Use Option<Symbol> in ModuleKind::Def.

This way, `None` represents "crate root without a name" instead of
`kw::Empty`. This changes makes it impossible to forget to handle the
exceptional case.

Author: nnethercote

Diff for: compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -131,7 +131,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 let expn_id = self.cstore().expn_that_defined_untracked(def_id, self.tcx.sess);
                 return Some(self.new_module(
                     parent,
-                    ModuleKind::Def(def_kind, def_id, self.tcx.item_name(def_id)),
+                    ModuleKind::Def(def_kind, def_id, Some(self.tcx.item_name(def_id))),
                     expn_id,
                     self.def_span(def_id),
                     // FIXME: Account for `#[no_implicit_prelude]` attributes.
@@ -594,7 +594,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                         // HACK(eddyb) unclear how good this is, but keeping `$crate`
                         // in `source` breaks `tests/ui/imports/import-crate-var.rs`,
                         // while the current crate doesn't have a valid `crate_name`.
-                        if crate_name != kw::Empty {
+                        if let Some(crate_name) = crate_name {
                             // `crate_name` should not be interpreted as relative.
                             module_path.push(Segment::from_ident_and_id(
                                 Ident { name: kw::PathRoot, span: source.ident.span },
@@ -603,7 +603,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                             source.ident.name = crate_name;
                         }
                         if rename.is_none() {
-                            ident.name = crate_name;
+                            ident.name = sym::dummy;
                         }
 
                         self.r.dcx().emit_err(errors::CrateImported { span: item.span });
@@ -775,7 +775,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             ItemKind::Mod(.., ref mod_kind) => {
                 let module = self.r.new_module(
                     Some(parent),
-                    ModuleKind::Def(def_kind, def_id, ident.name),
+                    ModuleKind::Def(def_kind, def_id, Some(ident.name)),
                     expansion.to_expn_id(),
                     item.span,
                     parent.no_implicit_prelude
@@ -811,7 +811,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             ItemKind::Enum(_, _) | ItemKind::Trait(..) => {
                 let module = self.r.new_module(
                     Some(parent),
-                    ModuleKind::Def(def_kind, def_id, ident.name),
+                    ModuleKind::Def(def_kind, def_id, Some(ident.name)),
                     expansion.to_expn_id(),
                     item.span,
                     parent.no_implicit_prelude,

Diff for: compiler/rustc_resolve/src/diagnostics.rs

@@ -2439,7 +2439,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         let Res::Def(DefKind::Macro(MacroKind::Bang), _) = binding.res() else {
             return None;
         };
-        let module_name = crate_module.kind.name().unwrap();
+        let module_name = crate_module.kind.name().unwrap_or(kw::Empty);
         let import_snippet = match import.kind {
             ImportKind::Single { source, target, .. } if source != target => {
                 format!("{source} as {target}")

Diff for: compiler/rustc_resolve/src/lib.rs

@@ -503,18 +503,18 @@ enum ModuleKind {
     ///
     /// * A normal module – either `mod from_file;` or `mod from_block { }` –
     ///   or the crate root (which is conceptually a top-level module).
-    ///   Note that the crate root's [name][Self::name] will be [`kw::Empty`].
+    ///   The crate root will have `None` for the symbol.
     /// * A trait or an enum (it implicitly contains associated types, methods and variant
     ///   constructors).
-    Def(DefKind, DefId, Symbol),
+    Def(DefKind, DefId, Option<Symbol>),
 }
 
 impl ModuleKind {
     /// Get name of the module.
     fn name(&self) -> Option<Symbol> {
-        match self {
+        match *self {
             ModuleKind::Block => None,
-            ModuleKind::Def(.., name) => Some(*name),
+            ModuleKind::Def(.., name) => name,
         }
     }
 }
@@ -1402,7 +1402,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         let mut module_self_bindings = FxHashMap::default();
         let graph_root = arenas.new_module(
             None,
-            ModuleKind::Def(DefKind::Mod, root_def_id, kw::Empty),
+            ModuleKind::Def(DefKind::Mod, root_def_id, None),
             ExpnId::root(),
             crate_span,
             attr::contains_name(attrs, sym::no_implicit_prelude),
@@ -1411,7 +1411,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         );
         let empty_module = arenas.new_module(
             None,
-            ModuleKind::Def(DefKind::Mod, root_def_id, kw::Empty),
+            ModuleKind::Def(DefKind::Mod, root_def_id, None),
             ExpnId::root(),
             DUMMY_SP,
             true,
@@ -2286,7 +2286,8 @@ fn module_to_string(mut module: Module<'_>) -> Option<String> {
     loop {
         if let ModuleKind::Def(.., name) = module.kind {
             if let Some(parent) = module.parent {
-                names.push(name);
+                // `unwrap` is safe: the presence of a parent means it's not the crate root.
+                names.push(name.unwrap());
                 module = parent
             } else {
                 break;

Diff for: compiler/rustc_resolve/src/macros.rs

@@ -168,7 +168,7 @@ impl<'ra, 'tcx> ResolverExpand for Resolver<'ra, 'tcx> {
         hygiene::update_dollar_crate_names(|ctxt| {
             let ident = Ident::new(kw::DollarCrate, DUMMY_SP.with_ctxt(ctxt));
             match self.resolve_crate_root(ident).kind {
-                ModuleKind::Def(.., name) if name != kw::Empty => name,
+                ModuleKind::Def(.., name) if let Some(name) = name => name,
                 _ => kw::Crate,
             }
         });
@@ -1068,11 +1068,12 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             );
             if fallback_binding.ok().and_then(|b| b.res().opt_def_id()) != Some(def_id) {
                 let location = match parent_scope.module.kind {
-                    ModuleKind::Def(_, _, name) if name == kw::Empty => {
-                        "the crate root".to_string()
-                    }
                     ModuleKind::Def(kind, def_id, name) => {
-                        format!("{} `{name}`", kind.descr(def_id))
+                        if let Some(name) = name {
+                            format!("{} `{name}`", kind.descr(def_id))
+                        } else {
+                            "the crate root".to_string()
+                        }
                     }
                     ModuleKind::Block => "this scope".to_string(),
                 };