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Explain each variant of TAIT usage with examples

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This commit is contained in:
Oli Scherer 2021-03-15 11:02:35 +00:00
parent 6109d73112
commit c2683aa569
1 changed files with 44 additions and 1 deletions
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45
compiler/rustc_typeck/src/check/check.rs
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@ -88,16 +88,59 @@ pub(super) fn check_fn<'a, 'tcx>(
let declared_ret_ty = fn_sig.output();
let feature = match tcx.hir().get(fn_id) {
// TAIT usage in function return position.
// Example:
//
// ```rust
// type Foo = impl Debug;
// fn bar() -> Foo { 42 }
// ```
Node::Item(hir::Item { kind: ItemKind::Fn(..), .. }) |
// TAIT usage in associated function return position.
//
// Example with a free type alias:
//
// ```rust
// type Foo = impl Debug;
// impl SomeTrait for SomeType {
// fn bar() -> Foo { 42 }
// }
// ```
//
// Example with an associated TAIT:
//
// ```rust
// impl SomeTrait for SomeType {
// type Foo = impl Debug;
// fn bar() -> Self::Foo { 42 }
// }
// ```
Node::ImplItem(hir::ImplItem {
kind: hir::ImplItemKind::Fn(..), ..
}) => None,
// I don't know if TAIT uses in trait declarations make sense at all
// Forbid TAIT in trait declarations for now.
// Examples:
//
// ```rust
// type Foo = impl Debug;
// trait Bar {
// fn bar() -> Foo;
// }
// trait Bop {
// type Bop: PartialEq<Foo>;
// }
// ```
Node::TraitItem(hir::TraitItem {
kind: hir::TraitItemKind::Fn(..),
..
}) |
// Forbid TAIT in closure return position for now.
// Example:
//
// ```rust
// type Foo = impl Debug;
// let x = |y| -> Foo { 42 + y };
// ```
Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(..), .. }) => Some(sym::type_alias_impl_trait),
node => bug!("Item being checked wasn't a function/closure: {:?}", node),
};