bjoernager/rust
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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
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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))
}