From a632bada94a62b99843fd0e412a39d9ccb2ad6d8 Mon Sep 17 00:00:00 2001
From: phi-go <github@philipp-goerz.com>
Date: Mon, 17 Apr 2023 16:24:03 +0200
Subject: [PATCH] redo frontend work

This commit is a port of the changes done by @semtexzv, as described
in this comment:
https://github.com/rust-lang/rfcs/pull/1888#issuecomment-1186593662

All locations that need further work are marked with:
FIXME(explicit_tail_calls)
---
 compiler/rustc_ast/src/ast.rs                 |  3 ++
 compiler/rustc_ast/src/mut_visit.rs           |  3 ++
 compiler/rustc_ast/src/visit.rs               |  3 ++
 compiler/rustc_ast_lowering/src/expr.rs       |  4 +++
 compiler/rustc_ast_passes/src/feature_gate.rs |  4 +++
 .../rustc_ast_pretty/src/pprust/state/expr.rs |  5 +++
 .../src/assert/context.rs                     |  1 +
 compiler/rustc_feature/src/active.rs          |  2 ++
 compiler/rustc_hir/src/hir.rs                 |  5 +++
 compiler/rustc_hir/src/intravisit.rs          |  3 ++
 compiler/rustc_hir_pretty/src/lib.rs          | 12 +++++--
 compiler/rustc_hir_typeck/src/expr.rs         | 24 +++++++++++++
 .../rustc_hir_typeck/src/expr_use_visitor.rs  |  4 +++
 .../drop_ranges/cfg_build.rs                  |  2 ++
 .../src/mem_categorization.rs                 |  1 +
 compiler/rustc_middle/src/thir.rs             |  4 +++
 compiler/rustc_middle/src/thir/visit.rs       |  1 +
 .../src/build/expr/as_place.rs                |  1 +
 .../src/build/expr/as_rvalue.rs               |  1 +
 .../src/build/expr/category.rs                |  3 +-
 .../rustc_mir_build/src/build/expr/into.rs    |  4 +++
 .../rustc_mir_build/src/check_unsafety.rs     |  1 +
 compiler/rustc_mir_build/src/thir/cx/expr.rs  |  1 +
 compiler/rustc_mir_build/src/thir/print.rs    |  9 +++++
 compiler/rustc_parse/src/parser/expr.rs       | 11 ++++++
 compiler/rustc_passes/src/hir_stats.rs        |  4 +--
 compiler/rustc_passes/src/liveness.rs         |  7 ++++
 compiler/rustc_passes/src/naked_functions.rs  |  1 +
 compiler/rustc_span/src/symbol.rs             |  1 +
 compiler/rustc_ty_utils/src/consts.rs         |  4 +++
 .../language-features/explicit-tail-calls.md  | 34 +++++++++++++++++++
 .../clippy_lints/src/loops/never_loop.rs      |  6 ++++
 .../matches/significant_drop_in_scrutinee.rs  |  1 +
 .../clippy/clippy_lints/src/utils/author.rs   |  5 +++
 .../clippy/clippy_utils/src/eager_or_lazy.rs  |  1 +
 .../clippy/clippy_utils/src/hir_utils.rs      |  3 ++
 src/tools/clippy/clippy_utils/src/sugg.rs     |  2 ++
 src/tools/clippy/clippy_utils/src/visitors.rs |  4 +++
 src/tools/rustfmt/src/expr.rs                 |  3 ++
 src/tools/rustfmt/src/utils.rs                |  1 +
 src/tools/tidy/src/ui_tests.rs                |  2 +-
 .../explicit-tail-calls/closure-disallowed.rs | 14 ++++++++
 tests/ui/explicit-tail-calls/wrong-expr.rs    | 10 ++++++
 .../ui/explicit-tail-calls/wrong-expr.stderr  |  7 ++++
 tests/ui/explicit-tail-calls/wrong-type.rs    | 14 ++++++++
 .../feature-gate-explicit_tail_calls.rs       |  7 ++++
 .../feature-gate-explicit_tail_calls.stderr   | 12 +++++++
 47 files changed, 248 insertions(+), 7 deletions(-)
 create mode 100644 src/doc/unstable-book/src/language-features/explicit-tail-calls.md
 create mode 100644 tests/ui/explicit-tail-calls/closure-disallowed.rs
 create mode 100644 tests/ui/explicit-tail-calls/wrong-expr.rs
 create mode 100644 tests/ui/explicit-tail-calls/wrong-expr.stderr
 create mode 100644 tests/ui/explicit-tail-calls/wrong-type.rs
 create mode 100644 tests/ui/feature-gates/feature-gate-explicit_tail_calls.rs
 create mode 100644 tests/ui/feature-gates/feature-gate-explicit_tail_calls.stderr

