id);
+                            unsized_return = fcx.is_unsized_return(id);
+                        }
                     }
                     _ => {
                         err = fcx.report_mismatched_types(cause, expected, found, coercion_error);
@@ -1282,7 +1290,16 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                     .filter(|e| fcx.is_assign_to_bool(e, self.expected_ty()))
                     .is_some();
 
-                err.emit_unless(assign_to_bool);
+                if unsized_return {
+                    fcx.tcx.sess.delay_span_bug(
+                        cause.span,
+                        &format!(
+                            "elided E0308 in favor of more detailed E0277 or E0746: {:?}",
+                            cause.code
+                        ),
+                    );
+                }
+                err.emit_unless(assign_to_bool || unsized_return);
 
                 self.final_ty = Some(fcx.tcx.types.err);
             }
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index baf9ae1ac29..8f531ea6199 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -4964,6 +4964,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    fn is_unsized_return(&self, blk_id: hir::HirId) -> bool {
+        if let Some((fn_decl, _)) = self.get_fn_decl(blk_id) {
+            if let hir::FunctionRetTy::Return(ty) = fn_decl.output {
+                let ty = AstConv::ast_ty_to_ty(self, ty);
+                if let ty::Dynamic(..) = ty.kind {
+                    return true;
+                }
+            }
+        }
+        false
+    }
+
     /// A possible error is to forget to add a return type that is needed:
     ///
     /// ```
diff --git a/src/test/ui/error-codes/E0746.rs b/src/test/ui/error-codes/E0746.rs
index ad257b01e1b..c9ab455a4c5 100644
--- a/src/test/ui/error-codes/E0746.rs
+++ b/src/test/ui/error-codes/E0746.rs
@@ -5,13 +5,12 @@ impl Trait for u32 {}
 
 fn foo() -> dyn Trait { Struct }
 //~^ ERROR E0746
-//~| ERROR E0308
 
 fn bar() -> dyn Trait { //~ ERROR E0746
     if true {
-        return 0; //~ ERROR E0308
+        return 0;
     }
-    42 //~ ERROR E0308
+    42
 }
 
 fn main() {}
diff --git a/src/test/ui/error-codes/E0746.stderr b/src/test/ui/error-codes/E0746.stderr
index baafcd27c29..44bd0d7ed7d 100644
--- a/src/test/ui/error-codes/E0746.stderr
+++ b/src/test/ui/error-codes/E0746.stderr
@@ -1,14 +1,3 @@
-error[E0308]: mismatched types
-  --> $DIR/E0746.rs:6:25
-   |
-LL | fn foo() -> dyn Trait { Struct }
-   |             ---------   ^^^^^^ expected trait `Trait`, found struct `Struct`
-   |             |
-   |             expected `(dyn Trait + 'static)` because of return type
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                    found struct `Struct`
-
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
   --> $DIR/E0746.rs:6:13
    |
@@ -22,7 +11,7 @@ LL | fn foo() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/E0746.rs:10:13
+  --> $DIR/E0746.rs:9:13
    |
 LL | fn bar() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -33,30 +22,5 @@ help: you can use the `impl Trait` feature in the return type because all the re
 LL | fn bar() -> impl Trait {
    |             ^^^^^^^^^^
 
-error[E0308]: mismatched types
-  --> $DIR/E0746.rs:12:16
-   |
-LL | fn bar() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-LL |     if true {
-LL |         return 0;
-   |                ^ expected trait `Trait`, found integer
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                      found type `{integer}`
-
-error[E0308]: mismatched types
-  --> $DIR/E0746.rs:14:5
-   |
-LL | fn bar() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-...
-LL |     42
-   |     ^^ expected trait `Trait`, found integer
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                      found type `{integer}`
-
-error: aborting due to 5 previous errors
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
index 80168ca8257..b70a51dc825 100644
--- a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
+++ b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
@@ -12,25 +12,23 @@ fn bar() -> (usize, dyn Trait) { (42, Struct) }
 //~| ERROR E0308
 fn bap() -> Trait { Struct }
 //~^ ERROR E0746
-//~| ERROR E0308
 fn ban() -> dyn Trait { Struct }
 //~^ ERROR E0746
-//~| ERROR E0308
 fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0277
 // Suggest using `Box<dyn Trait>`
 fn bal() -> dyn Trait { //~ ERROR E0746
     if true {
-        return Struct; //~ ERROR E0308
+        return Struct;
     }
-    42 //~ ERROR E0308
+    42
 }
 
 // Suggest using `impl Trait`
 fn bat() -> dyn Trait { //~ ERROR E0746
     if true {
-        return 0; //~ ERROR E0308
+        return 0;
     }
-    42 //~ ERROR E0308
+    42
 }
 
 fn main() {}
diff --git a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
index ce4c141a0af..a09ce2bb298 100644
--- a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
+++ b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
@@ -38,17 +38,6 @@ LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
    = note: required because it appears within the type `(usize, (dyn Trait + 'static))`
    = note: the return type of a function must have a statically known size
 
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:21
-   |
-LL | fn bap() -> Trait { Struct }
-   |             -----   ^^^^^^ expected trait `Trait`, found struct `Struct`
-   |             |
-   |             expected `(dyn Trait + 'static)` because of return type
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                    found struct `Struct`
-
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:13
    |
@@ -61,19 +50,8 @@ help: you can use the `impl Trait` feature in the return type because all the re
 LL | fn bap() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:25
-   |
-LL | fn ban() -> dyn Trait { Struct }
-   |             ---------   ^^^^^^ expected trait `Trait`, found struct `Struct`
-   |             |
-   |             expected `(dyn Trait + 'static)` because of return type
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                    found struct `Struct`
-
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:15:13
    |
 LL | fn ban() -> dyn Trait { Struct }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -85,7 +63,7 @@ LL | fn ban() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:17:13
    |
 LL | fn bak() -> dyn Trait { unimplemented!() }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -94,26 +72,14 @@ LL | fn bak() -> dyn Trait { unimplemented!() }
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: the return type of a function must have a statically known size
 
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:16
-   |
-LL | fn bal() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-LL |     if true {
-LL |         return Struct;
-   |                ^^^^^^ expected trait `Trait`, found struct `Struct`
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                    found struct `Struct`
-
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:21:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
    |
 LL | fn bal() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
 help: if all the returned values were of the same type you could use `impl Trait` as the return type
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:5
    |
 LL |         return Struct;
    |                ^^^^^^
@@ -132,20 +98,8 @@ LL |     }
 LL |     Box::new(42)
    |
 
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
-   |
-LL | fn bal() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-...
-LL |     42
-   |     ^^ expected trait `Trait`, found integer
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                      found type `{integer}`
-
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:13
+  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:27:13
    |
 LL | fn bat() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -156,31 +110,7 @@ help: you can use the `impl Trait` feature in the return type because all the re
 LL | fn bat() -> impl Trait {
    |             ^^^^^^^^^^
 
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:31:16
-   |
-LL | fn bat() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-LL |     if true {
-LL |         return 0;
-   |                ^ expected trait `Trait`, found integer
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                      found type `{integer}`
-
-error[E0308]: mismatched types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:5
-   |
-LL | fn bat() -> dyn Trait {
-   |             --------- expected `(dyn Trait + 'static)` because of return type
-...
-LL |     42
-   |     ^^ expected trait `Trait`, found integer
-   |
-   = note: expected trait object `(dyn Trait + 'static)`
-                      found type `{integer}`
-
-error: aborting due to 15 previous errors
+error: aborting due to 9 previous errors
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.