Auto merge of #84401 - crlf0710:impl_main_by_path, r=petrochenkov

Implement RFC 1260 with feature_name `imported_main`.

This is the second extraction part of #84062 plus additional adjustments.
This (mostly) implements RFC 1260.

However there's still one test case failure in the extern crate case. Maybe `LocalDefId` doesn't work here? I'm not sure.

cc https://github.com/rust-lang/rust/issues/28937
r? `@petrochenkov`

compiler/rustc_middle/src/mir/mono.rs

@@ -6,7 +6,7 @@ use rustc_data_structures::base_n;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
+use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc_hir::{HirId, ItemId};
 use rustc_session::config::OptLevel;
 use rustc_span::source_map::Span;
@@ -93,7 +93,7 @@ impl<'tcx> MonoItem<'tcx> {
                 // indicator, then we'll be creating a globally shared version.
                 if tcx.codegen_fn_attrs(instance.def_id()).contains_extern_indicator()
                     || !instance.def.generates_cgu_internal_copy(tcx)
-                    || Some(instance.def_id()) == entry_def_id.map(LocalDefId::to_def_id)
+                    || Some(instance.def_id()) == entry_def_id
                 {
                     return InstantiationMode::GloballyShared { may_conflict: false };
                 }

compiler/rustc_middle/src/query/mod.rs

@@ -1194,7 +1194,7 @@ rustc_queries! {
 
     /// Identifies the entry-point (e.g., the `main` function) for a given
     /// crate, returning `None` if there is no entry point (such as for library crates).
-    query entry_fn(_: CrateNum) -> Option<(LocalDefId, EntryFnType)> {
+    query entry_fn(_: CrateNum) -> Option<(DefId, EntryFnType)> {
         desc { "looking up the entry function of a crate" }
     }
     query plugin_registrar_fn(_: CrateNum) -> Option<DefId> {

compiler/rustc_middle/src/ty/context.rs

@@ -20,8 +20,8 @@ use crate::ty::TyKind::*;
 use crate::ty::{
     self, AdtDef, AdtKind, Binder, BindingMode, BoundVar, CanonicalPolyFnSig, Const, ConstVid,
     DefIdTree, ExistentialPredicate, FloatTy, FloatVar, FloatVid, GenericParamDefKind, InferConst,
-    InferTy, IntTy, IntVar, IntVid, List, ParamConst, ParamTy, PolyFnSig, Predicate,
-    PredicateInner, PredicateKind, ProjectionTy, Region, RegionKind, ReprOptions,
+    InferTy, IntTy, IntVar, IntVid, List, MainDefinition, ParamConst, ParamTy, PolyFnSig,
+    Predicate, PredicateInner, PredicateKind, ProjectionTy, Region, RegionKind, ReprOptions,
     TraitObjectVisitor, Ty, TyKind, TyS, TyVar, TyVid, TypeAndMut, UintTy, Visibility,
 };
 use rustc_ast as ast;
@@ -1025,6 +1025,8 @@ pub struct GlobalCtxt<'tcx> {
     layout_interner: ShardedHashMap<&'tcx Layout, ()>,
 
     output_filenames: Arc<OutputFilenames>,
+
+    pub main_def: Option<MainDefinition>,
 }
 
 impl<'tcx> TyCtxt<'tcx> {
@@ -1185,6 +1187,7 @@ impl<'tcx> TyCtxt<'tcx> {
             const_stability_interner: Default::default(),
             alloc_map: Lock::new(interpret::AllocMap::new()),
             output_filenames: Arc::new(output_filenames.clone()),
+            main_def: resolutions.main_def,
         }
     }
 

compiler/rustc_middle/src/ty/mod.rs

@@ -124,6 +124,20 @@ pub struct ResolverOutputs {
     /// Extern prelude entries. The value is `true` if the entry was introduced
     /// via `extern crate` item and not `--extern` option or compiler built-in.
     pub extern_prelude: FxHashMap<Symbol, bool>,
+    pub main_def: Option<MainDefinition>,
+}
+
+#[derive(Clone, Copy)]
+pub struct MainDefinition {
+    pub res: Res<ast::NodeId>,
+    pub is_import: bool,
+    pub span: Span,
+}
+
+impl MainDefinition {
+    pub fn opt_fn_def_id(self) -> Option<DefId> {
+        if let Res::Def(DefKind::Fn, def_id) = self.res { Some(def_id) } else { None }
+    }
 }
 
 /// The "header" of an impl is everything outside the body: a Self type, a trait