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index ab0409efb3b29..71609979d435a 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -1270,6 +1270,7 @@ impl Expr {
             ExprKind::Break(..) => ExprPrecedence::Break,
             ExprKind::Continue(..) => ExprPrecedence::Continue,
             ExprKind::Ret(..) => ExprPrecedence::Ret,
+            ExprKind::Become(..) => ExprPrecedence::Ret,
             ExprKind::InlineAsm(..) => ExprPrecedence::InlineAsm,
             ExprKind::MacCall(..) => ExprPrecedence::Mac,
             ExprKind::Struct(..) => ExprPrecedence::Struct,
@@ -1465,6 +1466,8 @@ pub enum ExprKind {
     Continue(Option<Label>),
     /// A `return`, with an optional value to be returned.
     Ret(Option<P<Expr>>),
+    /// A `become`, with the required value to be returned.
+    Become(P<Expr>),
 
     /// Output of the `asm!()` macro.
     InlineAsm(P<InlineAsm>),
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 694d31d8f1fd4..1947a5f7e95cf 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -1451,6 +1451,9 @@ pub fn noop_visit_expr<T: MutVisitor>(
         ExprKind::Ret(expr) => {
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
+        ExprKind::Become(expr) => {
+            vis.visit_expr(expr);
+        }
         ExprKind::Yeet(expr) => {
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index ac9b321b71c6f..20daf2c01d67a 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -902,6 +902,9 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Ret(optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
+        ExprKind::Become(expression) => {
+            visitor.visit_expr(expression);
+        }
         ExprKind::Yeet(optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 0d212b3e130a2..fdc13035bcb95 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -284,6 +284,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let e = e.as_ref().map(|x| self.lower_expr(x));
                     hir::ExprKind::Ret(e)
                 }
+                ExprKind::Become(e) => {
+                    let e = self.lower_expr(e);
+                    hir::ExprKind::Become(e)
+                }
                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
                 ExprKind::InlineAsm(asm) => {
                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 17bcd24ee39fd..2ad254b1e37f1 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -600,6 +600,10 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
     gate_all!(associated_const_equality, "associated const equality is incomplete");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
+    gate_all!(
+        explicit_tail_calls,
+        "explicit tails calls using the `become` keyword is experimental"
+    );
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
 
diff --git a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs b/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
index 776bf54244ed1..99a793b176b92 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
@@ -522,6 +522,11 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
                 }
             }
+            ast::ExprKind::Become(expr) => {
+                self.word("become");
+                self.word(" ");
+                self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
+            }
             ast::ExprKind::Yeet(result) => {
                 self.word("do");
                 self.word(" ");
diff --git a/compiler/rustc_builtin_macros/src/assert/context.rs b/compiler/rustc_builtin_macros/src/assert/context.rs
index c9e3cd486f855..03c80069a5d5f 100644
--- a/compiler/rustc_builtin_macros/src/assert/context.rs
+++ b/compiler/rustc_builtin_macros/src/assert/context.rs
@@ -308,6 +308,7 @@ impl<'cx, 'a> Context<'cx, 'a> {
             | ExprKind::Match(_, _)
             | ExprKind::Path(_, _)
             | ExprKind::Ret(_)
+            | ExprKind::Become(_)
             | ExprKind::Try(_)
             | ExprKind::TryBlock(_)
             | ExprKind::Type(_, _)
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 48f5bd1cb5048..c187ce29d9f0b 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -389,6 +389,8 @@ 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 guaranteeing tail call elimination using the `become` keyword.
+    (active, explicit_tail_calls, "1.70.0", Some(66666), None), // FIXME(explicit_tail_calls)
     /// Allows using `efiapi`, `sysv64` and `win64` as calling convention
     /// for functions with varargs.
     (active, extended_varargs_abi_support, "1.65.0", Some(100189), None),
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index b274e62807988..9a6f1afc7c3a3 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -1714,6 +1714,7 @@ impl Expr<'_> {
             ExprKind::Break(..) => ExprPrecedence::Break,
             ExprKind::Continue(..) => ExprPrecedence::Continue,
             ExprKind::Ret(..) => ExprPrecedence::Ret,
+            ExprKind::Become(..) => ExprPrecedence::Ret,
             ExprKind::InlineAsm(..) => ExprPrecedence::InlineAsm,
             ExprKind::Struct(..) => ExprPrecedence::Struct,
             ExprKind::Repeat(..) => ExprPrecedence::Repeat,
@@ -1770,6 +1771,7 @@ impl Expr<'_> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
             | ExprKind::Assign(..)
@@ -1859,6 +1861,7 @@ impl Expr<'_> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
             | ExprKind::Assign(..)
@@ -2018,6 +2021,8 @@ pub enum ExprKind<'hir> {
     Continue(Destination),
     /// A `return`, with an optional value to be returned.
     Ret(Option<&'hir Expr<'hir>>),
+    /// A `become`, with a value to be returned.
+    Become(&'hir Expr<'hir>),
 
