Auto merge of #16588 - compiler-errors:async-and-const-bounds, r=Veykril

internal: Parse (nightly) `const` and `async` trait bounds

Both of these bound modifiers were added recently:

* `const` trait bounds: #119099
* `async` trait bounds: #120392

The latter will certainly will not do the right thing; namely, `async Fn` needs to be mapped to the `AsyncFn` trait. IDK how to do that, so advice would be appreciated, though perhaps we could land this first so the parser isn't complaining about these bounds?

Author: bors

crates/parser/src/grammar/generic_params.rs

@@ -157,6 +157,16 @@ fn type_bound(p: &mut Parser<'_>) -> bool {
                     p.bump_any();
                     p.expect(T![const]);
                 }
+                // test const_trait_bound
+                // const fn foo(_: impl const Trait) {}
+                T![const] => {
+                    p.bump_any();
+                }
+                // test async_trait_bound
+                // fn async_foo(_: impl async Fn(&i32)) {}
+                T![async] => {
+                    p.bump_any();
+                }
                 _ => (),
             }
             if paths::is_use_path_start(p) {

crates/parser/test_data/parser/inline/ok/0211_async_trait_bound.rast

@@ -0,0 +1,43 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "async_foo"
+    PARAM_LIST
+      L_PAREN "("
+      PARAM
+        WILDCARD_PAT
+          UNDERSCORE "_"
+        COLON ":"
+        WHITESPACE " "
+        IMPL_TRAIT_TYPE
+          IMPL_KW "impl"
+          WHITESPACE " "
+          TYPE_BOUND_LIST
+            TYPE_BOUND
+              ASYNC_KW "async"
+              WHITESPACE " "
+              PATH_TYPE
+                PATH
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "Fn"
+                    PARAM_LIST
+                      L_PAREN "("
+                      PARAM
+                        REF_TYPE
+                          AMP "&"
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "i32"
+                      R_PAREN ")"
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0211_async_trait_bound.rs

@@ -0,0 +1 @@
+fn async_foo(_: impl async Fn(&i32)) {}

crates/parser/test_data/parser/inline/ok/0212_const_trait_bound.rast

@@ -0,0 +1,34 @@
+SOURCE_FILE
+  FN
+    CONST_KW "const"
+    WHITESPACE " "
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      PARAM
+        WILDCARD_PAT
+          UNDERSCORE "_"
+        COLON ":"
+        WHITESPACE " "
+        IMPL_TRAIT_TYPE
+          IMPL_KW "impl"
+          WHITESPACE " "
+          TYPE_BOUND_LIST
+            TYPE_BOUND
+              CONST_KW "const"
+              WHITESPACE " "
+              PATH_TYPE
+                PATH
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "Trait"
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0212_const_trait_bound.rs

@@ -0,0 +1 @@
+const fn foo(_: impl const Trait) {}

crates/syntax/rust.ungram

@@ -614,7 +614,7 @@ TypeBoundList =
 
 TypeBound =
   Lifetime
-| ('?' | '~' 'const')? Type
+| ('~' 'const' | 'const')? 'async'? '?'? Type
 
 //************************//
 //        Patterns        //

crates/syntax/src/ast/generated/nodes.rs

@@ -1410,9 +1410,10 @@ pub struct TypeBound {
 }
 impl TypeBound {
     pub fn lifetime(&self) -> Option<Lifetime> { support::child(&self.syntax) }
-    pub fn question_mark_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![?]) }
     pub fn tilde_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![~]) }
     pub fn const_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![const]) }
+    pub fn async_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![async]) }
+    pub fn question_mark_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![?]) }
     pub fn ty(&self) -> Option<Type> { support::child(&self.syntax) }
 }