Rollup merge of rust-lang#128932 - bvanjoi:issue-128813, r=petrochenkov

skip updating when external binding is existed

Fixes rust-lang#128813

For following code:

```rs
extern crate core;

fn f() {
    use ::core;
}

macro_rules! m {
    () => {
        extern crate std as core;
    };
}

m!();

fn main() {}
```

- In the first loop, we define `extern crate core` and `use ::core` will be referred to `core` (yes, it does not consider if there are some macros that are not expanded. Ideally, this should be delayed if there are some unexpanded macros in the root, but I didn't change it like that because it seems like a huge change).
- Then `m` is expanded, which makes `extern_prelude('core')` return `std` rather than `core`, causing the inconsistency.

r? `@petrochenkov`

Author: matthiaskrgr

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -896,7 +896,8 @@ 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()) {
+            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.is_import() {
                     self.r.dcx().emit_err(
                         errors::MacroExpandedExternCrateCannotShadowExternArguments {
@@ -913,14 +914,21 @@ 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.
-            entry.binding = Some(imported_binding);
             if orig_name.is_some() {
                 entry.introduced_by_item = true;
             }
+            // Binding from `extern crate` item in source code can replace
+            // a binding from `--extern` on command line here.
+            if !entry.is_import() {
+                entry.binding = Some(imported_binding)
+            } else if ident.name != kw::Underscore {
+                self.r.dcx().span_delayed_bug(
+                    item.span,
+                    format!("it had been define the external module '{ident}' multiple times"),
+                );
+            }
         }
         self.r.define(parent, ident, TypeNS, imported_binding);
     }

src/tools/tidy/src/issues.txt

@@ -1289,8 +1289,7 @@ ui/imports/auxiliary/issue-52891.rs
 ui/imports/auxiliary/issue-55811.rs
 ui/imports/auxiliary/issue-56125.rs
 ui/imports/auxiliary/issue-59764.rs
-ui/imports/auxiliary/issue-85992-extern-1.rs
-ui/imports/auxiliary/issue-85992-extern-2.rs
+ui/imports/auxiliary/issue-85992-extern.rs
 ui/imports/issue-109148.rs
 ui/imports/issue-109343.rs
 ui/imports/issue-113953.rs

tests/ui/imports/auxiliary/issue-85992-extern-2.rs renamed to tests/ui/imports/auxiliary/empty.rs

@@ -1,6 +1,6 @@
 #[macro_export]
 macro_rules! m {
    () => {
-        use issue_85992_extern_2::Outcome;
+        use empty::Outcome;
    }
 }

tests/ui/imports/auxiliary/issue-85992-extern-1.rs renamed to tests/ui/imports/auxiliary/issue-85992-extern.rs

@@ -1,11 +1,11 @@
 //@ edition: 2021
-//@ compile-flags: --extern issue_85992_extern_1 --extern issue_85992_extern_2
-//@ aux-build: issue-85992-extern-1.rs
-//@ aux-build: issue-85992-extern-2.rs
+//@ compile-flags: --extern issue_85992_extern --extern empty
+//@ aux-build: issue-85992-extern.rs
+//@ aux-build: empty.rs
 
-issue_85992_extern_1::m!();
+issue_85992_extern::m!();
 
-use crate::issue_85992_extern_2;
-//~^ ERROR unresolved import `crate::issue_85992_extern_2`
+use crate::empty;
+//~^ ERROR unresolved import `crate::empty`
 
 fn main() {}

tests/ui/imports/issue-85992.rs

@@ -1,8 +1,8 @@
-error[E0432]: unresolved import `crate::issue_85992_extern_2`
+error[E0432]: unresolved import `crate::empty`
   --> $DIR/issue-85992.rs:8:5
    |
-LL | use crate::issue_85992_extern_2;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `issue_85992_extern_2` in the root
+LL | use crate::empty;
+   |     ^^^^^^^^^^^^ no `empty` in the root
 
 error: aborting due to 1 previous error
 

tests/ui/imports/issue-85992.stderr

@@ -0,0 +1,18 @@
+//@ edition: 2021
+
+// issue#128813
+
+extern crate core;
+
+macro_rules! m {
+    () => {
+        extern crate std as core;
+        //~^ ERROR: the name `core` is defined multiple times
+    };
+}
+
+m!();
+
+fn main() {
+    use ::core;
+}

tests/ui/imports/multiple-extern-by-macro-for-buitlin.rs

@@ -0,0 +1,22 @@
+error[E0259]: the name `core` is defined multiple times
+  --> $DIR/multiple-extern-by-macro-for-buitlin.rs:9:9
+   |
+LL | extern crate core;
+   | ------------------ previous import of the extern crate `core` here
+...
+LL |         extern crate std as core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ `core` reimported here
+...
+LL | m!();
+   | ---- in this macro invocation
+   |
+   = note: `core` must be defined only once in the type namespace of this module
+   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: you can use `as` to change the binding name of the import
+   |
+LL |         extern crate std as other_core;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0259`.

tests/ui/imports/multiple-extern-by-macro-for-buitlin.stderr

@@ -0,0 +1,19 @@
+//@ edition: 2021
+//@ aux-build: empty.rs
+
+// issue#128813
+
+extern crate empty;
+
+macro_rules! m {
+    () => {
+        extern crate std as empty;
+        //~^ ERROR: the name `empty` is defined multiple times
+    };
+}
+
+m!();
+
+fn main() {
+    use ::empty;
+}

tests/ui/imports/multiple-extern-by-macro-for-custom.rs

@@ -0,0 +1,22 @@
+error[E0259]: the name `empty` is defined multiple times
+  --> $DIR/multiple-extern-by-macro-for-custom.rs:10:9
+   |
+LL | extern crate empty;
+   | ------------------- previous import of the extern crate `empty` here
+...
+LL |         extern crate std as empty;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ `empty` reimported here
+...
+LL | m!();
+   | ---- in this macro invocation
+   |
+   = note: `empty` must be defined only once in the type namespace of this module
+   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: you can use `as` to change the binding name of the import
+   |
+LL |         extern crate std as other_empty;
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0259`.