     /// Inline assembly (from `asm!`), with its outputs and inputs.
     InlineAsm(&'hir InlineAsm<'hir>),
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 234256ab553c5..9b5555fcf44ed 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -783,6 +783,9 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::Ret(ref optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
+        ExprKind::Become(ref expression) => {
+            visitor.visit_expr(expression);
+        }
         ExprKind::InlineAsm(ref asm) => {
             visitor.visit_inline_asm(asm, expression.hir_id);
         }
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 74f5b359021d2..3af06d8b64555 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -1080,9 +1080,10 @@ impl<'a> State<'a> {
     // parses as the erroneous construct `if (return {})`, not `if (return) {}`.
     fn cond_needs_par(expr: &hir::Expr<'_>) -> bool {
         match expr.kind {
-            hir::ExprKind::Break(..) | hir::ExprKind::Closure { .. } | hir::ExprKind::Ret(..) => {
-                true
-            }
+            hir::ExprKind::Break(..)
+            | hir::ExprKind::Closure { .. }
+            | hir::ExprKind::Ret(..)
+            | hir::ExprKind::Become(..) => true,
             _ => contains_exterior_struct_lit(expr),
         }
     }
@@ -1547,6 +1548,11 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
                 }
             }
+            hir::ExprKind::Become(expr) => {
+                self.word("become");
+                self.word(" ");
+                self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
+            }
             hir::ExprKind::InlineAsm(asm) => {
                 self.word("asm!");
                 self.print_inline_asm(asm);
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index ffc73d64fc04b..a73a03ec8c15c 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -318,6 +318,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             ExprKind::Ret(ref expr_opt) => self.check_expr_return(expr_opt.as_deref(), expr),
+            ExprKind::Become(ref become_expr) => self.check_expr_become(become_expr, expr),
             ExprKind::Let(let_expr) => self.check_expr_let(let_expr),
             ExprKind::Loop(body, _, source, _) => {
                 self.check_expr_loop(body, source, expected, expr)
@@ -811,6 +812,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.tcx.types.never
     }
 
+    fn check_expr_become(
+        &self,
+        _expr: &'tcx hir::Expr<'tcx>,
+        _base_expr: &'tcx hir::Expr<'tcx>,
+    ) -> Ty<'tcx> {
+        bug!("become is not supported yet"); // FIXME(explicit_tail_calls)
+        // // FIXME: Checks
+        // // 1: That the the expr is a func call
+        // // 2: That the calling context is a function, not a closure
+        // // 3: That the function arguments & ret type match
+
+        // match &expr.kind {
+        //     hir::ExprKind::Call(..) => {}
+        //     _ => {
+        //         let mut err = self.tcx.sess.struct_span_err(expr.span, "become must always call a function");
+        //         err.span_label(expr.span, "this is not a function call");
+        //         err.emit();
+        //     }
+        // }
+        // // Defer other type checking to return. might not be totally correct, but it'll work for now
+        // self.check_expr_return(Some(expr), _base_expr)
+    }
+
     /// `explicit_return` is `true` if we're checking an explicit `return expr`,
     /// and `false` if we're checking a trailing expression.
     pub(super) fn check_return_expr(
diff --git a/compiler/rustc_hir_typeck/src/expr_use_visitor.rs b/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
index ee1c6fbfd650f..8621ae5c557e4 100644
--- a/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
+++ b/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
@@ -325,6 +325,10 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                 }
             }
 
