Fix requires_lto targets needing lto set in cargo

Targets that set `requires_lto = true` were not actually using lto when
compiling with cargo by default. They needed an extra `lto = true` in
`Cargo.toml` to work.

Fix this by letting lto take precedence over the `embed_bitcode` flag
when lto is required by a target.

If both these flags would be supplied by the user, an error is
generated. However, this did not happen when lto was requested by the
target instead of the user.

Author: Flakebi

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -410,7 +410,7 @@ struct CompiledModules {
 
 fn need_bitcode_in_object(tcx: TyCtxt<'_>) -> bool {
     let sess = tcx.sess;
-    sess.opts.cg.embed_bitcode
+    (sess.lto() != config::Lto::No || sess.opts.cg.embed_bitcode)
         && tcx.crate_types().contains(&CrateType::Rlib)
         && sess.opts.output_types.contains_key(&OutputType::Exe)
 }

src/doc/rustc/src/platform-support/amdgcn-amd-amdhsa.md

@@ -56,11 +56,6 @@ Build the library as `cdylib`:
 # Cargo.toml
 [lib]
 crate-type = ["cdylib"]
-
-[profile.dev]
-lto = true # LTO must be explicitly enabled for now
-[profile.release]
-lto = true
 ```
 
 The target-cpu must be from the list [supported by LLVM] (or printed with `rustc --target amdgcn-amd-amdhsa --print target-cpus`).

tests/run-make-cargo/amdgpu-lto/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "amdgpu_lto"
+version = "0.1.0"
+edition = "2024"
+
+[lib]
+path = "lib.rs"
+crate-type = ["cdylib"]

tests/run-make-cargo/amdgpu-lto/lib.rs

@@ -0,0 +1,9 @@
+#![no_std]
+
+#[panic_handler]
+fn panic_handler(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+#[unsafe(no_mangle)]
+fn foo() {}

tests/run-make-cargo/amdgpu-lto/rmake.rs

@@ -0,0 +1,27 @@
+// Check that compiling for the amdgpu target which has requires_lto=true works with a default
+// cargo configuration.
+
+//@ needs-llvm-components: amdgpu
+
+#![deny(warnings)]
+
+use run_make_support::{cargo, path};
+
+fn main() {
+    let target_dir = path("target");
+
+    cargo()
+        .args(&[
+            "build",
+            "--release",
+            "--lib",
+            "--manifest-path",
+            "Cargo.toml",
+            "-Zbuild-std=core",
+            "--target",
+            "amdgcn-amd-amdhsa",
+        ])
+        .env("RUSTFLAGS", "-Ctarget-cpu=gfx900")
+        .env("CARGO_TARGET_DIR", &target_dir)
+        .run();
+}