Don't walk the bodies of free constants for reachability.

compiler/rustc_middle/src/mir/interpret/queries.rs

@@ -1,4 +1,6 @@
-use super::{ErrorHandled, EvalToConstValueResult, EvalToValTreeResult, GlobalId};
+use super::{
+    ErrorHandled, EvalToAllocationRawResult, EvalToConstValueResult, EvalToValTreeResult, GlobalId,
+};
 
 use crate::mir;
 use crate::query::TyCtxtEnsure;
@@ -12,7 +14,7 @@ use rustc_span::{Span, DUMMY_SP};
 
 impl<'tcx> TyCtxt<'tcx> {
     /// Evaluates a constant without providing any generic parameters. This is useful to evaluate consts
-    /// that can't take any generic arguments like statics, const items or enum discriminants. If a
+    /// that can't take any generic arguments like const items or enum discriminants. If a
     /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
     #[instrument(skip(self), level = "debug")]
     pub fn const_eval_poly(self, def_id: DefId) -> EvalToConstValueResult<'tcx> {
@@ -26,6 +28,25 @@ impl<'tcx> TyCtxt<'tcx> {
         let param_env = self.param_env(def_id).with_reveal_all_normalized(self);
         self.const_eval_global_id(param_env, cid, None)
     }
+
+    /// Evaluates a constant without providing any generic parameters. This is useful to evaluate consts
+    /// that can't take any generic arguments like const items or enum discriminants. If a
+    /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
+    #[instrument(skip(self), level = "debug")]
+    pub fn const_eval_poly_to_alloc(self, def_id: DefId) -> EvalToAllocationRawResult<'tcx> {
+        // In some situations def_id will have generic parameters within scope, but they aren't allowed
+        // to be used. So we can't use `Instance::mono`, instead we feed unresolved generic parameters
+        // into `const_eval` which will return `ErrorHandled::ToGeneric` if any of them are
+        // encountered.
+        let args = GenericArgs::identity_for_item(self, def_id);
+        let instance = ty::Instance::new(def_id, args);
+        let cid = GlobalId { instance, promoted: None };
+        let param_env = self.param_env(def_id).with_reveal_all_normalized(self);
+        let inputs = self.erase_regions(param_env.and(cid));
+        // The query doesn't know where it is being invoked, so we need to fix the span.
+        self.eval_to_allocation_raw(inputs)
+    }
+
     /// Resolves and evaluates a constant.
     ///
     /// The constant can be located on a trait like `<A as B>::C`, in which case the given
@@ -177,7 +198,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
 impl<'tcx> TyCtxtEnsure<'tcx> {
     /// Evaluates a constant without providing any generic parameters. This is useful to evaluate consts
-    /// that can't take any generic arguments like statics, const items or enum discriminants. If a
+    /// that can't take any generic arguments like const items or enum discriminants. If a
     /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
     #[instrument(skip(self), level = "debug")]
     pub fn const_eval_poly(self, def_id: DefId) {

compiler/rustc_passes/src/reachable.rs

@@ -14,7 +14,7 @@ use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::Node;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
 use rustc_middle::middle::privacy::{self, Level};
-use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc};
+use rustc_middle::mir::interpret::{ConstAllocation, ErrorHandled, GlobalAlloc};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, ExistentialTraitRef, TyCtxt};
 use rustc_privacy::DefIdVisitor;
@@ -185,8 +185,18 @@ impl<'tcx> ReachableContext<'tcx> {
                     // Reachable constants will be inlined into other crates
                     // unconditionally, so we need to make sure that their
                     // contents are also reachable.
-                    hir::ItemKind::Const(_, _, init) => {
-                        self.visit_nested_body(init);
+                    hir::ItemKind::Const(..) => {
+                        match self.tcx.const_eval_poly_to_alloc(item.owner_id.def_id.into()) {
+                            Ok(alloc) => {
+                                let alloc = self.tcx.global_alloc(alloc.alloc_id).unwrap_memory();
+                                self.propagate_from_alloc(alloc);
+                            }
+                            Err(ErrorHandled::TooGeneric(span)) => span_bug!(
+                                span,
+                                "generic constants aren't implemented in reachability"
+                            ),
+                            Err(ErrorHandled::Reported(..)) => {}
+                        }
                     }
                     hir::ItemKind::Static(..) => {
                         if let Ok(alloc) = self.tcx.eval_static_initializer(item.owner_id.def_id) {

tests/codegen/dont_codegen_private_const_fn_only_used_in_const_eval.rs

@@ -13,4 +13,4 @@ const fn bar() {}
 
 pub const BAR: () = bar();
 
-// CHECK: define{{.*}}bar{{.*}}
+// CHECK-NOT: define{{.*}}bar{{.*}}