+            hir::ExprKind::Become(ref expr) => {
+                self.consume_expr(expr);
+            }
+
             hir::ExprKind::Assign(lhs, rhs, _) => {
                 self.mutate_expr(lhs);
                 self.consume_expr(rhs);
diff --git a/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs b/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
index 3e9a9ce1b3114..a29aad1c368b7 100644
--- a/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
+++ b/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
@@ -214,6 +214,7 @@ impl<'a, 'tcx> DropRangeVisitor<'a, 'tcx> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::InlineAsm(..)
             | ExprKind::Struct(..)
             | ExprKind::Repeat(..)
@@ -490,6 +491,7 @@ impl<'a, 'tcx> Visitor<'tcx> for DropRangeVisitor<'a, 'tcx> {
             | ExprKind::Path(..)
             | ExprKind::Repeat(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Struct(..)
             | ExprKind::Tup(..)
             | ExprKind::Type(..)
diff --git a/compiler/rustc_hir_typeck/src/mem_categorization.rs b/compiler/rustc_hir_typeck/src/mem_categorization.rs
index 6c861b5930a1e..c6b91f21b6c33 100644
--- a/compiler/rustc_hir_typeck/src/mem_categorization.rs
+++ b/compiler/rustc_hir_typeck/src/mem_categorization.rs
@@ -361,6 +361,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
             | hir::ExprKind::AssignOp(..)
             | hir::ExprKind::Closure { .. }
             | hir::ExprKind::Ret(..)
+            | hir::ExprKind::Become(..)
             | hir::ExprKind::Unary(..)
             | hir::ExprKind::Yield(..)
             | hir::ExprKind::MethodCall(..)
diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index 7d79a13d3fde3..d5f47ffe46d1c 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -411,6 +411,10 @@ pub enum ExprKind<'tcx> {
     Return {
         value: Option<ExprId>,
     },
+    /// A `become` expression.
+    Become {
+        value: ExprId,
+    },
     /// An inline `const` block, e.g. `const {}`.
     ConstBlock {
         did: DefId,
diff --git a/compiler/rustc_middle/src/thir/visit.rs b/compiler/rustc_middle/src/thir/visit.rs
index 5614528c4cb25..eefae9721c166 100644
--- a/compiler/rustc_middle/src/thir/visit.rs
+++ b/compiler/rustc_middle/src/thir/visit.rs
@@ -100,6 +100,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[value])
             }
         }
+        Become { value } => visitor.visit_expr(&visitor.thir()[value]),
         ConstBlock { did: _, substs: _ } => {}
         Repeat { value, count: _ } => {
             visitor.visit_expr(&visitor.thir()[value]);
diff --git a/compiler/rustc_mir_build/src/build/expr/as_place.rs b/compiler/rustc_mir_build/src/build/expr/as_place.rs
index fb775766c6541..d92eea96e4309 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_place.rs
@@ -549,6 +549,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }
diff --git a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs b/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
index 8631749a524be..7e5718f6ea634 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
@@ -513,6 +513,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::InlineAsm { .. }
             | ExprKind::PlaceTypeAscription { .. }
             | ExprKind::ValueTypeAscription { .. } => {
diff --git a/compiler/rustc_mir_build/src/build/expr/category.rs b/compiler/rustc_mir_build/src/build/expr/category.rs
index d33401f07645e..5da060c7b38b7 100644
--- a/compiler/rustc_mir_build/src/build/expr/category.rs
+++ b/compiler/rustc_mir_build/src/build/expr/category.rs
@@ -81,7 +81,8 @@ impl Category {
             | ExprKind::Block { .. }
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
-            | ExprKind::Return { .. } =>
+            | ExprKind::Return { .. }
+            | ExprKind::Become { .. } =>
             // FIXME(#27840) these probably want their own
             // category, like "nonterminating"
             {
diff --git a/compiler/rustc_mir_build/src/build/expr/into.rs b/compiler/rustc_mir_build/src/build/expr/into.rs
index 05a723a6b6750..ccffe6ee67f89 100644
--- a/compiler/rustc_mir_build/src/build/expr/into.rs
+++ b/compiler/rustc_mir_build/src/build/expr/into.rs
@@ -495,6 +495,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.unit()
             }
 
+            ExprKind::Become { .. } => {
+                bug!("expr_into_dest not yet implemented for `become`."); // FIXME(explicit_tail_calls)
+            }
+
             // Avoid creating a temporary
             ExprKind::VarRef { .. }
             | ExprKind::UpvarRef { .. }
diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index 03a7f2d70faeb..780ea0cdcbb7e 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -316,6 +316,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             | ExprKind::Closure { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::Yield { .. }
             | ExprKind::Loop { .. }
             | ExprKind::Let { .. }
diff --git a/compiler/rustc_mir_build/src/thir/cx/expr.rs b/compiler/rustc_mir_build/src/thir/cx/expr.rs
index 8e2e92e6f6a94..b8ed2fdcddc8a 100644
--- a/compiler/rustc_mir_build/src/thir/cx/expr.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/expr.rs
@@ -686,6 +686,7 @@ impl<'tcx> Cx<'tcx> {
                 ExprKind::Repeat { value: self.mirror_expr(v), count: *count }
             }
             hir::ExprKind::Ret(ref v) => ExprKind::Return { value: v.map(|v| self.mirror_expr(v)) },
+            hir::ExprKind::Become(v) => ExprKind::Become { value: self.mirror_expr(v) },
             hir::ExprKind::Break(dest, ref value) => match dest.target_id {
                 Ok(target_id) => ExprKind::Break {
                     label: region::Scope { id: target_id.local_id, data: region::ScopeData::Node },
diff --git a/compiler/rustc_mir_build/src/thir/print.rs b/compiler/rustc_mir_build/src/thir/print.rs
index ed61d6ee78b1d..f87dd3e244e4b 100644
--- a/compiler/rustc_mir_build/src/thir/print.rs
+++ b/compiler/rustc_mir_build/src/thir/print.rs
@@ -421,6 +421,15 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
 
