Syntatically accept become expressions

compiler/rustc_ast/src/ast.rs

@@ -1295,6 +1295,7 @@ impl Expr {
             ExprKind::Yield(..) => ExprPrecedence::Yield,
             ExprKind::Yeet(..) => ExprPrecedence::Yeet,
             ExprKind::FormatArgs(..) => ExprPrecedence::FormatArgs,
+            ExprKind::Become(..) => ExprPrecedence::Become,
             ExprKind::Err => ExprPrecedence::Err,
         }
     }
@@ -1515,6 +1516,11 @@ pub enum ExprKind {
     /// with an optional value to be returned.
     Yeet(Option<P<Expr>>),
 
+    /// A tail call return, with the value to be returned.
+    ///
+    /// While `.0` must be a function call, we check this later, after parsing.
+    Become(P<Expr>),
+
     /// Bytes included via `include_bytes!`
     /// Added for optimization purposes to avoid the need to escape
     /// large binary blobs - should always behave like [`ExprKind::Lit`]

compiler/rustc_ast/src/mut_visit.rs

@@ -1457,6 +1457,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
         ExprKind::Yeet(expr) => {
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
+        ExprKind::Become(expr) => vis.visit_expr(expr),
         ExprKind::InlineAsm(asm) => vis.visit_inline_asm(asm),
         ExprKind::FormatArgs(fmt) => vis.visit_format_args(fmt),
         ExprKind::OffsetOf(container, fields) => {

compiler/rustc_ast/src/util/parser.rs

@@ -245,6 +245,7 @@ pub enum ExprPrecedence {
     Ret,
     Yield,
     Yeet,
+    Become,
 
     Range,
 
@@ -298,7 +299,8 @@ impl ExprPrecedence {
             | ExprPrecedence::Continue
             | ExprPrecedence::Ret
             | ExprPrecedence::Yield
-            | ExprPrecedence::Yeet => PREC_JUMP,
+            | ExprPrecedence::Yeet
+            | ExprPrecedence::Become => PREC_JUMP,
 
             // `Range` claims to have higher precedence than `Assign`, but `x .. x = x` fails to
             // parse, instead of parsing as `(x .. x) = x`. Giving `Range` a lower precedence

compiler/rustc_ast/src/visit.rs

@@ -908,6 +908,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Yeet(optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
+        ExprKind::Become(expr) => visitor.visit_expr(expr),
         ExprKind::MacCall(mac) => visitor.visit_mac_call(mac),
         ExprKind::Paren(subexpression) => visitor.visit_expr(subexpression),
         ExprKind::InlineAsm(asm) => visitor.visit_inline_asm(asm),