Implement explicit tail calls

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 +275,10 @@ 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);
+                    hir::ExprKind::Become(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_borrowck/src/invalidation.rs

@@ -137,6 +137,12 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
                 }
                 self.mutate_place(location, *destination, Deep);
             }
+            TerminatorKind::TailCall { func, args, .. } => {
+                self.consume_operand(location, func);
+                for arg in args {
+                    self.consume_operand(location, arg);
+                }
+            }
             TerminatorKind::Assert { cond, expected: _, msg, target: _, unwind: _ } => {
                 self.consume_operand(location, cond);
                 use rustc_middle::mir::AssertKind;

compiler/rustc_borrowck/src/lib.rs

@@ -719,6 +719,12 @@ impl<'cx, 'tcx, R> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx, R> for MirBorro
                 }
                 self.mutate_place(loc, (*destination, span), Deep, flow_state);
             }
+            TerminatorKind::TailCall { func, args, .. } => {
+                self.consume_operand(loc, (func, span), flow_state);
+                for arg in args {
+                    self.consume_operand(loc, (arg, span), flow_state);
+                }
+            }
             TerminatorKind::Assert { cond, expected: _, msg, target: _, unwind: _ } => {
                 self.consume_operand(loc, (cond, span), flow_state);
                 use rustc_middle::mir::AssertKind;
@@ -803,7 +809,11 @@ impl<'cx, 'tcx, R> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx, R> for MirBorro
                 }
             }
 
-            TerminatorKind::Resume | TerminatorKind::Return | TerminatorKind::GeneratorDrop => {
+            // FIXME(explicit_tail_calls): do we need to do something similar before the tail call?
+            TerminatorKind::Resume
+            | TerminatorKind::Return
+            | TerminatorKind::TailCall { .. }
+            | TerminatorKind::GeneratorDrop => {
                 // Returning from the function implicitly kills storage for all locals and statics.
                 // Often, the storage will already have been killed by an explicit
                 // StorageDead, but we don't always emit those (notably on unwind paths),

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1370,7 +1370,15 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 }
                 // FIXME: check the values
             }
-            TerminatorKind::Call { func, args, destination, from_hir_call, target, .. } => {
+            TerminatorKind::Call { func, args, .. }
+            | TerminatorKind::TailCall { func, args, .. } => {
+                let from_hir_call = matches!(
+                    &term.kind,
+                    TerminatorKind::Call { from_hir_call: true, .. }
+                        // tail calls don't support overloaded operators
+                        | TerminatorKind::TailCall { .. }
+                );
+
                 self.check_operand(func, term_location);
                 for arg in args {
                     self.check_operand(arg, term_location);
@@ -1426,7 +1434,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     ConstraintCategory::Boring,
                 );
                 let sig = self.normalize(sig, term_location);
-                self.check_call_dest(body, term, &sig, *destination, *target, term_location);
+
+                if let TerminatorKind::Call { destination, target, .. } = term.kind {
+                    self.check_call_dest(body, term, &sig, destination, target, term_location);
+                }
 
                 // The ordinary liveness rules will ensure that all
                 // regions in the type of the callee are live here. We
@@ -1444,7 +1455,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                         .add_element(region_vid, term_location);
                 }
 
-                self.check_call_inputs(body, term, &sig, args, term_location, *from_hir_call);
+                self.check_call_inputs(body, term, &sig, args, term_location, from_hir_call);
             }
             TerminatorKind::Assert { cond, msg, .. } => {
                 self.check_operand(cond, term_location);
@@ -1637,6 +1648,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     span_mirbug!(self, block_data, "return on cleanup block")
                 }
             }
+            TerminatorKind::TailCall { .. } => {
+                if is_cleanup {
+                    span_mirbug!(self, block_data, "tailcall on cleanup block")
+                }
+            }
             TerminatorKind::GeneratorDrop { .. } => {
                 if is_cleanup {
                     span_mirbug!(self, block_data, "generator_drop in cleanup block")

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_codegen_cranelift/src/base.rs

