Auto merge of #142289 - fmease:maybe-perf-gen-args, r=compiler-errors

[perf] `GenericArgs`-related: Change asserts to debug asserts & use more slice interning over iterable interning

1. The 1st commit yields the following perf gains: [#142289 (comment)](https://github.com/rust-lang/rust/pull/142289#issuecomment-2964041303).
2. The 2nd commit might also have a minor positive perf impact, however that one wasn't tested in isolation.

For reference, the initial approach https://github.com/rust-lang/rust/commit/c7e6accd79d91fe5dec01a81499a08f9db280440 (results: https://github.com/rust-lang/rust/pull/142289#issuecomment-2961076587) had a lot more changes (apart from what's now contained in commit 1 and 2) which seemed to be perf irrelevant (cf. the partial countercheck in https://github.com/rust-lang/rust/commit/6f82bf1cfece61d32714fbfeecf8c5cf1356b3ae (results: https://github.com/rust-lang/rust/pull/142289#issuecomment-2968393647).

3 files changed, 5 insertions, 6 deletions

diff --git a/compiler/rustc_hir_analysis/src/delegation.rs b/compiler/rustc_hir_analysis/src/delegation.rs
index b907ec090e5..f5821aed03f 100644
--- a/compiler/rustc_hir_analysis/src/delegation.rs
+++ b/compiler/rustc_hir_analysis/src/delegation.rs
@@ -276,8 +276,7 @@ fn create_generic_args<'tcx>(
                 tcx.impl_trait_header(parent).unwrap().trait_ref.instantiate_identity().args;
 
             let trait_args = ty::GenericArgs::identity_for_item(tcx, sig_id);
-            let method_args =
-                tcx.mk_args_from_iter(trait_args.iter().skip(callee_generics.parent_count));
+            let method_args = tcx.mk_args(&trait_args[callee_generics.parent_count..]);
             let method_args = build_generic_args(tcx, sig_id, def_id, method_args);
 
             tcx.mk_args_from_iter(parent_args.iter().chain(method_args))
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 6a47000fc85..b4d8b2e7176 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -4,7 +4,7 @@
 
 pub mod tls;
 
-use std::assert_matches::{assert_matches, debug_assert_matches};
+use std::assert_matches::debug_assert_matches;
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 use std::env::VarError;
@@ -283,9 +283,9 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         def_id: DefId,
         args: ty::GenericArgsRef<'tcx>,
     ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
-        assert_matches!(self.def_kind(def_id), DefKind::AssocTy | DefKind::AssocConst);
+        debug_assert_matches!(self.def_kind(def_id), DefKind::AssocTy | DefKind::AssocConst);
         let trait_def_id = self.parent(def_id);
-        assert_matches!(self.def_kind(trait_def_id), DefKind::Trait);
+        debug_assert_matches!(self.def_kind(trait_def_id), DefKind::Trait);
         let trait_generics = self.generics_of(trait_def_id);
         (
             ty::TraitRef::new_from_args(self, trait_def_id, args.truncate_to(self, trait_generics)),
diff --git a/compiler/rustc_middle/src/ty/generic_args.rs b/compiler/rustc_middle/src/ty/generic_args.rs
index 5e038f91675..7d34d8df3f3 100644
--- a/compiler/rustc_middle/src/ty/generic_args.rs
+++ b/compiler/rustc_middle/src/ty/generic_args.rs
@@ -588,7 +588,7 @@ impl<'tcx> GenericArgs<'tcx> {
     }
 
     pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> GenericArgsRef<'tcx> {
-        tcx.mk_args_from_iter(self.iter().take(generics.count()))
+        tcx.mk_args(&self[..generics.count()])
     }
 
     pub fn print_as_list(&self) -> String {