Rollup merge of #149860 - Delta17920:fix/149821-root-cause, r=petrochenkov

Fix: Prevent macro-expanded extern crates from shadowing extern arguments

prevents an ICE by fixing a logic bug in `build_reduced_graph.rs`.
the bug caused the compiler to correctly detect and report a shadowing error for a macro-expanded `extern crate` but then continue processing the invalid item, corrupting the resolver's internal state (`extern_prelude`) and leading to a crash in later resolution passes the fix adds an early return after the shadowing error is reported to ensure the invalid item is not added to the resolution graph.

Fixes #149821

Author: jhpratt

compiler/rustc_resolve/src/ident.rs

@@ -771,36 +771,39 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         } else {
             None
         };
-        // Skip ambiguity errors for extern flag bindings "overridden"
-        // by extern item bindings.
-        // FIXME: Remove with lang team approval.
-        let issue_145575_hack = Some(binding) == extern_prelude_flag_binding
-            && extern_prelude_item_binding.is_some()
-            && extern_prelude_item_binding != Some(innermost_binding);
-        if let Some(kind) = ambiguity_error_kind
-            && !issue_145575_hack
-        {
-            let misc = |f: Flags| {
-                if f.contains(Flags::MISC_SUGGEST_CRATE) {
-                    AmbiguityErrorMisc::SuggestCrate
-                } else if f.contains(Flags::MISC_SUGGEST_SELF) {
-                    AmbiguityErrorMisc::SuggestSelf
-                } else if f.contains(Flags::MISC_FROM_PRELUDE) {
-                    AmbiguityErrorMisc::FromPrelude
-                } else {
-                    AmbiguityErrorMisc::None
-                }
-            };
-            self.ambiguity_errors.push(AmbiguityError {
-                kind,
-                ident: orig_ident,
-                b1: innermost_binding,
-                b2: binding,
-                warning: false,
-                misc1: misc(innermost_flags),
-                misc2: misc(flags),
-            });
-            return true;
+        if let Some(kind) = ambiguity_error_kind {
+            // Skip ambiguity errors for extern flag bindings "overridden"
+            // by extern item bindings.
+            // FIXME: Remove with lang team approval.
+            let issue_145575_hack = Some(binding) == extern_prelude_flag_binding
+                && extern_prelude_item_binding.is_some()
+                && extern_prelude_item_binding != Some(innermost_binding);
+
+            if issue_145575_hack {
+                self.issue_145575_hack_applied = true;
+            } else {
+                let misc = |f: Flags| {
+                    if f.contains(Flags::MISC_SUGGEST_CRATE) {
+                        AmbiguityErrorMisc::SuggestCrate
+                    } else if f.contains(Flags::MISC_SUGGEST_SELF) {
+                        AmbiguityErrorMisc::SuggestSelf
+                    } else if f.contains(Flags::MISC_FROM_PRELUDE) {
+                        AmbiguityErrorMisc::FromPrelude
+                    } else {
+                        AmbiguityErrorMisc::None
+                    }
+                };
+                self.ambiguity_errors.push(AmbiguityError {
+                    kind,
+                    ident: orig_ident,
+                    b1: innermost_binding,
+                    b2: binding,
+                    warning: false,
+                    misc1: misc(innermost_flags),
+                    misc2: misc(flags),
+                });
+                return true;
+            }
         }
 
         false

compiler/rustc_resolve/src/imports.rs

@@ -1170,7 +1170,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             return;
                         }
                         if let Some(initial_res) = initial_res {
-                            if res != initial_res {
+                            if res != initial_res && !this.issue_145575_hack_applied {
                                 span_bug!(import.span, "inconsistent resolution for an import");
                             }
                         } else if this.privacy_errors.is_empty() {

compiler/rustc_resolve/src/lib.rs

@@ -1188,6 +1188,7 @@ pub struct Resolver<'ra, 'tcx> {
     privacy_errors: Vec<PrivacyError<'ra>> = Vec::new(),
     /// Ambiguity errors are delayed for deduplication.
     ambiguity_errors: Vec<AmbiguityError<'ra>> = Vec::new(),
+    issue_145575_hack_applied: bool = false,
     /// `use` injections are delayed for better placement and deduplication.
     use_injections: Vec<UseError<'tcx>> = Vec::new(),
     /// Crate-local macro expanded `macro_export` referred to by a module-relative path.

tests/ui/resolve/ice-inconsistent-resolution-149821.rs

@@ -0,0 +1,17 @@
+//@ edition: 2024
+
+mod m {
+    use crate::*;
+    use core;
+}
+
+macro_rules! define_other_core {
+    () => {
+        extern crate std as core;
+        //~^ ERROR macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+    };
+}
+
+define_other_core! {}
+
+fn main() {}

tests/ui/resolve/ice-inconsistent-resolution-149821.stderr

@@ -0,0 +1,13 @@
+error: macro-expanded `extern crate` items cannot shadow names passed with `--extern`
+  --> $DIR/ice-inconsistent-resolution-149821.rs:10:9
+   |
+LL |         extern crate std as core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | define_other_core! {}
+   | --------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+