hir: Add Become expression kind (explicit tail calls experiment)

compiler/rustc_ast_lowering/src/expr.rs

@@ -277,9 +277,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 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))
+                    hir::ExprKind::Become(sub_expr)
                 }
                 ExprKind::InlineAsm(asm) => {
                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))

compiler/rustc_hir/src/hir.rs

@@ -1719,6 +1719,7 @@ impl Expr<'_> {
             ExprKind::Break(..) => ExprPrecedence::Break,
             ExprKind::Continue(..) => ExprPrecedence::Continue,
             ExprKind::Ret(..) => ExprPrecedence::Ret,
+            ExprKind::Become(..) => ExprPrecedence::Become,
             ExprKind::InlineAsm(..) => ExprPrecedence::InlineAsm,
             ExprKind::OffsetOf(..) => ExprPrecedence::OffsetOf,
             ExprKind::Struct(..) => ExprPrecedence::Struct,
@@ -1776,6 +1777,7 @@ impl Expr<'_> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
             | ExprKind::Assign(..)
@@ -1866,6 +1868,7 @@ impl Expr<'_> {
             | ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
+            | ExprKind::Become(..)
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
             | ExprKind::Assign(..)
@@ -2025,6 +2028,8 @@ pub enum ExprKind<'hir> {
     Continue(Destination),
     /// A `return`, with an optional value to be returned.
     Ret(Option<&'hir Expr<'hir>>),
+    /// A `become`, with the value to be returned.
+    Become(&'hir Expr<'hir>),
 
     /// Inline assembly (from `asm!`), with its outputs and inputs.
     InlineAsm(&'hir InlineAsm<'hir>),

compiler/rustc_hir/src/intravisit.rs

@@ -791,6 +791,7 @@ 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 expr) => visitor.visit_expr(expr),
         ExprKind::InlineAsm(ref asm) => {
             visitor.visit_inline_asm(asm, expression.hir_id);
         }

compiler/rustc_hir_pretty/src/lib.rs

@@ -1554,6 +1554,11 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(expr, parser::PREC_JUMP);
                 }
             }
+            hir::ExprKind::Become(result) => {
+                self.word("become");
+                self.word(" ");
+                self.print_expr_maybe_paren(result, parser::PREC_JUMP);
+            }
             hir::ExprKind::InlineAsm(asm) => {
                 self.word("asm!");
                 self.print_inline_asm(asm);

compiler/rustc_hir_typeck/messages.ftl

@@ -78,8 +78,8 @@ hir_typeck_note_edition_guide = for more on editions, read https://doc.rust-lang
 hir_typeck_op_trait_generic_params = `{$method_name}` must not have any generic parameters
 
 hir_typeck_return_stmt_outside_of_fn_body =
-    return statement outside of function body
-    .encl_body_label = the return is part of this body...
+    {$statement_kind} statement outside of function body
+    .encl_body_label = the {$statement_kind} is part of this body...
     .encl_fn_label = ...not the enclosing function body
 
 hir_typeck_struct_expr_non_exhaustive =

compiler/rustc_hir_typeck/src/errors.rs

@@ -2,7 +2,10 @@
 use std::borrow::Cow;
 
 use crate::fluent_generated as fluent;
-use rustc_errors::{AddToDiagnostic, Applicability, Diagnostic, MultiSpan, SubdiagnosticMessage};
+use rustc_errors::{
+    AddToDiagnostic, Applicability, Diagnostic, DiagnosticArgValue, IntoDiagnosticArg, MultiSpan,
+    SubdiagnosticMessage,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::{
@@ -31,6 +34,24 @@ pub struct ReturnStmtOutsideOfFnBody {
     pub encl_body_span: Option<Span>,
     #[label(hir_typeck_encl_fn_label)]
     pub encl_fn_span: Option<Span>,
+    pub statement_kind: ReturnLikeStatementKind,
+}
+
+pub enum ReturnLikeStatementKind {
+    Return,
+    Become,
+}
+
+impl IntoDiagnosticArg for ReturnLikeStatementKind {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        let kind = match self {
+            Self::Return => "return",
+            Self::Become => "become",
+        }
+        .into();
+
+        DiagnosticArgValue::Str(kind)
+    }
 }
 
