From 9917ef9a6674619a72c16efaf1ca3777730d0012 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 18 Oct 2023 02:27:03 +0000
Subject: [PATCH] Don't compare host param by name

---
 compiler/rustc_hir_analysis/src/bounds.rs    | 9 +++++----
 compiler/rustc_hir_typeck/src/callee.rs      | 7 +++++--
 compiler/rustc_middle/src/ty/generic_args.rs | 5 -----
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_hir_analysis/src/bounds.rs b/compiler/rustc_hir_analysis/src/bounds.rs
index af1a4e5d99e98..b6688e0ce29e0 100644
--- a/compiler/rustc_hir_analysis/src/bounds.rs
+++ b/compiler/rustc_hir_analysis/src/bounds.rs
@@ -46,10 +46,11 @@ impl<'tcx> Bounds<'tcx> {
     ) {
         self.push_trait_bound_inner(tcx, trait_ref, span, polarity);
 
-        // if we have a host param, we push an unconst trait bound in addition
-        // to the const one.
-        // FIXME(effects) we should find a better way than name matching
-        if tcx.features().effects && trait_ref.skip_binder().args.host_effect_param().is_some() {
+        // push a non-const (`host = true`) version of the bound if it is `~const`.
+        if tcx.features().effects
+            && let Some(host_effect_idx) = tcx.generics_of(trait_ref.def_id()).host_effect_index
+            && trait_ref.skip_binder().args.const_at(host_effect_idx) != tcx.consts.true_
+        {
             let generics = tcx.generics_of(trait_ref.def_id());
             let Some(host_index) = generics.host_effect_index else { return };
             let trait_ref = trait_ref.map_bound(|mut trait_ref| {
diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index 1c23ccd157940..78d30f3aa12ec 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -786,8 +786,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 tcx.consts.false_
             }
             Some(hir::ConstContext::ConstFn) => {
-                let args = ty::GenericArgs::identity_for_item(tcx, context);
-                args.host_effect_param().expect("ConstContext::Maybe must have host effect param")
+                let host_idx = tcx
+                    .generics_of(context)
+                    .host_effect_index
+                    .expect("ConstContext::Maybe must have host effect param");
+                ty::GenericArgs::identity_for_item(tcx, context).const_at(host_idx)
             }
             None => tcx.consts.true_,
         };
diff --git a/compiler/rustc_middle/src/ty/generic_args.rs b/compiler/rustc_middle/src/ty/generic_args.rs
index 72390e4bbb03b..a861af4785945 100644
--- a/compiler/rustc_middle/src/ty/generic_args.rs
+++ b/compiler/rustc_middle/src/ty/generic_args.rs
@@ -11,7 +11,6 @@ use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 use rustc_serialize::{self, Decodable, Encodable};
-use rustc_span::sym;
 use rustc_type_ir::WithCachedTypeInfo;
 use smallvec::SmallVec;
 
@@ -452,10 +451,6 @@ impl<'tcx> GenericArgs<'tcx> {
         tcx.mk_args_from_iter(self.iter().take(generics.count()))
     }
 
-    pub fn host_effect_param(&'tcx self) -> Option<ty::Const<'tcx>> {
-        self.consts().rfind(|x| matches!(x.kind(), ty::ConstKind::Param(p) if p.name == sym::host))
-    }
-
     pub fn print_as_list(&self) -> String {
         let v = self.iter().map(|arg| arg.to_string()).collect::<Vec<_>>();
         format!("[{}]", v.join(", "))