                 print_indented!(self, "}", depth_lvl);
             }
+            Become { .. } => {
+                bug!("print_expr: `Become` is not implemented yet."); // FIXME(explicit_tail_calls)
+                // print_indented!(self, "Become {", depth_lvl);
+                // print_indented!(self, "value:", depth_lvl + 1);
+
+                // self.print_expr(*value, depth_lvl + 2);
+
+                // print_indented!(self, "}", depth_lvl);
+            }
             ConstBlock { did, substs } => {
                 print_indented!(self, "ConstBlock {", depth_lvl);
                 print_indented!(self, format!("did: {:?}", did), depth_lvl + 1);
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 03c82fbd329fc..9a1bd1213a542 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1400,6 +1400,9 @@ impl<'a> Parser<'a> {
             self.parse_try_block(lo)
         } else if self.eat_keyword(kw::Return) {
             self.parse_expr_return()
+        } else if self.eat_keyword(kw::Become) {
+            self.sess.gated_spans.gate(sym::explicit_tail_calls, self.prev_token.span);
+            self.parse_expr_become()
         } else if self.eat_keyword(kw::Continue) {
             self.parse_expr_continue(lo)
         } else if self.eat_keyword(kw::Break) {
@@ -1699,6 +1702,14 @@ impl<'a> Parser<'a> {
         self.maybe_recover_from_bad_qpath(expr)
     }
 
+    /// Parse `"become" expr?`.
+    fn parse_expr_become(&mut self) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let kind = ExprKind::Become(self.parse_expr()?);
+        let expr = self.mk_expr(lo.to(self.prev_token.span), kind);
+        self.maybe_recover_from_bad_qpath(expr)
+    }
+
     /// Parse `"do" "yeet" expr?`.
     fn parse_expr_yeet(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
diff --git a/compiler/rustc_passes/src/hir_stats.rs b/compiler/rustc_passes/src/hir_stats.rs
index 47e032758f23d..b515937dd680d 100644
--- a/compiler/rustc_passes/src/hir_stats.rs
+++ b/compiler/rustc_passes/src/hir_stats.rs
@@ -302,7 +302,7 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
             [
                 ConstBlock, Array, Call, MethodCall, Tup, Binary, Unary, Lit, Cast, Type,
                 DropTemps, Let, If, Loop, Match, Closure, Block, Assign, AssignOp, Field, Index,
-                Path, AddrOf, Break, Continue, Ret, InlineAsm, Struct, Repeat, Yield, Err
+                Path, AddrOf, Break, Continue, Ret, Become, InlineAsm, Struct, Repeat, Yield, Err
             ]
         );
         hir_visit::walk_expr(self, e)
@@ -567,7 +567,7 @@ 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,
+                AssignOp, Field, Index, Range, Underscore, Path, AddrOf, Break, Continue, Ret, Become,
                 InlineAsm, FormatArgs, MacCall, Struct, Repeat, Paren, Try, Yield, Yeet, IncludedBytes, Err
             ]
         );
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index a8471ce3b6fb2..5c4c47b1a819b 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -467,6 +467,7 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             | hir::ExprKind::Lit(_)
             | hir::ExprKind::ConstBlock(..)
             | hir::ExprKind::Ret(..)
+            | hir::ExprKind::Become(..)
             | hir::ExprKind::Block(..)
             | hir::ExprKind::Assign(..)
             | hir::ExprKind::AssignOp(..)
@@ -965,6 +966,11 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 self.propagate_through_opt_expr(o_e.as_deref(), self.exit_ln)
             }
 
