From 198fc5dd20e4260c39eecd358fa9e5da7be19312 Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Thu, 15 May 2025 21:27:02 +0000
Subject: [PATCH] Hack together inline-always-overrides

---
 compiler/rustc_codegen_llvm/src/attributes.rs |  5 +++++
 compiler/rustc_middle/src/mir/mono.rs         |  2 ++
 compiler/rustc_middle/src/ty/context.rs       | 12 ++++++++++--
 compiler/rustc_session/src/options.rs         |  3 +++
 4 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index 8070ea0b3e927..2e4eaacb4f54d 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -35,6 +35,11 @@ pub(crate) fn inline_attr<'ll, 'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: ty::Instance<'tcx>,
 ) -> Option<&'ll Attribute> {
+    if tcx.has_inline_always_override(instance) {
+        eprintln!("Applying override");
+        return Some(AttributeKind::AlwaysInline.create_attr(cx.llcx));
+    }
+
     // `optnone` requires `noinline`
     let codegen_fn_attrs = tcx.codegen_fn_attrs(instance.def_id());
     let inline = match (codegen_fn_attrs.inline, &codegen_fn_attrs.optimize) {
diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 6b45b2dc3ff50..feab672c8b834 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -206,6 +206,8 @@ impl<'tcx> MonoItem<'tcx> {
         if codegen_fn_attrs.inline.always() {
             return InstantiationMode::LocalCopy;
         }
+        // FIXME: Ideally we'd check has_inline_always_override here, but we can't because symbol names
+        // depend on instantiation mode so instantiation mode can't depend on symbol name.
 
         // #[inline(never)] functions in general are poor candidates for inlining and thus since
         // LocalCopy generally increases code size for the benefit of optimizations from inlining,
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 3c5c21a7a89c2..1110e80f88d8c 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -80,8 +80,8 @@ use crate::traits::solve::{
 use crate::ty::predicate::ExistentialPredicateStableCmpExt as _;
 use crate::ty::{
     self, AdtDef, AdtDefData, AdtKind, Binder, Clause, Clauses, Const, GenericArg, GenericArgs,
-    GenericArgsRef, GenericParamDefKind, List, ListWithCachedTypeInfo, ParamConst, ParamTy,
-    Pattern, PatternKind, PolyExistentialPredicate, PolyFnSig, Predicate, PredicateKind,
+    GenericArgsRef, GenericParamDefKind, Instance, List, ListWithCachedTypeInfo, ParamConst,
+    ParamTy, Pattern, PatternKind, PolyExistentialPredicate, PolyFnSig, Predicate, PredicateKind,
     PredicatePolarity, Region, RegionKind, ReprOptions, TraitObjectVisitor, Ty, TyKind, TyVid,
     ValTree, ValTreeKind, Visibility,
 };
@@ -3552,6 +3552,14 @@ impl<'tcx> TyCtxt<'tcx> {
         self.get_diagnostic_attr(def_id, sym::do_not_recommend).is_some()
     }
 
+    pub fn has_inline_always_override(self, instance: Instance<'tcx>) -> bool {
+        let Some(overrides) = &self.sess.opts.unstable_opts.inline_always_overrides else {
+            return false;
+        };
+        let symbol_name = self.symbol_name(instance).name;
+        overrides.iter().any(|o| symbol_name.starts_with(o))
+    }
+
     /// Whether this def is one of the special bin crate entrypoint functions that must have a
     /// monomorphization and also not be internalized in the bin crate.
     pub fn is_entrypoint(self, def_id: DefId) -> bool {
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 6dd90546de1b0..d88738729d941 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -2368,6 +2368,9 @@ options! {
         - hash collisions when creating dep-nodes"),
     indirect_branch_cs_prefix: bool = (false, parse_bool, [TRACKED TARGET_MODIFIER],
         "add `cs` prefix to `call` and `jmp` to indirect thunks (default: no)"),
+    inline_always_overrides: Option<Vec<String>> = (None, parse_opt_comma_list, [TRACKED],
+        "comma-separated list of full paths to functions to treat as if they are inline(always)"
+    ),
     inline_llvm: bool = (true, parse_bool, [TRACKED],
         "enable LLVM inlining (default: yes)"),
     inline_mir: Option<bool> = (None, parse_opt_bool, [TRACKED],