Add support for become expr/tail calls

crates/parser/src/grammar/expressions/atom.rs

@@ -58,6 +58,7 @@ pub(super) const ATOM_EXPR_FIRST: TokenSet =
         T![match],
         T![move],
         T![return],
+        T![become],
         T![static],
         T![try],
         T![unsafe],
@@ -102,6 +103,7 @@ pub(super) fn atom_expr(
         T![try] => try_block_expr(p, None),
         T![match] => match_expr(p),
         T![return] => return_expr(p),
+        T![become] => become_expr(p),
         T![yield] => yield_expr(p),
         T![do] if p.nth_at_contextual_kw(1, T![yeet]) => yeet_expr(p),
         T![continue] => continue_expr(p),
@@ -621,6 +623,18 @@ fn return_expr(p: &mut Parser<'_>) -> CompletedMarker {
     m.complete(p, RETURN_EXPR)
 }
 
+// test become_expr
+// fn foo() {
+//     become foo();
+// }
+fn become_expr(p: &mut Parser<'_>) -> CompletedMarker {
+    assert!(p.at(T![become]));
+    let m = p.start();
+    p.bump(T![become]);
+    expr(p);
+    m.complete(p, BECOME_EXPR)
+}
+
 // test yield_expr
 // fn foo() {
 //     yield;

crates/parser/test_data/parser/inline/ok/0209_become_expr.rast

@@ -0,0 +1,31 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        EXPR_STMT
+          BECOME_EXPR
+            BECOME_KW "become"
+            WHITESPACE " "
+            CALL_EXPR
+              PATH_EXPR
+                PATH
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "foo"
+              ARG_LIST
+                L_PAREN "("
+                R_PAREN ")"
+          SEMICOLON ";"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0209_become_expr.rs

@@ -0,0 +1,3 @@
+fn foo() {
+    become foo();
+}