Export __rdl_* symbols to the allocator shim when doing LTO

Author: bjorn3

compiler/rustc_codegen_ssa/src/back/lto.rs

@@ -1,6 +1,7 @@
 use std::ffi::CString;
 use std::sync::Arc;
 
+use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_data_structures::memmap::Mmap;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo, SymbolExportLevel};
@@ -95,6 +96,19 @@ pub(super) fn exported_symbols_for_lto(
             .filter_map(|&(s, info): &(ExportedSymbol<'_>, SymbolExportInfo)| {
                 if info.level.is_below_threshold(export_threshold) || info.used {
                     Some(symbol_name_for_instance_in_crate(tcx, s, cnum))
+                } else if export_threshold == SymbolExportLevel::C
+                    && info.rustc_std_internal_symbol
+                    && let Some(AllocatorKind::Default) = allocator_kind_for_codegen(tcx)
+                {
+                    // Export the __rdl_* exports for usage by the allocator shim when not using
+                    // #[global_allocator]. Most of the conditions above are only used to avoid
+                    // unnecessary expensive symbol_name_for_instance_in_crate calls.
+                    let sym = symbol_name_for_instance_in_crate(tcx, s, cnum);
+                    if sym.contains("__rdl_") || sym.contains("__rg_oom") {
+                        Some(sym)
+                    } else {
+                        None
+                    }
                 } else {
                     None
                 }