Stop collecting unmentioned constants

This avoids generating useless dead LLVM IR.

Author: Mark-Simulacrum

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 { .. } => {

tests/codegen-units/partitioning/vtable-through-const.rs

@@ -35,7 +35,6 @@ mod mod1 {
         }
     }
 
-    //~ MONO_ITEM fn mod1::id::<i64> @@ vtable_through_const-mod1.volatile[Internal]
     fn id<T>(x: T) -> T {
         x
     }
@@ -50,8 +49,6 @@ mod mod1 {
         fn do_something_else(&self) {}
     }
 
-    //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2>::do_something @@ vtable_through_const-mod1.volatile[External]
-    //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2>::do_something_else @@ vtable_through_const-mod1.volatile[External]
     impl Trait2 for NeedsDrop {}
 
     pub trait Trait2Gen<T> {
@@ -93,8 +90,6 @@ pub fn main() {
     // Same as above
     //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[External]
     //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[External]
-    //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[External]
-    //~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[External]
     mod1::TRAIT1_GEN_REF.do_something(0u8);
 
     //~ MONO_ITEM fn mod1::id::<char> @@ vtable_through_const-mod1.volatile[External]