create -Z force-allocator-shim codegen option

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -299,13 +299,12 @@ fn link_rlib<'a, B: ArchiveBuilder<'a>>(
         }
     }
 
-    match flavor {
-        RlibFlavor::Normal => {}
-        RlibFlavor::StaticlibBase => {
-            let obj = codegen_results.allocator_module.as_ref().and_then(|m| m.object.as_ref());
-            if let Some(obj) = obj {
-                ab.add_file(obj);
-            }
+    if flavor == RlibFlavor::StaticlibBase
+        || sess.opts.debugging_opts.force_allocator_shim.unwrap_or(false)
+    {
+        let obj = codegen_results.allocator_module.as_ref().and_then(|m| m.object.as_ref());
+        if let Some(obj) = obj {
+            ab.add_file(obj);
         }
     }
 

compiler/rustc_interface/src/tests.rs

@@ -732,6 +732,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(drop_tracking, true);
     tracked!(dual_proc_macros, true);
     tracked!(fewer_names, Some(true));
+    tracked!(force_allocator_shim, Some(false));
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));
     tracked!(function_sections, Some(false));

compiler/rustc_metadata/src/creader.rs

@@ -809,7 +809,9 @@ impl<'a> CrateLoader<'a> {
         // if our compilation session actually needs an allocator based on what
         // we're emitting.
         let all_rlib = self.sess.crate_types().iter().all(|ct| matches!(*ct, CrateType::Rlib));
-        if all_rlib {
+        let force_allocator_shim =
+            self.sess.opts.debugging_opts.force_allocator_shim.unwrap_or(false);
+        if all_rlib && !force_allocator_shim {
             return;
         }
 

compiler/rustc_session/src/options.rs

@@ -1222,6 +1222,8 @@ options! {
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),
+    force_allocator_shim: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "force generation of an allocator shim"),
     force_unstable_if_unmarked: bool = (false, parse_bool, [TRACKED],
         "force all crates to be `rustc_private` unstable (default: no)"),
     fuel: Option<(String, u64)> = (None, parse_optimization_fuel, [TRACKED],

src/test/run-make/force-allocator-shim/Makefile

@@ -0,0 +1,12 @@
+-include ../../run-make-fulldeps/tools.mk
+
+# Tests that the `-Z force-allocator-shim` flag causes an allocator shim to be
+# generated when expected.
+
+all:
+	$(RUSTC) foo.rs --crate-type rlib -Z force-allocator-shim
+	nm $(TMPDIR)/libfoo.rlib | $(CGREP) "__rust_alloc" && \
+	nm $(TMPDIR)/libfoo.rlib | $(CGREP) "__rust_alloc_error_handler" && \
+	nm $(TMPDIR)/libfoo.rlib | $(CGREP) "__rust_alloc_zeroed" && \
+	nm $(TMPDIR)/libfoo.rlib | $(CGREP) "__rust_dealloc" && \
+	nm $(TMPDIR)/libfoo.rlib | $(CGREP) "__rust_realloc"

src/test/run-make/force-allocator-shim/foo.rs

@@ -0,0 +1 @@
+pub fn foo() {}