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Allow impl Fn() -> impl Trait in return position

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This allows writing the following function signatures:
```rust
fn f0() -> impl Fn() -> impl Trait;
fn f3() -> &'static dyn Fn() -> impl Trait;
```

These signatures were already allowed for common traits and associated
types, there is no reason why `Fn*` traits should be special in this
regard.
This commit is contained in:
Maybe Waffle 2022-02-02 15:09:44 +03:00
parent 1481fd964b
commit 8b494f427c
6 changed files with 73 additions and 67 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
compiler/rustc_ast_lowering/src/path.rs
View File

@ -191,7 +191,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
}
GenericArgs::Parenthesized(ref data) => match parenthesized_generic_args {
ParenthesizedGenericArgs::Ok => self.lower_parenthesized_parameter_data(data),
ParenthesizedGenericArgs::Ok => {
self.lower_parenthesized_parameter_data(data, itctx)
}
ParenthesizedGenericArgs::Err => {
// Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
let sub = if !data.inputs.is_empty() {
@ -344,6 +346,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
fn lower_parenthesized_parameter_data(
&mut self,
data: &ParenthesizedArgs,
itctx: ImplTraitContext,
) -> (GenericArgsCtor<'hir>, bool) {
// Switch to `PassThrough` mode for anonymous lifetimes; this
// means that we permit things like `&Ref<T>`, where `Ref` has
@ -355,6 +358,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
self.lower_ty_direct(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
}));
let output_ty = match output {
// Only allow `impl Trait` in return position. i.e.:
// ```rust
// fn f(_: impl Fn() -> impl Debug) -> impl Fn() -> impl Debug
// // disallowed --^^^^^^^^^^ allowed --^^^^^^^^^^
// ```
FnRetTy::Ty(ty) if matches!(itctx, ImplTraitContext::ReturnPositionOpaqueTy { .. }) => {
self.lower_ty(&ty, itctx)
}
FnRetTy::Ty(ty) => {
self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
}

16
src/test/ui/impl-trait/impl_fn_associativity.rs Normal file
View File

@ -0,0 +1,16 @@
// run-pass
use std::fmt::Debug;
fn f_debug() -> impl Fn() -> impl Debug {
|| ()
}
fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
|| f_debug()
}
fn main() {
// Check that `ff_debug` is `() -> (() -> Debug)` and not `(() -> ()) -> Debug`
let debug = ff_debug()()();
assert_eq!(format!("{:?}", debug), "()");
}

3
src/test/ui/impl-trait/nested_impl_trait.rs
View File

@ -25,8 +25,7 @@ fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
}
fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
//~^ `impl Trait` only allowed in function and inherent method return types
|| 5
|| 5u8
}
fn main() {}

8
src/test/ui/impl-trait/nested_impl_trait.stderr
View File

@ -40,12 +40,6 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
| ^^^^^^^^^^^^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/nested_impl_trait.rs:27:42
|
LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
| ^^^^^^^^^^^^^^
error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
--> $DIR/nested_impl_trait.rs:5:46
|
@ -64,7 +58,7 @@ LL | fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
= help: the trait `Into<U>` is implemented for `T`
= note: required for `impl Into<u32>` to implement `Into<impl Debug>`
error: aborting due to 8 previous errors
error: aborting due to 7 previous errors
Some errors have detailed explanations: E0277, E0562, E0666.
For more information about an error, try `rustc --explain E0277`.

6
src/test/ui/impl-trait/where-allowed.rs
View File

@ -39,9 +39,8 @@ fn in_dyn_Fn_return_in_parameters(_: &dyn Fn() -> impl Debug) { panic!() }
fn in_dyn_Fn_parameter_in_return() -> &'static dyn Fn(impl Debug) { panic!() }
//~^ ERROR `impl Trait` only allowed in function and inherent method return types
// Disallowed
// Allowed
fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
//~^ ERROR `impl Trait` only allowed in function and inherent method return types
// Disallowed
fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
@ -57,9 +56,8 @@ fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
//~^ ERROR `impl Trait` only allowed in function and inherent method return types
//~| ERROR nested `impl Trait` is not allowed
// Disallowed
// Allowed
fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() }
//~^ ERROR `impl Trait` only allowed in function and inherent method return types
// Disallowed
fn in_Fn_parameter_in_generics<F: Fn(impl Debug)> (_: F) { panic!() }