 #[derive(Diagnostic)]

compiler/rustc_hir_typeck/src/expr.rs

@@ -5,6 +5,7 @@
 use crate::cast;
 use crate::coercion::CoerceMany;
 use crate::coercion::DynamicCoerceMany;
+use crate::errors::ReturnLikeStatementKind;
 use crate::errors::TypeMismatchFruTypo;
 use crate::errors::{AddressOfTemporaryTaken, ReturnStmtOutsideOfFnBody, StructExprNonExhaustive};
 use crate::errors::{
@@ -324,6 +325,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             ExprKind::Ret(ref expr_opt) => self.check_expr_return(expr_opt.as_deref(), expr),
+            ExprKind::Become(call) => self.check_expr_become(call, expr),
             ExprKind::Let(let_expr) => self.check_expr_let(let_expr),
             ExprKind::Loop(body, _, source, _) => {
                 self.check_expr_loop(body, source, expected, expr)
@@ -735,47 +737,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         if self.ret_coercion.is_none() {
-            let mut err = ReturnStmtOutsideOfFnBody {
-                span: expr.span,
-                encl_body_span: None,
-                encl_fn_span: None,
-            };
-
-            let encl_item_id = self.tcx.hir().get_parent_item(expr.hir_id);
-
-            if let Some(hir::Node::Item(hir::Item {
-                kind: hir::ItemKind::Fn(..),
-                span: encl_fn_span,
-                ..
-            }))
-            | Some(hir::Node::TraitItem(hir::TraitItem {
-                kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)),
-                span: encl_fn_span,
-                ..
-            }))
-            | Some(hir::Node::ImplItem(hir::ImplItem {
-                kind: hir::ImplItemKind::Fn(..),
-                span: encl_fn_span,
-                ..
-            })) = self.tcx.hir().find_by_def_id(encl_item_id.def_id)
-            {
-                // We are inside a function body, so reporting "return statement
-                // outside of function body" needs an explanation.
-
-                let encl_body_owner_id = self.tcx.hir().enclosing_body_owner(expr.hir_id);
-
-                // If this didn't hold, we would not have to report an error in
-                // the first place.
-                assert_ne!(encl_item_id.def_id, encl_body_owner_id);
-
-                let encl_body_id = self.tcx.hir().body_owned_by(encl_body_owner_id);
-                let encl_body = self.tcx.hir().body(encl_body_id);
-
-                err.encl_body_span = Some(encl_body.value.span);
-                err.encl_fn_span = Some(*encl_fn_span);
-            }
-
-            self.tcx.sess.emit_err(err);
+            self.emit_return_outside_of_fn_body(expr, ReturnLikeStatementKind::Return);
 
             if let Some(e) = expr_opt {
                 // We still have to type-check `e` (issue #86188), but calling
@@ -815,6 +777,38 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.tcx.types.never
     }
 
