Auto merge of #95604 - nbdd0121:used2, r=petrochenkov

Generate synthetic object file to ensure all exported and used symbols participate in the linking

Fix #50007 and #47384

This is the synthetic object file approach that I described in https://github.com/rust-lang/rust/pull/95363#issuecomment-1079932354, allowing all exported and used symbols to be linked while still allowing them to be GCed.

Related #93791, #95363

r? `@petrochenkov`
cc `@carbotaniuman`

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -12,6 +12,7 @@ use std::{env, mem, str};
 
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
 use rustc_middle::middle::dependency_format::Linkage;
+use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo, SymbolExportKind};
 use rustc_middle::ty::TyCtxt;
 use rustc_serialize::{json, Encoder};
 use rustc_session::config::{self, CrateType, DebugInfo, LinkerPluginLto, Lto, OptLevel, Strip};
@@ -1518,22 +1519,13 @@ impl<'a> L4Bender<'a> {
     }
 }
 
-pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<String> {
-    if let Some(ref exports) = tcx.sess.target.override_export_symbols {
-        return exports.iter().map(ToString::to_string).collect();
-    }
-
-    let mut symbols = Vec::new();
-
-    let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
-    for &(symbol, level) in tcx.exported_symbols(LOCAL_CRATE).iter() {
-        if level.is_below_threshold(export_threshold) {
-            symbols.push(symbol_export::symbol_name_for_instance_in_crate(
-                tcx,
-                symbol,
-                LOCAL_CRATE,
-            ));
-        }
+fn for_each_exported_symbols_include_dep<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    crate_type: CrateType,
+    mut callback: impl FnMut(ExportedSymbol<'tcx>, SymbolExportInfo, CrateNum),
+) {
+    for &(symbol, info) in tcx.exported_symbols(LOCAL_CRATE).iter() {
+        callback(symbol, info, LOCAL_CRATE);
     }
 
     let formats = tcx.dependency_formats(());
@@ -1543,16 +1535,52 @@ pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<St
         let cnum = CrateNum::new(index + 1);
         // For each dependency that we are linking to statically ...
         if *dep_format == Linkage::Static {
-            // ... we add its symbol list to our export list.
-            for &(symbol, level) in tcx.exported_symbols(cnum).iter() {
-                if !level.is_below_threshold(export_threshold) {
-                    continue;
-                }
-
-                symbols.push(symbol_export::symbol_name_for_instance_in_crate(tcx, symbol, cnum));
+            for &(symbol, info) in tcx.exported_symbols(cnum).iter() {
+                callback(symbol, info, cnum);
             }
         }
     }
+}
+
+pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<String> {
+    if let Some(ref exports) = tcx.sess.target.override_export_symbols {
+        return exports.iter().map(ToString::to_string).collect();
+    }
+
+    let mut symbols = Vec::new();
+
+    let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
+    for_each_exported_symbols_include_dep(tcx, crate_type, |symbol, info, cnum| {
+        if info.level.is_below_threshold(export_threshold) {
+            symbols.push(symbol_export::symbol_name_for_instance_in_crate(tcx, symbol, cnum));
+        }
+    });
+
+    symbols
+}
+
+pub(crate) fn linked_symbols(
+    tcx: TyCtxt<'_>,
+    crate_type: CrateType,
+) -> Vec<(String, SymbolExportKind)> {
+    match crate_type {
+        CrateType::Executable | CrateType::Cdylib => (),
+        CrateType::Staticlib | CrateType::ProcMacro | CrateType::Rlib | CrateType::Dylib => {
+            return Vec::new();
+        }
+    }
+
+    let mut symbols = Vec::new();
+
+    let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
+    for_each_exported_symbols_include_dep(tcx, crate_type, |symbol, info, cnum| {
+        if info.level.is_below_threshold(export_threshold) || info.used {
+            symbols.push((
+                symbol_export::symbol_name_for_instance_in_crate(tcx, symbol, cnum),
+                info.kind,
+            ));
+        }
+    });
 
     symbols
 }