+            hir::ExprKind::Become(ref expr) => {
+                // Ignore succ and subst exit_ln.
+                self.propagate_through_expr(expr, self.exit_ln)
+            }
+
             hir::ExprKind::Break(label, ref opt_expr) => {
                 // Find which label this break jumps to
                 let target = match label.target_id {
@@ -1412,6 +1418,7 @@ fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
         | hir::ExprKind::DropTemps(..)
         | hir::ExprKind::Unary(..)
         | hir::ExprKind::Ret(..)
+        | hir::ExprKind::Become(..)
         | hir::ExprKind::Break(..)
         | hir::ExprKind::Continue(..)
         | hir::ExprKind::Lit(_)
diff --git a/compiler/rustc_passes/src/naked_functions.rs b/compiler/rustc_passes/src/naked_functions.rs
index c398467f03ef8..ce9104b7ff056 100644
--- a/compiler/rustc_passes/src/naked_functions.rs
+++ b/compiler/rustc_passes/src/naked_functions.rs
@@ -203,6 +203,7 @@ impl<'tcx> CheckInlineAssembly<'tcx> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Struct(..)
             | ExprKind::Repeat(..)
             | ExprKind::Yield(..) => {
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 6ce0b66ef6a45..17d16e06fc294 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -677,6 +677,7 @@ symbols! {
         expf32,
         expf64,
         explicit_generic_args_with_impl_trait,
+        explicit_tail_calls,
         export_name,
         expr,
         extended_key_value_attributes,
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index b67607a4db63e..22a6494b4a004 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -244,6 +244,9 @@ fn recurse_build<'tcx>(
         ExprKind::Closure { .. } | ExprKind::Return { .. } => {
             error(GenericConstantTooComplexSub::ClosureAndReturnNotSupported(node.span))?
         }
+        ExprKind::Become { .. } => {
+            bug!("Become is not yet implemented."); // FIXME(explicit_tail_calls)
+        }
         // let expressions imply control flow
         ExprKind::Match { .. } | ExprKind::If { .. } | ExprKind::Let { .. } => {
             error(GenericConstantTooComplexSub::ControlFlowNotSupported(node.span))?
@@ -336,6 +339,7 @@ impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
             | thir::ExprKind::Break { .. }
             | thir::ExprKind::Continue { .. }
             | thir::ExprKind::Return { .. }
+            | thir::ExprKind::Become { .. }
             | thir::ExprKind::Array { .. }
             | thir::ExprKind::Tuple { .. }
             | thir::ExprKind::Adt(_)
diff --git a/src/doc/unstable-book/src/language-features/explicit-tail-calls.md b/src/doc/unstable-book/src/language-features/explicit-tail-calls.md
new file mode 100644
index 0000000000000..2a5920bcf6963
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/explicit-tail-calls.md
@@ -0,0 +1,34 @@
+# `explicit_tail_calls`
+<!-- FIXME(explicit_tail_calls) check description and issue number -->
+
+The tracking issue for this feature is: FIXME [#ISSUE]
+
+[#ISSUE]: https://github.com/rust-lang/rust/issues/66666
+
+------------------------
+
+Currently, each function call might create a new stack frame.
+This makes it impossible to have a long chain of deeply
+recursive functions without exhausting the stack. While the
+compiler might optimize this, it is in no way guaranteed.
+
+The `become <expr>` forces the compiler to perform something called
+tail-call elimination, which does not increase stack space when performing
+recursive function calls. The `<expr>` inside `become` statement
+must always be a call to a function of same type (same arguments and return type).
+
+Example:
+```rust
+#![feature(become_stmt)]
+fn fib(n: usize, left: usize, right: usize) -> usize {
+    match n {
+        0 => 0,
+        1 => b,
+        rec => become fib(n - 1, right, left + right)
+    }
+}
+fn main() {
+    let value = fib(30, 0, 1);
+    assert_eq!(value, 514229);
+}
+```
diff --git a/src/tools/clippy/clippy_lints/src/loops/never_loop.rs b/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
index f0a1b1dfe5628..80e79fa7d5a19 100644
--- a/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
+++ b/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
@@ -206,6 +206,12 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
                 NeverLoopResult::AlwaysBreak,
             )
         }),
+        ExprKind::Become(e) => {
+            combine_seq(
+                never_loop_expr(e, ignore_ids, main_loop_id),
+                NeverLoopResult::AlwaysBreak,
+            )
+        }
         ExprKind::InlineAsm(asm) => asm
             .operands
             .iter()
diff --git a/src/tools/clippy/clippy_lints/src/matches/significant_drop_in_scrutinee.rs b/src/tools/clippy/clippy_lints/src/matches/significant_drop_in_scrutinee.rs
index 04225beeb704b..0832ed7789ef0 100644
--- a/src/tools/clippy/clippy_lints/src/matches/significant_drop_in_scrutinee.rs
+++ b/src/tools/clippy/clippy_lints/src/matches/significant_drop_in_scrutinee.rs
@@ -329,6 +329,7 @@ impl<'a, 'tcx> Visitor<'tcx> for SigDropHelper<'a, 'tcx> {
             ExprKind::Field(..) |
             ExprKind::Index(..) |
             ExprKind::Ret(..) |
+            ExprKind::Become(..) |
             ExprKind::Repeat(..) |
             ExprKind::Yield(..) => walk_expr(self, ex),
             ExprKind::AddrOf(_, _, _) |
diff --git a/src/tools/clippy/clippy_lints/src/utils/author.rs b/src/tools/clippy/clippy_lints/src/utils/author.rs
index 2dac807c420e3..3d30d1a747b17 100644
--- a/src/tools/clippy/clippy_lints/src/utils/author.rs
+++ b/src/tools/clippy/clippy_lints/src/utils/author.rs
@@ -554,6 +554,11 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
                 kind!("Ret({value})");
                 value.if_some(|e| self.expr(e));
             },
+            ExprKind::Become(value) => {
+                bind!(self, value);
+                kind!("Ret({value})");
+                self.expr(value);
+            },
             ExprKind::InlineAsm(_) => {
                 kind!("InlineAsm(_)");
                 out!("// unimplemented: `ExprKind::InlineAsm` is not further destructured at the moment");
diff --git a/src/tools/clippy/clippy_utils/src/eager_or_lazy.rs b/src/tools/clippy/clippy_utils/src/eager_or_lazy.rs
index 28c8571706135..a96721dff6c49 100644
--- a/src/tools/clippy/clippy_utils/src/eager_or_lazy.rs
+++ b/src/tools/clippy/clippy_utils/src/eager_or_lazy.rs
@@ -191,6 +191,7 @@ fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessS
                 ExprKind::Break(..)
                 | ExprKind::Continue(_)
                 | ExprKind::Ret(_)
+                | ExprKind::Become(_)
                 | ExprKind::InlineAsm(_)
                 | ExprKind::Yield(..)
                 | ExprKind::Err(_) => {
diff --git a/src/tools/clippy/clippy_utils/src/hir_utils.rs b/src/tools/clippy/clippy_utils/src/hir_utils.rs
index 3ee7147828bd5..d6defebd079f4 100644
--- a/src/tools/clippy/clippy_utils/src/hir_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/hir_utils.rs
@@ -755,6 +755,9 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
                     self.hash_expr(e);
                 }
             },
+            ExprKind::Become(ref e) => {
+                self.hash_expr(e);
+            },
             ExprKind::Path(ref qpath) => {
                 self.hash_qpath(qpath);
             },
diff --git a/src/tools/clippy/clippy_utils/src/sugg.rs b/src/tools/clippy/clippy_utils/src/sugg.rs
index a5a4a921d94ec..7c2adc7d77c85 100644
--- a/src/tools/clippy/clippy_utils/src/sugg.rs
+++ b/src/tools/clippy/clippy_utils/src/sugg.rs
@@ -146,6 +146,7 @@ impl<'a> Sugg<'a> {
             | hir::ExprKind::Path(..)
             | hir::ExprKind::Repeat(..)
             | hir::ExprKind::Ret(..)
+            | hir::ExprKind::Become(..)
             | hir::ExprKind::Struct(..)
             | hir::ExprKind::Tup(..)
             | hir::ExprKind::Err(_) => Sugg::NonParen(get_snippet(expr.span)),
@@ -208,6 +209,7 @@ impl<'a> Sugg<'a> {
             | ast::ExprKind::Path(..)
             | ast::ExprKind::Repeat(..)
             | ast::ExprKind::Ret(..)
+            | ast::ExprKind::Become(..)
             | ast::ExprKind::Yeet(..)
             | ast::ExprKind::FormatArgs(..)
             | ast::ExprKind::Struct(..)
diff --git a/src/tools/clippy/clippy_utils/src/visitors.rs b/src/tools/clippy/clippy_utils/src/visitors.rs
index 1dc19bac98444..2020c9784cd93 100644
--- a/src/tools/clippy/clippy_utils/src/visitors.rs
+++ b/src/tools/clippy/clippy_utils/src/visitors.rs
@@ -648,6 +648,10 @@ pub fn for_each_unconsumed_temporary<'tcx, B>(
                 helper(typeck, consume, e, f)?;
             },
 
