Hack together inline-always-overrides

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -367,7 +367,12 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
         (InlineAttr::None, _) if instance.def.requires_inline(cx.tcx) => InlineAttr::Hint,
         (inline, _) => inline,
     };
-    to_add.extend(inline_attr(cx, inline));
+    if cx.tcx.has_inline_always_override(instance) {
+        eprintln!("Applying override");
+        to_add.extend(inline_attr(cx, InlineAttr::Always));
+    } else {
+        to_add.extend(inline_attr(cx, inline));
+    }
 
     // The `uwtable` attribute according to LLVM is:
     //

compiler/rustc_middle/src/mir/mono.rs

@@ -209,6 +209,8 @@ 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,

compiler/rustc_middle/src/ty/context.rs

@@ -78,8 +78,8 @@ 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,
 };
@@ -3395,6 +3395,14 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn do_not_recommend_impl(self, def_id: DefId) -> bool {
         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))
+    }
 }
 
 /// Parameter attributes that can only be determined by examining the body of a function instead

compiler/rustc_session/src/options.rs

@@ -2249,6 +2249,9 @@ options! {
         - hashes of green query instances
         - hash collisions of query keys
         - hash collisions when creating dep-nodes"),
+    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],