#[cold] on match arms

compiler/rustc_middle/src/thir.rs

@@ -568,6 +568,7 @@ pub struct Arm<'tcx> {
     pub lint_level: LintLevel,
     pub scope: region::Scope,
     pub span: Span,
+    pub is_cold: bool,
 }
 
 #[derive(Copy, Clone, Debug, HashStable)]

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -14,7 +14,7 @@ use rustc_middle::middle::region;
 use rustc_middle::mir::{self, *};
 use rustc_middle::thir::{self, *};
 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
-use rustc_span::{BytePos, Pos, Span, Symbol};
+use rustc_span::{BytePos, Pos, Span, Symbol, sym};
 use tracing::{debug, instrument};
 
 use crate::builder::ForGuard::{self, OutsideGuard, RefWithinGuard};
@@ -458,7 +458,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         opt_scrutinee_place,
                     );
 
-                    let arm_block = this.bind_pattern(
+                    let mut arm_block = this.bind_pattern(
                         outer_source_info,
                         branch,
                         &built_match_tree.fake_borrow_temps,
@@ -467,6 +467,34 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         EmitStorageLive::Yes,
                     );
 
+                    if arm.is_cold
+                        && let Some(cold_path_def) = this.tcx.get_diagnostic_item(sym::cold_path)
+                    {
+                        let cold_path_ty = this.tcx.type_of(cold_path_def).instantiate_identity();
+                        let cold_path = Operand::Constant(Box::new(ConstOperand {
+                            span: arm.span,
+                            user_ty: None,
+                            const_: Const::from_value(ConstValue::ZeroSized, cold_path_ty),
+                        }));
+                        let new_arm_block = this.cfg.start_new_block();
+                        let cold_path_result =
+                            this.local_decls.push(LocalDecl::new(this.tcx.types.unit, arm.span));
+                        this.cfg.terminate(
+                            arm_block,
+                            this.source_info(arm.span),
+                            TerminatorKind::Call {
+                                func: cold_path,
+                                args: Box::new([]),
+                                destination: Place::from(cold_path_result),
+                                target: Some(new_arm_block),
+                                unwind: UnwindAction::Unreachable,
+                                call_source: CallSource::Misc,
+                                fn_span: arm.span,
+                            },
+                        );
+                        arm_block = new_arm_block;
+                    }
+
                     this.fixed_temps_scope = old_dedup_scope;
 
                     if let Some(source_scope) = scope {

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

@@ -1005,13 +1005,15 @@ impl<'tcx> ThirBuildCx<'tcx> {
     }
 
     fn convert_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) -> ArmId {
+        let attrs = self.tcx.hir().attrs(arm.hir_id);
         let arm = Arm {
             pattern: self.pattern_from_hir(&arm.pat),
             guard: arm.guard.as_ref().map(|g| self.mirror_expr(g)),
             body: self.mirror_expr(arm.body),
             lint_level: LintLevel::Explicit(arm.hir_id),
             scope: region::Scope { local_id: arm.hir_id.local_id, data: region::ScopeData::Node },
             span: arm.span,
+            is_cold: attrs.iter().any(|a| a.name_or_empty() == sym::cold),
         };
         self.thir.arms.push(arm)
     }

compiler/rustc_mir_build/src/thir/print.rs

@@ -623,7 +623,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
         print_indented!(self, "Arm {", depth_lvl);
 
         let arm = &self.thir.arms[arm_id];
-        let Arm { pattern, guard, body, lint_level, scope, span } = arm;
+        let Arm { pattern, guard, body, lint_level, scope, span, is_cold } = arm;
 
         print_indented!(self, "pattern: ", depth_lvl + 1);
         self.print_pat(pattern, depth_lvl + 2);
@@ -640,6 +640,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
         print_indented!(self, format!("lint_level: {:?}", lint_level), depth_lvl + 1);
         print_indented!(self, format!("scope: {:?}", scope), depth_lvl + 1);
         print_indented!(self, format!("span: {:?}", span), depth_lvl + 1);
+        print_indented!(self, format!("is_cold: {:?}", is_cold), depth_lvl + 1);
         print_indented!(self, "}", depth_lvl);
     }
 

library/core/src/intrinsics/mod.rs

@@ -1475,6 +1475,7 @@ pub const unsafe fn assume(b: bool) {
 /// This intrinsic does not have a stable counterpart.
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
+#[rustc_diagnostic_item = "cold_path"]
 #[rustc_nounwind]
 #[miri::intrinsic_fallback_is_spec]
 #[cold]

tests/ui/thir-print/thir-tree-match.stdout

@@ -143,6 +143,7 @@ body:
                                                                         lint_level: Explicit(HirId(DefId(0:16 ~ thir_tree_match[fcf8]::has_match).14))
                                                                         scope: Node(14)
                                                                         span: $DIR/thir-tree-match.rs:17:9: 17:40 (#0)
+                                                                        is_cold: false
                                                                     }
                                                                     Arm {
                                                                         pattern: 
@@ -195,6 +196,7 @@ body:
                                                                         lint_level: Explicit(HirId(DefId(0:16 ~ thir_tree_match[fcf8]::has_match).20))
                                                                         scope: Node(20)
                                                                         span: $DIR/thir-tree-match.rs:18:9: 18:32 (#0)
+                                                                        is_cold: false
                                                                     }
                                                                     Arm {
                                                                         pattern: 
@@ -239,6 +241,7 @@ body:
                                                                         lint_level: Explicit(HirId(DefId(0:16 ~ thir_tree_match[fcf8]::has_match).26))
                                                                         scope: Node(26)
                                                                         span: $DIR/thir-tree-match.rs:19:9: 19:28 (#0)
+                                                                        is_cold: false
                                                                     }
                                                                 ]
                                                             }