make extern crate x as y under macro as an error when y is module

Author: bvanjoi

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -895,8 +895,12 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
         self.r.potentially_unused_imports.push(import);
         let imported_binding = self.r.import(binding, import);
         if parent == self.r.graph_root {
-            if let Some(entry) = self.r.extern_prelude.get(&ident.normalize_to_macros_2_0()) {
-                if expansion != LocalExpnId::ROOT && orig_name.is_some() && !entry.is_import() {
+            let ident = ident.normalize_to_macros_2_0();
+            if let Some(entry) = self.r.extern_prelude.get(&ident) {
+                if expansion != LocalExpnId::ROOT
+                    && orig_name.is_some()
+                    && entry.binding.map_or(true, |binding| binding.module().is_some())
+                {
                     self.r.dcx().emit_err(
                         errors::MacroExpandedExternCrateCannotShadowExternArguments {
                             span: item.span,
@@ -912,7 +916,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
             let entry = self
                 .r
                 .extern_prelude
-                .entry(ident.normalize_to_macros_2_0())
+                .entry(ident)
                 .or_insert(ExternPreludeEntry { binding: None, introduced_by_item: true });
             // Binding from `extern crate` item in source code can replace
             // a binding from `--extern` on command line here.

tests/ui/imports/multiple-extern-core-mod-by-macro.rs

@@ -0,0 +1,20 @@
+//@ edition: 2021
+
+// issue#128813
+
+extern crate core;
+
+fn f() {
+    use ::core;
+}
+
+macro_rules! m {
+    () => {
+        extern crate std as core;
+        //~^ ERROR: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+    };
+}
+
+m!();
+
+fn main() {}

tests/ui/imports/multiple-extern-core-mod-by-macro.stderr

@@ -0,0 +1,13 @@
+error: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+  --> $DIR/multiple-extern-core-mod-by-macro.rs:13:9
+   |
+LL |         extern crate std as core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | m!();
+   | ---- in this macro invocation
+   |
+   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+