Auto merge of #109010 - compiler-errors:rtn, r=eholk

Initial support for return type notation (RTN)

See: https://smallcultfollowing.com/babysteps/blog/2023/02/13/return-type-notation-send-bounds-part-2/

1. Only supports `T: Trait<method(): Send>` style bounds, not `<T as Trait>::method(): Send`. Checking validity and injecting an implicit binder for all of the late-bound method generics is harder to do for the latter.
    * I'd add this in a follow-up.
3. ~Doesn't support RTN in general type position, i.e. no `let x: <T as Trait>::method() = ...`~
    * I don't think we actually want this.
5. Doesn't add syntax for "eliding" the function args -- i.e. for now, we write `method(): Send` instead of `method(..): Send`.
    * May be a hazard if we try to add it in the future. I'll probably add it in a follow-up later, with a structured suggestion to change `method()` to `method(..)` once we add it.
7. ~I'm not in love with the feature gate name 😺~
    * I renamed it to `return_type_notation` :heavy_check_mark:

Follow-up PRs will probably add support for `where T::method(): Send` bounds. I'm not sure if we ever want to support return-type-notation in arbitrary type positions. I may also make the bounds require `..` in the args list later.

r? `@ghost`

3 files changed, 12 insertions, 6 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index cc0fc7b8358..9b6bfaadef0 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -167,6 +167,9 @@ pub enum GenericArgs {
     AngleBracketed(AngleBracketedArgs),
     /// The `(A, B)` and `C` in `Foo(A, B) -> C`.
     Parenthesized(ParenthesizedArgs),
+    /// Associated return type bounds, like `T: Trait<method(..): Send>`
+    /// which applies the `Send` bound to the return-type of `method`.
+    ReturnTypeNotation(Span),
 }
 
 impl GenericArgs {
@@ -178,6 +181,7 @@ impl GenericArgs {
         match self {
             AngleBracketed(data) => data.span,
             Parenthesized(data) => data.span,
+            ReturnTypeNotation(span) => *span,
         }
     }
 }
@@ -231,15 +235,15 @@ impl AngleBracketedArg {
     }
 }
 
-impl Into<Option<P<GenericArgs>>> for AngleBracketedArgs {
-    fn into(self) -> Option<P<GenericArgs>> {
-        Some(P(GenericArgs::AngleBracketed(self)))
+impl Into<P<GenericArgs>> for AngleBracketedArgs {
+    fn into(self) -> P<GenericArgs> {
+        P(GenericArgs::AngleBracketed(self))
     }
 }
 
-impl Into<Option<P<GenericArgs>>> for ParenthesizedArgs {
-    fn into(self) -> Option<P<GenericArgs>> {
-        Some(P(GenericArgs::Parenthesized(self)))
+impl Into<P<GenericArgs>> for ParenthesizedArgs {
+    fn into(self) -> P<GenericArgs> {
+        P(GenericArgs::Parenthesized(self))
     }
 }
 
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 46e46ab575e..514978f5569 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -561,6 +561,7 @@ pub fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vi
     match generic_args {
         GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
         GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
+        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }
 
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 608f87ab6eb..e5a0ad1f1e4 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -481,6 +481,7 @@ where
             walk_list!(visitor, visit_ty, &data.inputs);
             walk_fn_ret_ty(visitor, &data.output);
         }
+        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }