Auto merge of #98900 - lcnr:region-stuff, r=jackh726

const_generics: correctly deal with bound variables

removes the hack in `resolve` which was needed because we evaluated constants without caring about their bound variables.

Each commit should be fairly self-contained, even if they build on each other

r? `@jackh726`

compiler/rustc_trait_selection/src/traits/project.rs

@@ -635,13 +635,18 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn fold_const(&mut self, constant: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        if self.selcx.tcx().lazy_normalization() || !self.eager_inference_replacement {
+        let tcx = self.selcx.tcx();
+        if tcx.lazy_normalization() {
             constant
         } else {
             let constant = constant.super_fold_with(self);
-            debug!(?constant);
-            debug!("self.param_env: {:?}", self.param_env);
-            constant.eval(self.selcx.tcx(), self.param_env)
+            debug!(?constant, ?self.param_env);
+            with_replaced_escaping_bound_vars(
+                self.selcx.infcx(),
+                &mut self.universes,
+                constant,
+                |constant| constant.eval(tcx, self.param_env),
+            )
         }
     }
 
@@ -671,6 +676,41 @@ pub struct BoundVarReplacer<'me, 'tcx> {
     universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
 }
 
+/// Executes `f` on `value` after replacing all escaping bound variables with placeholders
+/// and then replaces these placeholders with the original bound variables in the result.
+///
+/// In most places, bound variables should be replaced right when entering a binder, making
+/// this function unnecessary. However, normalization currently does not do that, so we have
+/// to do this lazily.
+///
+/// You should not add any additional uses of this function, at least not without first
+/// discussing it with t-types.
+///
+/// FIXME(@lcnr): We may even consider experimenting with eagerly replacing bound vars during
+/// normalization as well, at which point this function will be unnecessary and can be removed.
+pub fn with_replaced_escaping_bound_vars<'a, 'tcx, T: TypeFoldable<'tcx>, R: TypeFoldable<'tcx>>(
+    infcx: &'a InferCtxt<'a, 'tcx>,
+    universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,
+    value: T,
+    f: impl FnOnce(T) -> R,
+) -> R {
+    if value.has_escaping_bound_vars() {
+        let (value, mapped_regions, mapped_types, mapped_consts) =
+            BoundVarReplacer::replace_bound_vars(infcx, universe_indices, value);
+        let result = f(value);
+        PlaceholderReplacer::replace_placeholders(
+            infcx,
+            mapped_regions,
+            mapped_types,
+            mapped_consts,
+            universe_indices,
+            result,
+        )
+    } else {
+        f(value)
+    }
+}
+
 impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
     /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
     /// use a binding level above `universe_indices.len()`, we fail.

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -6,7 +6,7 @@ use crate::infer::at::At;
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::{InferCtxt, InferOk};
 use crate::traits::error_reporting::InferCtxtExt;
-use crate::traits::project::needs_normalization;
+use crate::traits::project::{needs_normalization, BoundVarReplacer, PlaceholderReplacer};
 use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -283,11 +283,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 let tcx = self.infcx.tcx;
                 let infcx = self.infcx;
                 let (data, mapped_regions, mapped_types, mapped_consts) =
-                    crate::traits::project::BoundVarReplacer::replace_bound_vars(
-                        infcx,
-                        &mut self.universes,
-                        data,
-                    );
+                    BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
                 let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
@@ -313,8 +309,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 debug!("QueryNormalizer: result = {:#?}", result);
                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                 self.obligations.extend(obligations);
-
-                let res = crate::traits::project::PlaceholderReplacer::replace_placeholders(
+                let res = PlaceholderReplacer::replace_placeholders(
                     infcx,
                     mapped_regions,
                     mapped_types,
@@ -343,7 +338,13 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
         constant: ty::Const<'tcx>,
     ) -> Result<ty::Const<'tcx>, Self::Error> {
         let constant = constant.try_super_fold_with(self)?;
-        Ok(constant.eval(self.infcx.tcx, self.param_env))
+        debug!(?constant, ?self.param_env);
+        Ok(crate::traits::project::with_replaced_escaping_bound_vars(
+            self.infcx,
+            &mut self.universes,
+            constant,
+            |constant| constant.eval(self.infcx.tcx, self.param_env),
+        ))
     }
 
     fn try_fold_mir_const(