94
src/test/ui/impl-trait/where-allowed.stderr
View File

@ -1,5 +1,5 @@
error[E0666]: nested `impl Trait` is not allowed
--> $DIR/where-allowed.rs:47:51
--> $DIR/where-allowed.rs:46:51
|
LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
| --------^^^^^^^^^^-
@ -8,7 +8,7 @@ LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
| outer `impl Trait`
error[E0666]: nested `impl Trait` is not allowed
--> $DIR/where-allowed.rs:56:57
--> $DIR/where-allowed.rs:55:57
|
LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
| --------^^^^^^^^^^-
@ -17,7 +17,7 @@ LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic
| outer `impl Trait`
error[E0658]: `impl Trait` in type aliases is unstable
--> $DIR/where-allowed.rs:119:16
--> $DIR/where-allowed.rs:117:16
|
LL | type Out = impl Debug;
| ^^^^^^^^^^
@ -26,7 +26,7 @@ LL | type Out = impl Debug;
= help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
error[E0658]: `impl Trait` in type aliases is unstable
--> $DIR/where-allowed.rs:154:23
--> $DIR/where-allowed.rs:152:23
|
LL | type InTypeAlias<R> = impl Debug;
| ^^^^^^^^^^
@ -35,7 +35,7 @@ LL | type InTypeAlias<R> = impl Debug;
= help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
error[E0658]: `impl Trait` in type aliases is unstable
--> $DIR/where-allowed.rs:157:39
--> $DIR/where-allowed.rs:155:39
|
LL | type InReturnInTypeAlias<R> = fn() -> impl Debug;
| ^^^^^^^^^^
@ -85,80 +85,68 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
LL | fn in_dyn_Fn_parameter_in_return() -> &'static dyn Fn(impl Debug) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/where-allowed.rs:43:57
|
LL | fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
--> $DIR/where-allowed.rs:47:51
--> $DIR/where-allowed.rs:46:51
|
LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/where-allowed.rs:52:53
--> $DIR/where-allowed.rs:51:53
|
LL | fn in_impl_Fn_return_in_parameters(_: &impl Fn() -> impl Debug) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
--> $DIR/where-allowed.rs:56:57
--> $DIR/where-allowed.rs:55:57
|
LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/where-allowed.rs:61:59
|
LL | fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
--> $DIR/where-allowed.rs:65:38
--> $DIR/where-allowed.rs:63:38
|
LL | fn in_Fn_parameter_in_generics<F: Fn(impl Debug)> (_: F) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/where-allowed.rs:69:40
--> $DIR/where-allowed.rs:67:40
|
LL | fn in_Fn_return_in_generics<F: Fn() -> impl Debug> (_: F) { panic!() }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:82:32
--> $DIR/where-allowed.rs:80:32
|
LL | struct InBraceStructField { x: impl Debug }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in path
--> $DIR/where-allowed.rs:86:41
--> $DIR/where-allowed.rs:84:41
|
LL | struct InAdtInBraceStructField { x: Vec<impl Debug> }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:90:27
--> $DIR/where-allowed.rs:88:27
|
LL | struct InTupleStructField(impl Debug);
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:95:25
--> $DIR/where-allowed.rs:93:25
|
LL | InBraceVariant { x: impl Debug },
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:97:20
--> $DIR/where-allowed.rs:95:20
|
LL | InTupleVariant(impl Debug),
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
--> $DIR/where-allowed.rs:108:23
--> $DIR/where-allowed.rs:106:23
|
LL | fn in_return() -> impl Debug;
| ^^^^^^^^^^
@ -167,7 +155,7 @@ LL | fn in_return() -> impl Debug;
= help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `impl` method return
--> $DIR/where-allowed.rs:125:34
--> $DIR/where-allowed.rs:123:34
|
LL | fn in_trait_impl_return() -> impl Debug { () }
| ^^^^^^^^^^
@ -176,127 +164,127 @@ LL | fn in_trait_impl_return() -> impl Debug { () }
= help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `extern fn` param
--> $DIR/where-allowed.rs:138:33
--> $DIR/where-allowed.rs:136:33
|
LL | fn in_foreign_parameters(_: impl Debug);
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `extern fn` return
--> $DIR/where-allowed.rs:141:31
