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),

compiler/rustc_ast_lowering/src/expr.rs

@@ -275,6 +275,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ExprKind::Ret(e)
                 }
                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
+                ExprKind::Become(sub_expr) => {
+                    let sub_expr = self.lower_expr(sub_expr);
+
+                    // FIXME(explicit_tail_calls): Use `hir::ExprKind::Become` once we implemented it
+                    hir::ExprKind::Ret(Some(sub_expr))
+                }
                 ExprKind::InlineAsm(asm) => {
                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
                 }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -555,6 +555,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
+    gate_all!(explicit_tail_calls, "`become` expression is experimental");
 
     if !visitor.features.negative_bounds {
         for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -537,6 +537,11 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
                 }
             }
+            ast::ExprKind::Become(result) => {
+                self.word("become");
+                self.word(" ");
+                self.print_expr_maybe_paren(result, parser::PREC_JUMP);
+            }
             ast::ExprKind::InlineAsm(a) => {
                 // FIXME: This should have its own syntax, distinct from a macro invocation.
                 self.word("asm!");

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -320,6 +320,7 @@ impl<'cx, 'a> Context<'cx, 'a> {
             | ExprKind::Underscore
             | ExprKind::While(_, _, _)
             | ExprKind::Yeet(_)
+            | ExprKind::Become(_)
             | ExprKind::Yield(_) => {}
         }
     }

compiler/rustc_feature/src/active.rs

@@ -395,6 +395,8 @@ declare_features! (
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (active, exhaustive_patterns, "1.13.0", Some(51085), None),
+    /// Allows explicit tail calls via `become` expression.
+    (incomplete, explicit_tail_calls, "CURRENT_RUSTC_VERSION", Some(112788), None),
     /// Allows using `efiapi`, `sysv64` and `win64` as calling convention
     /// for functions with varargs.
     (active, extended_varargs_abi_support, "1.65.0", Some(100189), None),

compiler/rustc_parse/src/parser/expr.rs

@@ -1430,6 +1430,8 @@ impl<'a> Parser<'a> {
             self.parse_expr_yield()
         } else if self.is_do_yeet() {
             self.parse_expr_yeet()
+        } else if self.eat_keyword(kw::Become) {
+            self.parse_expr_become()
         } else if self.check_keyword(kw::Let) {
             self.parse_expr_let()
         } else if self.eat_keyword(kw::Underscore) {
@@ -1746,6 +1748,16 @@ impl<'a> Parser<'a> {
         self.maybe_recover_from_bad_qpath(expr)
     }
 
+    /// Parse `"become" expr`, with `"become"` token already eaten.
+    fn parse_expr_become(&mut self) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let kind = ExprKind::Become(self.parse_expr()?);
+        let span = lo.to(self.prev_token.span);
+        self.sess.gated_spans.gate(sym::explicit_tail_calls, span);
+        let expr = self.mk_expr(span, kind);
+        self.maybe_recover_from_bad_qpath(expr)
+    }
+
     /// Parse `"break" (('label (:? expr)?) | expr?)` with `"break"` token already eaten.
     /// If the label is followed immediately by a `:` token, the label and `:` are
     /// parsed as part of the expression (i.e. a labeled loop). The language team has

compiler/rustc_passes/src/hir_stats.rs

@@ -569,7 +569,8 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
                 Array, ConstBlock, Call, MethodCall, Tup, Binary, Unary, Lit, Cast, Type, Let,
                 If, While, ForLoop, Loop, Match, Closure, Block, Async, Await, TryBlock, Assign,
                 AssignOp, Field, Index, Range, Underscore, Path, AddrOf, Break, Continue, Ret,
-                InlineAsm, FormatArgs, OffsetOf, MacCall, Struct, Repeat, Paren, Try, Yield, Yeet, IncludedBytes, Err
+                InlineAsm, FormatArgs, OffsetOf, MacCall, Struct, Repeat, Paren, Try, Yield, Yeet,
+                Become, IncludedBytes, Err
             ]
         );
         ast_visit::walk_expr(self, e)

compiler/rustc_span/src/symbol.rs

@@ -686,6 +686,7 @@ symbols! {
         expf32,
         expf64,
         explicit_generic_args_with_impl_trait,
+        explicit_tail_calls,
         export_name,
         expr,
         extended_key_value_attributes,

tests/ui/feature-gates/feature-gate-explicit_tail_calls.rs

@@ -0,0 +1,9 @@
+pub fn you<T>() -> T {
+    become bottom(); //~ error: `become` expression is experimental
+}
+
+pub fn bottom<T>() -> T {
+    become you(); //~ error: `become` expression is experimental
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-explicit_tail_calls.stderr

@@ -0,0 +1,21 @@
+error[E0658]: `become` expression is experimental
+  --> $DIR/feature-gate-explicit_tail_calls.rs:2:5
+   |
+LL |     become bottom();
+   |     ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #112788 <https://github.com/rust-lang/rust/issues/112788> for more information
+   = help: add `#![feature(explicit_tail_calls)]` to the crate attributes to enable
+
+error[E0658]: `become` expression is experimental
+  --> $DIR/feature-gate-explicit_tail_calls.rs:6:5
+   |
+LL |     become you();
+   |     ^^^^^^^^^^^^
+   |
+   = note: see issue #112788 <https://github.com/rust-lang/rust/issues/112788> for more information
+   = help: add `#![feature(explicit_tail_calls)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.