From 928564c7bfb395e1e5bd34d8913b7627ee4d1130 Mon Sep 17 00:00:00 2001 From: Mark Rousskov Date: Sun, 22 Jun 2025 17:51:56 -0400 Subject: [PATCH] Stop collecting unmentioned constants This avoids generating useless dead LLVM IR. --- compiler/rustc_monomorphize/src/collector.rs | 14 ++++++++------ .../partitioning/vtable-through-const.rs | 5 ----- 2 files changed, 8 insertions(+), 11 deletions(-) diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs index 173030e0326e8..3eb9a9dc85037 100644 --- a/compiler/rustc_monomorphize/src/collector.rs +++ b/compiler/rustc_monomorphize/src/collector.rs @@ -1481,12 +1481,14 @@ impl<'v> RootCollector<'_, 'v> { // Const items only generate mono items if they are actually used somewhere. // Just declaring them is insufficient. - // But even just declaring them must collect the items they refer to - // unless their generics require monomorphization. - if !self.tcx.generics_of(id.owner_id).own_requires_monomorphization() - && let Ok(val) = self.tcx.const_eval_poly(id.owner_id.to_def_id()) - { - collect_const_value(self.tcx, val, self.output); + // If we're collecting items eagerly, then recurse into all constants. + // Otherwise the value is only collected when explicitly mentioned in other items. + if self.strategy == MonoItemCollectionStrategy::Eager { + if !self.tcx.generics_of(id.owner_id).own_requires_monomorphization() + && let Ok(val) = self.tcx.const_eval_poly(id.owner_id.to_def_id()) + { + collect_const_value(self.tcx, val, self.output); + } } } DefKind::Impl { .. } => { diff --git a/tests/codegen-units/partitioning/vtable-through-const.rs b/tests/codegen-units/partitioning/vtable-through-const.rs index aad9ccb634b02..7a070728843c7 100644 --- a/tests/codegen-units/partitioning/vtable-through-const.rs +++ b/tests/codegen-units/partitioning/vtable-through-const.rs @@ -35,7 +35,6 @@ mod mod1 { } } - //~ MONO_ITEM fn mod1::id:: @@ vtable_through_const-mod1.volatile[Internal] fn id(x: T) -> T { x } @@ -50,8 +49,6 @@ mod mod1 { fn do_something_else(&self) {} } - //~ MONO_ITEM fn ::do_something @@ vtable_through_const-mod1.volatile[External] - //~ MONO_ITEM fn ::do_something_else @@ vtable_through_const-mod1.volatile[External] impl Trait2 for NeedsDrop {} pub trait Trait2Gen { @@ -93,8 +90,6 @@ pub fn main() { // Same as above //~ MONO_ITEM fn >::do_something @@ vtable_through_const-mod1.volatile[External] //~ MONO_ITEM fn >::do_something_else @@ vtable_through_const-mod1.volatile[External] - //~ MONO_ITEM fn >::do_something @@ vtable_through_const-mod1.volatile[External] - //~ MONO_ITEM fn >::do_something_else @@ vtable_through_const-mod1.volatile[External] mod1::TRAIT1_GEN_REF.do_something(0u8); //~ MONO_ITEM fn mod1::id:: @@ vtable_through_const-mod1.volatile[External]