shared-generics: Do not share instantiations that cannot be created outside of the current crate

In Zed shared-generics loading takes up a significant chunk of time in incremental build, as rustc deserializes rmeta of all dependencies of a crate.
I've recently realized that shared-generics includes all instantiations of some_generic_function in the following snippet:
```rs
pub fn some_generic_function(_: impl Fn()) {}

pub fn non_generic_function() {
	some_generic_function(|| {});
	some_generic_function(|| {});
	some_generic_function(|| {});
	some_generic_function(|| {});
	some_generic_function(|| {});
	some_generic_function(|| {});
	some_generic_function(|| {});
}
```
even though none of these instantiations can actually be created from outside of `non_generic_function`.

This PR makes shared-generics account for visibilities of generic arguments; an item is only considered for exporting if it is reachable from the outside or if all of it's arguments are visible outside of the local crate.

This PR reduces incremental build time for Zed (touch edito.rs scenario) from 12.4s to 10.4s.

Author: osiewicz

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -320,6 +320,29 @@ fn exported_symbols_provider_local(
 
         let cgus = tcx.collect_and_partition_mono_items(()).codegen_units;
 
+        let visibilities = tcx.effective_visibilities(());
+        let is_local_to_current_crate = |ty: Ty<'_>| {
+            let no_refs = ty.peel_refs();
+            let root_def_id = match no_refs.kind() {
+                rustc_type_ir::Adt(adt_def, _) => adt_def.did(),
+                rustc_type_ir::Closure(closure, _) => *closure,
+                rustc_type_ir::FnDef(def_id, _) => *def_id,
+                rustc_type_ir::Coroutine(def_id, _) => *def_id,
+                rustc_type_ir::CoroutineClosure(def_id, _) => *def_id,
+                rustc_type_ir::CoroutineWitness(def_id, _) => *def_id,
+                rustc_type_ir::Foreign(def_id) => *def_id,
+                _ => {
+                    return false;
+                }
+            };
+            let Some(root_def_id) = root_def_id.as_local() else {
+                return false;
+            };
+
+            let is_local = visibilities.public_at_level(root_def_id).is_none();
+
+            is_local
+        };
         // The symbols created in this loop are sorted below it
         #[allow(rustc::potential_query_instability)]
         for (mono_item, data) in cgus.iter().flat_map(|cgu| cgu.items().iter()) {
@@ -348,7 +371,21 @@ fn exported_symbols_provider_local(
 
             match *mono_item {
                 MonoItem::Fn(Instance { def: InstanceKind::Item(def), args }) => {
-                    if args.non_erasable_generics().next().is_some() {
+                    let mut types = args.types();
+                    let has_generics = args.non_erasable_generics().next().is_some();
+                    let should_export = has_generics
+                        && (def.as_local().is_some_and(|local_did| {
+                            visibilities.public_at_level(local_did).is_some()
+                        }) || types.all(|arg| {
+                            arg.walk().all(|ty| {
+                                let Some(ty) = ty.as_type() else {
+                                    return true;
+                                };
+                                !is_local_to_current_crate(ty)
+                            })
+                        }));
+
+                    if should_export {
                         let symbol = ExportedSymbol::Generic(def, args);
                         symbols.push((
                             symbol,
@@ -361,16 +398,41 @@ fn exported_symbols_provider_local(
                     }
                 }
                 MonoItem::Fn(Instance { def: InstanceKind::DropGlue(_, Some(ty)), args }) => {
-                    // A little sanity-check
-                    assert_eq!(args.non_erasable_generics().next(), Some(GenericArgKind::Type(ty)));
-                    symbols.push((
-                        ExportedSymbol::DropGlue(ty),
-                        SymbolExportInfo {
-                            level: SymbolExportLevel::Rust,
-                            kind: SymbolExportKind::Text,
-                            used: false,
-                        },
-                    ));
+                    let Some(GenericArgKind::Type(typ)) = args.non_erasable_generics().next()
+                    else {
+                        bug!("Expected a type argument for drop glue");
+                    };
+                    let should_export = {
+                        let root_identifier = match typ.kind() {
+                            rustc_type_ir::Adt(def, args) => Some((def.did(), args)),
+                            rustc_type_ir::Closure(id, args) => Some((*id, args)),
+                            _ => None,
+                        };
+                        if let Some((did, args)) = root_identifier {
+                            did.as_local().is_some_and(|local_did| {
+                                visibilities.public_at_level(local_did).is_some()
+                            }) || args.types().all(|arg| {
+                                arg.walk().all(|ty| {
+                                    let Some(ty) = ty.as_type() else {
+                                        return true;
+                                    };
+                                    !is_local_to_current_crate(ty)
+                                })
+                            })
+                        } else {
+                            true
+                        }
+                    };
+                    if should_export {
+                        symbols.push((
+                            ExportedSymbol::DropGlue(ty),
+                            SymbolExportInfo {
+                                level: SymbolExportLevel::Rust,
+                                kind: SymbolExportKind::Text,
+                                used: false,
+                            },
+                        ));
+                    }
                 }
                 MonoItem::Fn(Instance {
                     def: InstanceKind::AsyncDropGlueCtorShim(_, Some(ty)),