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

Implement RFC 1260 with feature_name `imported_main`.

This is the second extraction part of rust-lang#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 rust-lang#28937
r? `@petrochenkov`

Author: bors

compiler/rustc_codegen_cranelift/src/driver/jit.rs

@@ -103,7 +103,7 @@ pub(super) fn run_jit(tcx: TyCtxt<'_>, backend_config: BackendConfig) -> ! {
     });
 
     let (main_def_id, entry_ty) = tcx.entry_fn(LOCAL_CRATE).unwrap();
-    let instance = Instance::mono(tcx, main_def_id.to_def_id()).polymorphize(tcx);
+    let instance = Instance::mono(tcx, main_def_id).polymorphize(tcx);
 
     match entry_ty {
         EntryFnType::Main => {

compiler/rustc_codegen_cranelift/src/main_shim.rs

@@ -13,7 +13,7 @@ pub(crate) fn maybe_create_entry_wrapper(
 ) {
     let (main_def_id, use_start_lang_item) = match tcx.entry_fn(LOCAL_CRATE) {
         Some((def_id, entry_ty)) => (
-            def_id.to_def_id(),
+            def_id,
             match entry_ty {
                 EntryFnType::Main => true,
                 EntryFnType::Start => false,

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -344,7 +344,7 @@ impl DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
             spflags |= DISPFlags::SPFlagOptimized;
         }
         if let Some((id, _)) = self.tcx.entry_fn(LOCAL_CRATE) {
-            if id.to_def_id() == def_id {
+            if id == def_id {
                 spflags |= DISPFlags::SPFlagMainSubprogram;
             }
         }

compiler/rustc_codegen_ssa/src/base.rs

@@ -15,7 +15,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::profiling::{get_resident_set_size, print_time_passes_entry};
 use rustc_data_structures::sync::{par_iter, ParallelIterator};
 use rustc_hir as hir;
-use rustc_hir::def_id::{LocalDefId, LOCAL_CRATE};
+use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::lang_items::LangItem;
 use rustc_index::vec::Idx;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
@@ -348,12 +348,29 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     cx: &'a Bx::CodegenCx,
 ) -> Option<Bx::Function> {
     let main_def_id = cx.tcx().entry_fn(LOCAL_CRATE).map(|(def_id, _)| def_id)?;
-    let instance = Instance::mono(cx.tcx(), main_def_id.to_def_id());
+    let main_is_local = main_def_id.is_local();
+    let instance = Instance::mono(cx.tcx(), main_def_id);
 
-    if !cx.codegen_unit().contains_item(&MonoItem::Fn(instance)) {
+    if main_is_local {
         // We want to create the wrapper in the same codegen unit as Rust's main
         // function.
-        return None;
+        if !cx.codegen_unit().contains_item(&MonoItem::Fn(instance)) {
+            return None;
+        }
+    } else {
+        // FIXME: Add support for non-local main fn codegen
+        let span = cx.tcx().main_def.unwrap().span;
+        let n = 28937;
+        cx.sess()
+            .struct_span_err(span, "entry symbol `main` from foreign crate is not yet supported.")
+            .note(&format!(
+                "see issue #{} <https://github.com/rust-lang/rust/issues/{}> \
+                 for more information",
+                n, n,
+            ))
+            .emit();
+        cx.sess().abort_if_errors();
+        bug!();
     }
 
     let main_llfn = cx.get_fn_addr(instance);
@@ -366,7 +383,7 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fn create_entry_fn<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         cx: &'a Bx::CodegenCx,
         rust_main: Bx::Value,
-        rust_main_def_id: LocalDefId,
+        rust_main_def_id: DefId,
         use_start_lang_item: bool,
     ) -> Bx::Function {
         // The entry function is either `int main(void)` or `int main(int argc, char **argv)`,

compiler/rustc_error_codes/src/error_codes/E0136.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 More than one `main` function was found.
 
 Erroneous code example:
 
-```compile_fail,E0136
+```compile_fail
 fn main() {
     // ...
 }

compiler/rustc_expand/src/expand.rs

@@ -20,7 +20,7 @@ use rustc_attr::{self as attr, is_builtin_attr};
 use rustc_data_structures::map_in_place::MapInPlace;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{Applicability, PResult};
+use rustc_errors::{Applicability, FatalError, PResult};
 use rustc_feature::Features;
 use rustc_parse::parser::{AttemptLocalParseRecovery, ForceCollect, Parser, RecoverComma};
 use rustc_parse::validate_attr;
@@ -414,6 +414,8 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                         kind.article(), kind.descr()
                     ),
                 );
+                // FIXME: this workaround issue #84569
+                FatalError.raise();
             }
         };
         self.cx.trace_macros_diag();

compiler/rustc_feature/src/active.rs

@@ -653,6 +653,9 @@ declare_features! (
     /// Allows unsizing coercions in `const fn`.
     (active, const_fn_unsize, "1.53.0", Some(64992), None),
 
+    /// Allows using imported `main` function
+    (active, imported_main, "1.53.0", Some(28937), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_interface/src/queries.rs

@@ -307,7 +307,7 @@ impl<'tcx> Queries<'tcx> {
             _ => return,
         };
 
-        let attrs = &*tcx.get_attrs(def_id.to_def_id());
+        let attrs = &*tcx.get_attrs(def_id);
         let attrs = attrs.iter().filter(|attr| tcx.sess.check_name(attr, sym::rustc_error));
         for attr in attrs {
             match attr.meta_item_list() {

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> {