+            ExprKind::Become(_expr) => {
+                panic!("Become is not yet implemented in for_each_unconsumed_temporary."); // FIXME(explicit_tail_calls)
+            }
+
             // Either drops temporaries, jumps out of the current expression, or has no sub expression.
             ExprKind::DropTemps(_)
             | ExprKind::Ret(_)
diff --git a/src/tools/rustfmt/src/expr.rs b/src/tools/rustfmt/src/expr.rs
index ac96bedf2fe86..33ee1a67083cb 100644
--- a/src/tools/rustfmt/src/expr.rs
+++ b/src/tools/rustfmt/src/expr.rs
@@ -232,6 +232,9 @@ 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)
diff --git a/src/tools/rustfmt/src/utils.rs b/src/tools/rustfmt/src/utils.rs
index a26375ee64384..b99bacd18dfd5 100644
--- a/src/tools/rustfmt/src/utils.rs
+++ b/src/tools/rustfmt/src/utils.rs
@@ -504,6 +504,7 @@ 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(..)
diff --git a/src/tools/tidy/src/ui_tests.rs b/src/tools/tidy/src/ui_tests.rs
index 29664c854c873..3e950e0043718 100644
--- a/src/tools/tidy/src/ui_tests.rs
+++ b/src/tools/tidy/src/ui_tests.rs
@@ -9,7 +9,7 @@ use std::path::{Path, PathBuf};
 
