Auto merge of rust-lang#114682 - bvanjoi:fix-36837, r=<try>

resolve: re-export ambiguity as warning

Fixes rust-lang#36837

Expose these ambiguous bindings as warnings instead of simply concealing them. This change introduces potential breaking alterations. For instance, code that previously compiled successfully may now fail to compile as expected:

```rs
// lib.rs
mod a {
  pub type C = i8;
}

mod b {
  pub type C = i16;
}

pub use a::*;
pub use b::*;

// main.rs
extern crate lib;

mod a {
    pub type C = i32;
}

use lib::*;
use a::*;

fn main() {
    let _: C = 1; // it will throw an ambiguity error but previous it will not.
}
```
r? `@petrochenkov`

compiler/rustc_errors/src/lib.rs

@@ -1841,6 +1841,9 @@ pub fn report_ambiguity_error<'a, G: EmissionGuarantee>(
     for help_msg in ambiguity.b1_help_msgs {
         db.help(help_msg);
     }
+    if ambiguity.extern_crate {
+        return;
+    }
     db.span_note(ambiguity.b2_span, ambiguity.b2_note_msg);
     for help_msg in ambiguity.b2_help_msgs {
         db.help(help_msg);

compiler/rustc_lint_defs/src/lib.rs

@@ -543,6 +543,8 @@ impl<HCX> ToStableHashKey<HCX> for LintId {
 
 #[derive(Debug)]
 pub struct AmbiguityErrorDiag {
+    /// Does this ambiguity binding come from a different crate?
+    pub extern_crate: bool,
     pub msg: String,
     pub span: Span,
     pub label_span: Span,

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1257,6 +1257,21 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         })
     }
 
+    fn get_ambiguity_module_children(
+        self,
+        id: DefIndex,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = AmbiguityModChild> + 'a {
+        iter::from_coroutine(move || {
+            let children = self.root.tables.ambiguity_module_children.get(self, id);
+            if !children.is_default() {
+                for child in children.decode((self, sess)) {
+                    yield child;
+                }
+            }
+        })
+    }
+
     fn is_ctfe_mir_available(self, id: DefIndex) -> bool {
         self.root.tables.mir_for_ctfe.get(self, id).is_some()
     }

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -11,7 +11,7 @@ use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_middle::arena::ArenaAllocatable;
-use rustc_middle::metadata::ModChild;
+use rustc_middle::metadata::{AmbiguityModChild, ModChild};
 use rustc_middle::middle::exported_symbols::ExportedSymbol;
 use rustc_middle::middle::stability::DeprecationEntry;
 use rustc_middle::query::ExternProviders;
@@ -581,6 +581,16 @@ impl CStore {
     ) -> Span {
         self.get_crate_data(cnum).get_proc_macro_quoted_span(id, sess)
     }
+
+    pub fn ambiguity_module_children_untracked(
+        &self,
+        def_id: DefId,
+        sess: &Session,
+    ) -> Vec<AmbiguityModChild> {
+        self.get_crate_data(def_id.krate)
+            .get_ambiguity_module_children(def_id.index, sess)
+            .collect()
+    }
 }
 
 impl CrateStore for CStore {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1547,6 +1547,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
             record_defaulted_array!(self.tables.module_children_reexports[def_id] <-
                 module_children.iter().filter(|child| !child.reexport_chain.is_empty()));
+
+            record_defaulted_array!(self.tables.ambiguity_module_children[def_id] <- tcx.ambiguity_module_children_local(local_def_id));
         }
     }
 

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -17,7 +17,7 @@ use rustc_hir::definitions::DefKey;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
-use rustc_middle::metadata::ModChild;
+use rustc_middle::metadata::{AmbiguityModChild, ModChild};
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
 use rustc_middle::middle::resolve_bound_vars::ObjectLifetimeDefault;
@@ -398,6 +398,7 @@ define_tables! {
     // That's why the encoded list needs to contain `ModChild` structures describing all the names
     // individually instead of `DefId`s.
     module_children_reexports: Table<DefIndex, LazyArray<ModChild>>,
+    ambiguity_module_children: Table<DefIndex, LazyArray<AmbiguityModChild>>,
 
 - optional:
     attributes: Table<DefIndex, LazyArray<ast::Attribute>>,

compiler/rustc_middle/src/metadata.rs

@@ -44,3 +44,6 @@ pub struct ModChild {
     /// Empty if the module child is a proper item.
     pub reexport_chain: SmallVec<[Reexport; 2]>,
 }
+
+/// Same as `ModChild`, however, it includes ambiguity error.
+pub type AmbiguityModChild = ModChild;

compiler/rustc_middle/src/ty/context.rs

@@ -8,7 +8,7 @@ use crate::arena::Arena;
 use crate::dep_graph::{DepGraph, DepKindStruct};
 use crate::infer::canonical::{CanonicalParamEnvCache, CanonicalVarInfo, CanonicalVarInfos};
 use crate::lint::struct_lint_level;
-use crate::metadata::ModChild;
+use crate::metadata::{AmbiguityModChild, ModChild};
 use crate::middle::codegen_fn_attrs::CodegenFnAttrs;
 use crate::middle::resolve_bound_vars;
 use crate::middle::stability;
@@ -2256,6 +2256,10 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn module_children_local(self, def_id: LocalDefId) -> &'tcx [ModChild] {
         self.resolutions(()).module_children.get(&def_id).map_or(&[], |v| &v[..])
     }
+
+    pub fn ambiguity_module_children_local(self, def_id: LocalDefId) -> &'tcx [AmbiguityModChild] {
+        self.resolutions(()).ambiguity_module_children.get(&def_id).map_or(&[], |v| &v[..])
+    }
 }
 
 /// Parameter attributes that can only be determined by examining the body of a function instead

compiler/rustc_middle/src/ty/mod.rs

@@ -18,7 +18,7 @@ pub use self::BorrowKind::*;
 pub use self::IntVarValue::*;
 pub use self::Variance::*;
 use crate::error::{OpaqueHiddenTypeMismatch, TypeMismatchReason};
-use crate::metadata::ModChild;
+use crate::metadata::{AmbiguityModChild, ModChild};
 use crate::middle::privacy::EffectiveVisibilities;
 use crate::mir::{Body, CoroutineLayout};
 use crate::query::Providers;
@@ -159,6 +159,7 @@ pub struct ResolverGlobalCtxt {
     pub extern_crate_map: FxHashMap<LocalDefId, CrateNum>,
     pub maybe_unused_trait_imports: FxIndexSet<LocalDefId>,
     pub module_children: LocalDefIdMap<Vec<ModChild>>,
+    pub ambiguity_module_children: LocalDefIdMap<Vec<AmbiguityModChild>>,
     pub glob_map: FxHashMap<LocalDefId, FxHashSet<Symbol>>,
     pub main_def: Option<MainDefinition>,
     pub trait_impls: FxIndexMap<DefId, Vec<LocalDefId>>,