Add LTONotSupported

src/errors.rs

@@ -14,6 +14,10 @@ pub(crate) struct LinkageConstOrMutType {
     pub span: Span
 }
 
+#[derive(SessionDiagnostic)]
+#[diag(codegen_gcc::lto_not_supported)]
+pub(crate) struct LTONotSupported {}
+
 #[derive(SessionDiagnostic)]
 #[diag(codegen_gcc::unwinding_inline_asm)]
 pub(crate) struct UnwindingInlineAsm {

src/lib.rs

@@ -61,6 +61,7 @@
 use std::any::Any;
 use std::sync::{Arc, Mutex};
 
+use crate::errors::LTONotSupported;
 use gccjit::{Context, OptimizationLevel, CType};
 use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleCodegen};
@@ -99,7 +100,7 @@ pub struct GccCodegenBackend {
 impl CodegenBackend for GccCodegenBackend {
     fn init(&self, sess: &Session) {
         if sess.lto() != Lto::No {
-            sess.warn("LTO is not supported. You may get a linker error.");
+            sess.emit_warning(LTONotSupported {});
         }
 
         let temp_dir = TempDir::new().expect("cannot create temporary directory");