Parse explicit tail calls

src/expr.rs

@@ -1741,6 +1741,8 @@ pub(crate) mod parsing {
             input.parse().map(Expr::Continue)
         } else if input.peek(Token![return]) {
             input.parse().map(Expr::Return)
+        } else if input.peek(Token![become]) {
+            expr_become(input)
         } else if input.peek(token::Bracket) {
             array_or_repeat(input)
         } else if input.peek(Token![let]) {
@@ -2393,6 +2395,16 @@ pub(crate) mod parsing {
         }
     }
 
+    #[cfg(feature = "full")]
+    fn expr_become(input: ParseStream) -> Result<Expr> {
+        let begin = input.fork();
+        input.parse::<Token![become]>()?;
+        if can_begin_expr(input) {
+            input.parse::<Expr>()?;
+        }
+        Ok(Expr::Verbatim(verbatim::between(&begin, input)))
+    }
+
     #[cfg(feature = "full")]
     #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
     impl Parse for ExprTryBlock {

tests/repo/mod.rs

@@ -32,19 +32,6 @@ static EXCLUDE_FILES: &[&str] = &[
     "tests/ui/attributes/unsafe/unsafe-attributes.rs",
     "tests/ui/rust-2024/unsafe-attributes/unsafe-attribute-marked.rs",
 
-    // TODO: explicit tail calls: `become _g()`
-    // https://github.com/dtolnay/syn/issues/1501
-    "src/tools/miri/tests/fail/tail_calls/cc-mismatch.rs",
-    "src/tools/miri/tests/fail/tail_calls/signature-mismatch-arg.rs",
-    "src/tools/miri/tests/pass/tail_call.rs",
-    "src/tools/rust-analyzer/crates/parser/test_data/parser/inline/ok/0209_become_expr.rs",
-    "tests/mir-opt/tail_call_drops.rs",
-    "tests/ui/explicit-tail-calls/ctfe-arg-good-borrow.rs",
-    "tests/ui/explicit-tail-calls/ctfe-arg-move.rs",
-    "tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs",
-    "tests/ui/explicit-tail-calls/drop-order.rs",
-    "tests/ui/explicit-tail-calls/return-lifetime-sub.rs",
-
     // TODO: non-lifetime binders: `where for<'a, T> &'a Struct<T>: Trait`
     // https://github.com/dtolnay/syn/issues/1435
     "src/tools/rustfmt/tests/source/issue_5721.rs",

tests/test_precedence.rs

@@ -238,7 +238,7 @@ fn librustc_parenthesize(mut librustc_expr: P<ast::Expr>) -> P<ast::Expr> {
 
     fn noop_visit_expr<T: MutVisitor>(e: &mut Expr, vis: &mut T) {
         match &mut e.kind {
-            ExprKind::AddrOf(BorrowKind::Raw, ..) => {}
+            ExprKind::AddrOf(BorrowKind::Raw, ..) | ExprKind::Become(..) => {}
             ExprKind::Struct(expr) => {
                 let StructExpr {
                     qself,