Rollup merge of #112790 - WaffleLapkin:syntactically, r=Nilstrieb

Syntactically accept `become` expressions (explicit tail calls experiment)

This adds `ast::ExprKind::Become`, implements parsing and properly gates the feature.

cc `@scottmcm`

src/expr.rs

@@ -232,6 +232,7 @@ pub(crate) fn format_expr(
         ast::ExprKind::Ret(Some(ref expr)) => {
             rewrite_unary_prefix(context, "return ", &**expr, shape)
         }
+        ast::ExprKind::Become(ref expr) => rewrite_unary_prefix(context, "become ", &**expr, shape),
         ast::ExprKind::Yeet(None) => Some("do yeet".to_owned()),
         ast::ExprKind::Yeet(Some(ref expr)) => {
             rewrite_unary_prefix(context, "do yeet ", &**expr, shape)

src/utils.rs

@@ -505,6 +505,7 @@ pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr
         | ast::ExprKind::Range(..)
         | ast::ExprKind::Repeat(..)
         | ast::ExprKind::Ret(..)
+        | ast::ExprKind::Become(..)
         | ast::ExprKind::Yeet(..)
         | ast::ExprKind::Tup(..)
         | ast::ExprKind::Type(..)