Send -fno-lto when linker plugin LTO is not requested to avoid having GCC do LTO when using rustc_codegen_gcc

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -900,7 +900,14 @@ impl<'a> Linker for GccLinker<'a> {
     fn linker_plugin_lto(&mut self) {
         match self.sess.opts.cg.linker_plugin_lto {
             LinkerPluginLto::Disabled => {
-                // Nothing to do
+                // GCC, when used as a linker, will perform LTO even without -flto when it sees
+                // object files with the GCC IR (which can happen when using rustc_codegen_gcc).
+                // Since the std is compiled with -Cembed-bitcode=yes, any program using the std
+                // will have the linker do LTO.
+                // So, we disable this with -fno-lto when linker plugin LTO is not requested.
+                if self.is_cc() && self.codegen_backend == "gcc" {
+                    self.cc_arg("-fno-lto");
+                }
             }
             LinkerPluginLto::LinkerPluginAuto => {
                 self.push_linker_plugin_lto_args(None);

tests/run-make/fno-lto/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/fno-lto/rmake.rs

@@ -0,0 +1,17 @@
+//@ needs-target-std
+//@ ignore-msvc
+//@ ignore-wasm
+//@ needs-backends: gcc
+
+use run_make_support::rustc;
+
+fn main() {
+    rustc().input("main.rs").print("link-args").run_unchecked().assert_stdout_contains("-fno-lto");
+
+    rustc()
+        .input("main.rs")
+        .arg("-Clinker-plugin-lto")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stdout_not_contains("-fno-lto");
+}

tests/run-make/linker-warning/rmake.rs

@@ -1,4 +1,5 @@
 //@ ignore-cross-compile (need to run fake linker)
+//@ ignore-backends: gcc
 
 use run_make_support::{Rustc, diff, regex, rustc};