@@ -433,6 +433,11 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                     )
                 });
             }
+            // FIXME(explicit_tail_calls): add support for tail calls to the cranelift backend, once cranelift supports tail calls
+            TerminatorKind::TailCall { fn_span, .. } => span_bug!(
+                *fn_span,
+                "tail calls are not yet supported in `rustc_codegen_cranelift` backend"
+            ),
             TerminatorKind::InlineAsm {
                 template,
                 operands,

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -566,6 +566,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
                     {
                         return None;
                     }
+                    TerminatorKind::TailCall { .. } => return None,
                     TerminatorKind::Call { .. } => {}
                 }
             }

compiler/rustc_codegen_gcc/src/builder.rs

@@ -1378,6 +1378,18 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         call
     }
 
+    fn tail_call(
+        &mut self,
+        _typ: Type<'gcc>,
+        _fn_attrs: Option<&CodegenFnAttrs>,
+        _fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
+        _func: RValue<'gcc>,
+        _: &[RValue<'gcc>],
+        _: Option<&Funclet>
+    ) { 
+        bug!("tail calls are not yet supported in `rustc_codegen_gcc` backend")
+    }
+
     fn zext(&mut self, value: RValue<'gcc>, dest_typ: Type<'gcc>) -> RValue<'gcc> {
         // FIXME(antoyo): this does not zero-extend.
         if value.get_type().is_bool() && dest_typ.is_i8(&self.cx) {

compiler/rustc_codegen_llvm/src/builder.rs

@@ -14,6 +14,7 @@ use rustc_codegen_ssa::traits::*;
 use rustc_codegen_ssa::MemFlags;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
+use rustc_middle::bug;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::ty::layout::{
     FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOfHelpers, TyAndLayout,
@@ -1217,6 +1218,29 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         call
     }
 
+    fn tail_call(
+        &mut self,
+        llty: Self::Type,
+        fn_attrs: Option<&CodegenFnAttrs>,
+        fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
+        llfn: Self::Value,
+        args: &[Self::Value],
+        funclet: Option<&Self::Funclet>,
+    ) {
+        let call = self.call(llty, fn_attrs, Some(fn_abi), llfn, args, funclet);
+
+        // Depending on the pass mode we neet to generate different return instructions for llvm
+        match &fn_abi.ret.mode {
+            abi::call::PassMode::Ignore | abi::call::PassMode::Indirect { .. } => self.ret_void(),
+            abi::call::PassMode::Direct(_) | abi::call::PassMode::Pair { .. } => self.ret(call),
+            mode @ abi::call::PassMode::Cast(..) => {
+                bug!("Encountered `PassMode::{mode:?}` during codegen")
+            }
+        }
+
+        unsafe { llvm::LLVMRustSetTailCallKind(call, llvm::TailCallKind::MustTail) };
+    }
+
     fn zext(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildZExt(self.llbuilder, val, dest_ty, UNNAMED) }
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -585,6 +585,17 @@ pub enum ThreadLocalMode {
     LocalExec,
 }
 
+/// LLVMRustTailCallKind
+#[derive(Copy, Clone)]
+#[repr(C)]
+pub enum TailCallKind {
+    None,
+    Tail,
+    MustTail,
+    NoTail,
+    Last,
+}
+
 /// LLVMRustChecksumKind
 #[derive(Copy, Clone)]
 #[repr(C)]
@@ -1195,6 +1206,7 @@ extern "C" {
         NameLen: size_t,
     ) -> Option<&Value>;
     pub fn LLVMSetTailCall(CallInst: &Value, IsTailCall: Bool);
+    pub fn LLVMRustSetTailCallKind(CallInst: &Value, TKC: TailCallKind);
 
     // Operations on attributes
     pub fn LLVMRustCreateAttrNoValue(C: &Context, attr: AttributeKind) -> &Attribute;

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -292,6 +292,7 @@ pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKi
                 | TerminatorKind::SwitchInt { .. }
                 | TerminatorKind::Yield { .. }
                 | TerminatorKind::FalseEdge { .. }
+                | TerminatorKind::TailCall { .. }
                 | TerminatorKind::FalseUnwind { .. } => { /* nothing to do */ }
                 TerminatorKind::Call { unwind, .. }
                 | TerminatorKind::InlineAsm { unwind, .. }