Auto merge of #107650 - compiler-errors:rollup-4pntchf, r=compiler-errors

Rollup of 8 pull requests

Successful merges:

 - #106887 (Make const/fn return params more suggestable)
 - #107519 (Add type alias for raw OS errors)
 - #107551 ( Replace `ConstFnMutClosure` with const closures )
 - #107595 (Retry opening proc-macro DLLs a few times on Windows.)
 - #107615 (Replace nbsp in all rustdoc code blocks)
 - #107621 (Intern external constraints in new solver)
 - #107631 (loudly tell people when they change `Cargo.lock`)
 - #107632 (Clarifying that .map() returns None if None.)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

2 files changed, 17 insertions, 6 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 8a6436b041b..28347b137f3 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2109,7 +2109,7 @@ impl<'a> Parser<'a> {
             ClosureBinder::NotPresent
         };
 
-        let constness = self.parse_constness(Case::Sensitive);
+        let constness = self.parse_closure_constness(Case::Sensitive);
 
         let movability =
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 2ea55f838a3..0cb88f3c3a9 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -732,9 +732,10 @@ impl<'a> Parser<'a> {
     fn check_const_closure(&self) -> bool {
         self.is_keyword_ahead(0, &[kw::Const])
             && self.look_ahead(1, |t| match &t.kind {
-                token::Ident(kw::Move | kw::Static | kw::Async, _)
-                | token::OrOr
-                | token::BinOp(token::Or) => true,
+                // async closures do not work with const closures, so we do not parse that here.
+                token::Ident(kw::Move | kw::Static, _) | token::OrOr | token::BinOp(token::Or) => {
+                    true
+                }
                 _ => false,
             })
     }
@@ -1198,8 +1199,18 @@ impl<'a> Parser<'a> {
 
     /// Parses constness: `const` or nothing.
     fn parse_constness(&mut self, case: Case) -> Const {
-        // Avoid const blocks to be parsed as const items
-        if self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
+        self.parse_constness_(case, false)
+    }
+
+    /// Parses constness for closures
+    fn parse_closure_constness(&mut self, case: Case) -> Const {
+        self.parse_constness_(case, true)
+    }
+
+    fn parse_constness_(&mut self, case: Case, is_closure: bool) -> Const {
+        // Avoid const blocks and const closures to be parsed as const items
+        if (self.check_const_closure() == is_closure)
+            && self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
             && self.eat_keyword_case(kw::Const, case)
         {
             Const::Yes(self.prev_token.uninterpolated_span())