+    fn check_expr_become(
+        &self,
+        call: &'tcx hir::Expr<'tcx>,
+        expr: &'tcx hir::Expr<'tcx>,
+    ) -> Ty<'tcx> {
+        match &self.ret_coercion {
+            Some(ret_coercion) => {
+                let ret_ty = ret_coercion.borrow().expected_ty();
+                let call_expr_ty = self.check_expr_with_hint(call, ret_ty);
+
+                // N.B. don't coerce here, as tail calls can't support most/all coercions
+                // FIXME(explicit_tail_calls): add a diagnostic note that `become` doesn't allow coercions
+                self.demand_suptype(expr.span, ret_ty, call_expr_ty);
+            }
+            None => {
+                self.emit_return_outside_of_fn_body(expr, ReturnLikeStatementKind::Become);
+
+                // Fallback to simply type checking `call` without hint/demanding the right types.
+                // Best effort to highlight more errors.
+                self.check_expr(call);
+            }
+        }
+
+        self.tcx.types.never
+    }
+
+    /// Check an expression that _is being returned_.
+    /// For example, this is called with `return_expr: $expr` when `return $expr`
+    /// is encountered.
+    ///
+    /// Note that this function must only be called in function bodies.
+    ///
     /// `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(
@@ -831,10 +825,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mut span = return_expr.span;
         // Use the span of the trailing expression for our cause,
         // not the span of the entire function
-        if !explicit_return {
-            if let ExprKind::Block(body, _) = return_expr.kind && let Some(last_expr) = body.expr {
+        if !explicit_return
+            && let ExprKind::Block(body, _) = return_expr.kind
+            && let Some(last_expr) = body.expr
+        {
                 span = last_expr.span;
-            }
         }
         ret_coercion.borrow_mut().coerce(
             self,
@@ -854,6 +849,55 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// Emit an error because `return` or `become` is used outside of a function body.
+    ///
+    /// `expr` is the `return` (`become`) "statement", `kind` is the kind of the statement
+    /// either `Return` or `Become`.
+    fn emit_return_outside_of_fn_body(&self, expr: &hir::Expr<'_>, kind: ReturnLikeStatementKind) {
+        let mut err = ReturnStmtOutsideOfFnBody {
+            span: expr.span,
+            encl_body_span: None,
+            encl_fn_span: None,
+            statement_kind: kind,
+        };
+
+        let encl_item_id = self.tcx.hir().get_parent_item(expr.hir_id);
+
+        if let Some(hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::Fn(..),
+            span: encl_fn_span,
+            ..
+        }))
+        | Some(hir::Node::TraitItem(hir::TraitItem {
+            kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)),
+            span: encl_fn_span,
+            ..
+        }))
+        | Some(hir::Node::ImplItem(hir::ImplItem {
+            kind: hir::ImplItemKind::Fn(..),
+            span: encl_fn_span,
+            ..
+        })) = self.tcx.hir().find_by_def_id(encl_item_id.def_id)
+        {
+            // We are inside a function body, so reporting "return statement
+            // outside of function body" needs an explanation.
+
+            let encl_body_owner_id = self.tcx.hir().enclosing_body_owner(expr.hir_id);
+
+            // If this didn't hold, we would not have to report an error in
+            // the first place.
+            assert_ne!(encl_item_id.def_id, encl_body_owner_id);
+
+            let encl_body_id = self.tcx.hir().body_owned_by(encl_body_owner_id);
+            let encl_body = self.tcx.hir().body(encl_body_id);
+
+            err.encl_body_span = Some(encl_body.value.span);
+            err.encl_fn_span = Some(*encl_fn_span);
+        }
+
+        self.tcx.sess.emit_err(err);
+    }
+
     fn point_at_return_for_opaque_ty_error(
         &self,
         errors: &mut Vec<traits::FulfillmentError<'tcx>>,

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -326,6 +326,10 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                 }
             }
 
+            hir::ExprKind::Become(call) => {
+                self.consume_expr(call);
+            }
+
             hir::ExprKind::Assign(lhs, rhs, _) => {
                 self.mutate_expr(lhs);
                 self.consume_expr(rhs);

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::OffsetOf(..)
             | ExprKind::Struct(..)
@@ -451,6 +452,8 @@ impl<'a, 'tcx> Visitor<'tcx> for DropRangeVisitor<'a, 'tcx> {
                 }
             }
 
+            ExprKind::Become(_call) => bug!("encountered a tail-call inside a generator"),
+
             ExprKind::Call(f, args) => {
                 self.visit_expr(f);
                 for arg in args {

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(..)

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -695,6 +695,11 @@ 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(call) => {
+                // FIXME(explicit_tail_calls): use `ExprKind::Become` once we implemented it
+                // Temporary transform `become` into a `return`, so we can write tests for code before this stage
+                ExprKind::Return { value: Some(self.mirror_expr(call)) }
+            }
             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 },

compiler/rustc_passes/src/hir_stats.rs

@@ -302,8 +302,8 @@ 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, OffsetOf, Struct, Repeat, Yield,
-                Err
+                Path, AddrOf, Break, Continue, Ret, Become, InlineAsm, OffsetOf, Struct, Repeat,
+                Yield, Err
             ]
         );
         hir_visit::walk_expr(self, e)

compiler/rustc_passes/src/liveness.rs

@@ -463,6 +463,7 @@ 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(..)
@@ -967,6 +968,11 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 self.propagate_through_opt_expr(o_e.as_deref(), self.exit_ln)
             }
 
+            hir::ExprKind::Become(ref e) => {
+                // Ignore succ and subst exit_ln.
+                self.propagate_through_expr(e, self.exit_ln)
+            }
+
             hir::ExprKind::Break(label, ref opt_expr) => {
                 // Find which label this break jumps to
                 let target = match label.target_id {
@@ -1408,6 +1414,7 @@ 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(_)

compiler/rustc_passes/src/naked_functions.rs

@@ -204,6 +204,7 @@ impl<'tcx> CheckInlineAssembly<'tcx> {
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
             | ExprKind::OffsetOf(..)
+            | ExprKind::Become(..)
             | ExprKind::Struct(..)
             | ExprKind::Repeat(..)
             | ExprKind::Yield(..) => {

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()