--> $DIR/where-allowed.rs:139:31
|
LL | fn in_foreign_return() -> impl Debug;
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `fn` pointer return
--> $DIR/where-allowed.rs:157:39
--> $DIR/where-allowed.rs:155:39
|
LL | type InReturnInTypeAlias<R> = fn() -> impl Debug;
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait
--> $DIR/where-allowed.rs:162:16
--> $DIR/where-allowed.rs:160:16
|
LL | impl PartialEq<impl Debug> for () {
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:167:24
--> $DIR/where-allowed.rs:165:24
|
LL | impl PartialEq<()> for impl Debug {
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:172:6
--> $DIR/where-allowed.rs:170:6
|
LL | impl impl Debug {
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:178:24
--> $DIR/where-allowed.rs:176:24
|
LL | impl InInherentImplAdt<impl Debug> {
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:184:11
--> $DIR/where-allowed.rs:182:11
|
LL | where impl Debug: Debug
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:191:15
--> $DIR/where-allowed.rs:189:15
|
LL | where Vec<impl Debug>: Debug
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in bound
--> $DIR/where-allowed.rs:198:24
--> $DIR/where-allowed.rs:196:24
|
LL | where T: PartialEq<impl Debug>
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
--> $DIR/where-allowed.rs:205:17
--> $DIR/where-allowed.rs:203:17
|
LL | where T: Fn(impl Debug)
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
--> $DIR/where-allowed.rs:212:22
--> $DIR/where-allowed.rs:210:22
|
LL | where T: Fn() -> impl Debug
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:218:40
--> $DIR/where-allowed.rs:216:40
|
LL | struct InStructGenericParamDefault<T = impl Debug>(T);
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:222:36
--> $DIR/where-allowed.rs:220:36
|
LL | enum InEnumGenericParamDefault<T = impl Debug> { Variant(T) }
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:226:38
--> $DIR/where-allowed.rs:224:38
|
LL | trait InTraitGenericParamDefault<T = impl Debug> {}
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:230:41
--> $DIR/where-allowed.rs:228:41
|
LL | type InTypeAliasGenericParamDefault<T = impl Debug> = T;
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:234:11
--> $DIR/where-allowed.rs:232:11
|
LL | impl <T = impl Debug> T {}
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
--> $DIR/where-allowed.rs:241:40
--> $DIR/where-allowed.rs:239:40
|
LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
| ^^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
--> $DIR/where-allowed.rs:247:29
--> $DIR/where-allowed.rs:245:29
|
LL | let _in_local_variable: impl Fn() = || {};
| ^^^^^^^^^
error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in closure return
--> $DIR/where-allowed.rs:249:46
--> $DIR/where-allowed.rs:247:46
|
LL | let _in_return_in_local_variable = || -> impl Fn() { || {} };
| ^^^^^^^^^
error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
--> $DIR/where-allowed.rs:234:7
--> $DIR/where-allowed.rs:232:7
|
LL | impl <T = impl Debug> T {}
| ^^^^^^^^^^^^^^
@ -306,7 +294,7 @@ LL | impl <T = impl Debug> T {}
= note: `#[deny(invalid_type_param_default)]` on by default
error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
--> $DIR/where-allowed.rs:241:36
--> $DIR/where-allowed.rs:239:36
|
LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
| ^^^^^^^^^^^^^^
@ -315,14 +303,14 @@ LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
= note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
error[E0118]: no nominal type found for inherent implementation
--> $DIR/where-allowed.rs:234:23
--> $DIR/where-allowed.rs:232:23
|
LL | impl <T = impl Debug> T {}
| ^ impl requires a nominal type
|
= note: either implement a trait on it or create a newtype to wrap it instead
error: aborting due to 49 previous errors
error: aborting due to 47 previous errors
Some errors have detailed explanations: E0118, E0562, E0658, E0666.
For more information about an error, try `rustc --explain E0118`.