 // FIXME: The following limits should be reduced eventually.
 const ENTRY_LIMIT: usize = 885;
-const ROOT_ENTRY_LIMIT: usize = 891;
+const ROOT_ENTRY_LIMIT: usize = 892;
 const ISSUES_ENTRY_LIMIT: usize = 1977;
 
 fn check_entries(tests_path: &Path, bad: &mut bool) {
diff --git a/tests/ui/explicit-tail-calls/closure-disallowed.rs b/tests/ui/explicit-tail-calls/closure-disallowed.rs
new file mode 100644
index 0000000000000..b18041727c1ab
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/closure-disallowed.rs
@@ -0,0 +1,14 @@
+#![feature(explicit_tail_calls)]
+
+fn a(v: u32) -> u32 {
+    let closure = |e: u32| -> u32 {
+        become a(e)
+        //~^ ERROR become statement cannot be used inside a closure
+    };
+
+    closure(v)
+}
+
+fn main() {
+    println!("{}", a(0));
+}
diff --git a/tests/ui/explicit-tail-calls/wrong-expr.rs b/tests/ui/explicit-tail-calls/wrong-expr.rs
new file mode 100644
index 0000000000000..e65bcff5370de
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/wrong-expr.rs
@@ -0,0 +1,10 @@
+#![feature(explicit_tail_calls)]
+
+fn b() -> u32 {
+    become b() + 1;
+    //~^ ERROR become must always call a function
+}
+
+fn main() {
+    println!("{}", b());
+}
diff --git a/tests/ui/explicit-tail-calls/wrong-expr.stderr b/tests/ui/explicit-tail-calls/wrong-expr.stderr
new file mode 100644
index 0000000000000..00ce24d69bedc
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/wrong-expr.stderr
@@ -0,0 +1,7 @@
+error: become must always call a function
+  --> $DIR/wrong-expr.rs:4:12
+   |
+LL |     become b() + 1;
+   |            ^^^^^^^ this is not a function call
+
+error: aborting due to previous error
diff --git a/tests/ui/explicit-tail-calls/wrong-type.rs b/tests/ui/explicit-tail-calls/wrong-type.rs
new file mode 100644
index 0000000000000..9a5388f728851
--- /dev/null
+++ b/tests/ui/explicit-tail-calls/wrong-type.rs
@@ -0,0 +1,14 @@
+#![feature(explicit_tail_calls)]
+
+fn a(v: u32) -> u32 {
+    v
+}
+
+fn b() -> u32 {
+    become a(0)
+    //~^ ERROR cannot become `a` because it has different arguments
+}
+
+fn main() {
+    println!("{}", b());
+}
diff --git a/tests/ui/feature-gates/feature-gate-explicit_tail_calls.rs b/tests/ui/feature-gates/feature-gate-explicit_tail_calls.rs
new file mode 100644
index 0000000000000..355e91252ac4a
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-explicit_tail_calls.rs
@@ -0,0 +1,7 @@
+fn other() {
+    println!("Called recursively");
+}
+
+fn main() {
+    become other(); //~ ERROR `become` statement is experimental
+}
diff --git a/tests/ui/feature-gates/feature-gate-explicit_tail_calls.stderr b/tests/ui/feature-gates/feature-gate-explicit_tail_calls.stderr
new file mode 100644
index 0000000000000..dce884361d014
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-explicit_tail_calls.stderr
@@ -0,0 +1,12 @@
+error[E0658]: `become` statement is experimental
+  --> $DIR/feature-gate-become_stmt.rs:6:5
+   |
+LL |     become other();
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: see issue #66666 <https://github.com/rust-lang/rust/issues/66666> for more information
+   = help: add `#![feature(become_stmt)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
\ No newline at end of file