Remove `Ident::empty`.

All uses have been removed. And it's nonsensical: an identifier by
definition has at least one char.

The commits adds an is-non-empty assertion to `Ident::new` to enforce
this, and converts some `Ident` constructions to use `Ident::new`.
Adding the assertion requires making `Ident::new` and
`Ident::with_dummy_span` non-const, which is no great loss.

The commit amends a couple of places that do path splitting to ensure no
empty identifiers are created.

2 files changed, 20 insertions, 9 deletions

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index cb328022c76..3460c53782f 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -549,7 +549,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                         source = module_path.pop().unwrap();
                         if rename.is_none() {
                             // Keep the span of `self`, but the name of `foo`
-                            ident = Ident { name: source.ident.name, span: self_span };
+                            ident = Ident::new(source.ident.name, self_span);
                         }
                     }
                 } else {
@@ -597,7 +597,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                         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 },
+                                Ident::new(kw::PathRoot, source.ident.span),
                                 self.r.next_node_id(),
                             ));
                             source.ident.name = crate_name;
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 7e516d82df1..4c47e9ed699 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -2157,13 +2157,24 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ns: Namespace,
         parent_scope: ParentScope<'ra>,
     ) -> Option<Res> {
-        let mut segments =
-            Vec::from_iter(path_str.split("::").map(Ident::from_str).map(Segment::from_ident));
-        if let Some(segment) = segments.first_mut() {
-            if segment.ident.name == kw::Empty {
-                segment.ident.name = kw::PathRoot;
-            }
-        }
+        let segments: Result<Vec<_>, ()> = path_str
+            .split("::")
+            .enumerate()
+            .map(|(i, s)| {
+                let sym = if s.is_empty() {
+                    if i == 0 {
+                        // For a path like `::a::b`, use `kw::PathRoot` as the leading segment.
+                        kw::PathRoot
+                    } else {
+                        return Err(()); // occurs in cases like `String::`
+                    }
+                } else {
+                    Symbol::intern(s)
+                };
+                Ok(Segment::from_ident(Ident::with_dummy_span(sym)))
+            })
+            .collect();
+        let Ok(segments) = segments else { return None };
 
         match self.maybe_resolve_path(&segments, Some(ns), &parent_scope, None) {
             PathResult::Module(ModuleOrUniformRoot::Module(module)) => Some(module.res().unwrap()),