Auto merge of #122505 - oli-obk:visit_nested_body2, r=tmiasko

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

follow-up to #122371

cc #119214

This avoids codegening items (e.g. functions) that are only used during const eval, but do not reach their final constant value (e.g. via function pointers).

r? `@tmiasko`

1 files changed, 23 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/mir/interpret/queries.rs b/compiler/rustc_middle/src/mir/interpret/queries.rs
index fe743fa4aac..95857e8579d 100644
--- a/compiler/rustc_middle/src/mir/interpret/queries.rs
+++ b/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;
@@ -13,7 +15,7 @@ use tracing::{debug, instrument};
 
 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> {
@@ -27,6 +29,24 @@ 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, DUMMY_SP)
     }
+
+    /// 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));
+        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 +197,7 @@ 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) {