extend doc comment for reachability set computation

compiler/rustc_passes/src/reachable.rs

@@ -1,9 +1,15 @@
-// Finds items that are externally reachable, to determine which items
-// need to have their metadata (and possibly their AST) serialized.
-// All items that can be referred to through an exported name are
-// reachable, and when a reachable thing is inline or generic, it
-// makes all other generics or inline functions that it references
-// reachable as well.
+//! Finds local items that are externally reachable, to determine which items
+//! need to have their metadata (and possibly their AST) serialized.
+//!
+//! This set is *not* transitively closed, i.e., in general the set only contains definitions that
+//! can be reached *directly* via an exported name, not private functions that can only be reached
+//! transitively.
+//!
+//! However, there's a catch: if an item is generic or cross-crate inlinable, then it will have its
+//! code generated by some downstream crate. Now if that item calls private monomorphic
+//! non-cross-crate-inlinable items, then those can be reached by the code generated by the
+//! downstream create! Therefore, when a reachable thing is cross-crate inlinable or generic, it
+//! makes all other functions that it references reachable as well.
 
 use hir::def_id::LocalDefIdSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -394,6 +400,7 @@ fn has_custom_linkage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
         || codegen_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
 }
 
+/// See module-level doc comment above.
 fn reachable_set(tcx: TyCtxt<'_>, (): ()) -> LocalDefIdSet {
     let effective_visibilities = &tcx.effective_visibilities(());