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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.
This commit is contained in:
Esteban Küber 2020-01-13 16:12:44 -08:00
parent 75eabb17ae
commit ea7e885204
6 changed files with 44 additions and 124 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
src/librustc_typeck/check/coercion.rs
View File

@ -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);
}

12
src/librustc_typeck/check/mod.rs
View File

@ -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:
///
/// ```

5
src/test/ui/error-codes/E0746.rs
View File

@ -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() {}

40
src/test/ui/error-codes/E0746.stderr
View File

@ -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: aborting due to 2 previous errors
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
For more information about this error, try `rustc --explain E0308`.

10
src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
View File

@ -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() {}

82
src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
View File

@ -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`.