Reserve gen keyword for gen {} blocks and gen fn in 2024 edition

compiler/rustc_parse/src/errors.rs

@@ -520,6 +520,14 @@ pub(crate) struct CatchAfterTry {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_gen_block)]
+#[help]
+pub(crate) struct GenBlock {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_comma_after_base_struct)]
 #[note]

compiler/rustc_parse/src/parser/expr.rs

@@ -1422,6 +1422,9 @@ impl<'a> Parser<'a> {
             } else if this.is_try_block() {
                 this.expect_keyword(kw::Try)?;
                 this.parse_try_block(lo)
+            } else if this.is_gen_block() {
+                this.expect_keyword(kw::Gen)?;
+                this.parse_gen_block(lo)
             } else if this.eat_keyword(kw::Return) {
                 this.parse_expr_return()
             } else if this.eat_keyword(kw::Continue) {
@@ -3040,6 +3043,14 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parses a `gen {...}` expression (`gen` token already eaten).
+    fn parse_gen_block(&mut self, _span_lo: Span) -> PResult<'a, P<Expr>> {
+        let (_attrs, _body) = self.parse_inner_attrs_and_block()?;
+
+        Err(errors::GenBlock { span: self.prev_token.span }
+            .into_diagnostic(&self.sess.span_diagnostic))
+    }
+
     fn is_do_catch_block(&self) -> bool {
         self.token.is_keyword(kw::Do)
             && self.is_keyword_ahead(1, &[kw::Catch])
@@ -3059,6 +3070,13 @@ impl<'a> Parser<'a> {
             && self.token.uninterpolated_span().at_least_rust_2018()
     }
 
+    fn is_gen_block(&self) -> bool {
+        self.token.is_keyword(kw::Gen)
+            && self
+                .look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace) || t.is_whole_block())
+            && self.token.uninterpolated_span().at_least_rust_2024()
+    }
+
     /// Parses an `async move? {...}` expression.
     fn parse_async_block(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;

compiler/rustc_parse/src/parser/item.rs

@@ -2297,9 +2297,9 @@ impl<'a> Parser<'a> {
         // `pub` is added in case users got confused with the ordering like `async pub fn`,
         // only if it wasn't preceded by `default` as `default pub` is invalid.
         let quals: &[Symbol] = if check_pub {
-            &[kw::Pub, kw::Const, kw::Async, kw::Unsafe, kw::Extern]
+            &[kw::Pub, kw::Gen, kw::Const, kw::Async, kw::Unsafe, kw::Extern]
         } else {
-            &[kw::Const, kw::Async, kw::Unsafe, kw::Extern]
+            &[kw::Gen, kw::Const, kw::Async, kw::Unsafe, kw::Extern]
         };
         self.check_keyword_case(kw::Fn, case) // Definitely an `fn`.
             // `$qual fn` or `$qual $qual`:
@@ -2353,6 +2353,9 @@ impl<'a> Parser<'a> {
         let async_start_sp = self.token.span;
         let asyncness = self.parse_asyncness(case);
 
+        let _gen_start_sp = self.token.span;
+        let genness = self.parse_genness(case);
+
         let unsafe_start_sp = self.token.span;
         let unsafety = self.parse_unsafety(case);
 
@@ -2368,6 +2371,10 @@ impl<'a> Parser<'a> {
             }
         }
 
+        if let Gen::Yes { span, .. } = genness {
+            self.sess.emit_err(errors::GenBlock { span });
+        }
+
         if !self.eat_keyword_case(kw::Fn, case) {
             // It is possible for `expect_one_of` to recover given the contents of
             // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't

compiler/rustc_parse/src/parser/mod.rs

@@ -11,6 +11,7 @@ mod stmt;
 mod ty;
 
 use crate::lexer::UnmatchedDelim;
+use ast::Gen;
 pub use attr_wrapper::AttrWrapper;
 pub use diagnostics::AttemptLocalParseRecovery;
 pub(crate) use expr::ForbiddenLetReason;
@@ -1126,6 +1127,16 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parses genness: `gen` or nothing.
+    fn parse_genness(&mut self, case: Case) -> Gen {
+        if self.token.span.at_least_rust_2024() && self.eat_keyword_case(kw::Gen, case) {
+            let span = self.prev_token.uninterpolated_span();
+            Gen::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
+        } else {
+            Gen::No
+        }
+    }
+
     /// Parses unsafety: `unsafe` or nothing.
     fn parse_unsafety(&mut self, case: Case) -> Unsafe {
         if self.eat_keyword_case(kw::Unsafe, case) {