Rollup merge of rust-lang#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 rust-lang#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
+