Rollup merge of #93969 - bjorn3:codegen_backend_dep_info, r=pnkfelix

Only add codegen backend to dep info if -Zbinary-dep-depinfo is used

I am currently migrating the cg_clif build system from using a binary linked to the codegen backend as rustc replacement to passing `-Zcodegen-backend` instead. Without this PR this would force cargo to rebuild the sysroot on any change to the codegen backend even if I explicitly specify that I want it to be preserved, which would make development of cg_clif a lot slower. If you still want to have changes to the codegen backend invalidate the cargo build cache you can explicitly specify `-Zbinary-dep-depinfo`.

cc ``@eddyb`` as the codegen backend was initially added to the depinfo for rust-gpu.

compiler/rustc_interface/src/passes.rs

@@ -629,11 +629,15 @@ fn write_out_deps(
         });
         files.extend(extra_tracked_files);
 
-        if let Some(ref backend) = sess.opts.debugging_opts.codegen_backend {
-            files.push(backend.to_string());
-        }
-
         if sess.binary_dep_depinfo() {
+            if let Some(ref backend) = sess.opts.debugging_opts.codegen_backend {
+                if backend.contains('.') {
+                    // If the backend name contain a `.`, it is the path to an external dynamic
+                    // library. If not, it is not a path.
+                    files.push(backend.to_string());
+                }
+            }
+
             boxed_resolver.borrow_mut().access(|resolver| {
                 for cnum in resolver.cstore().crates_untracked() {
                     let source = resolver.cstore().crate_source_untracked(cnum);

src/test/run-make-fulldeps/hotplug_codegen_backend/Makefile

@@ -2,10 +2,25 @@ include ../tools.mk
 
 # ignore-stage1
 
+# This test both exists as a check that -Zcodegen-backend is capable of loading external codegen
+# backends and that this external codegen backend is only included in the dep info if
+# -Zbinary-dep-depinfo is used.
+
 all:
 	/bin/echo || exit 0 # This test requires /bin/echo to exist
 	$(RUSTC) the_backend.rs --crate-name the_backend --crate-type dylib \
 		-o $(TMPDIR)/the_backend.dylib
+
+	$(RUSTC) some_crate.rs --crate-name some_crate --crate-type lib -o $(TMPDIR)/some_crate \
+		-Z codegen-backend=$(TMPDIR)/the_backend.dylib -Z unstable-options \
+		--emit link,dep-info
+	grep -x "This has been \"compiled\" successfully." $(TMPDIR)/libsome_crate.rlib
+	# don't declare a dependency on the codegen backend if -Zbinary-dep-depinfo isn't used.
+	grep -v "the_backend.dylib" $(TMPDIR)/some_crate.d
+
 	$(RUSTC) some_crate.rs --crate-name some_crate --crate-type lib -o $(TMPDIR)/some_crate \
-		-Z codegen-backend=$(TMPDIR)/the_backend.dylib -Z unstable-options
-	grep -x "This has been \"compiled\" successfully." $(TMPDIR)/some_crate
+		-Z codegen-backend=$(TMPDIR)/the_backend.dylib -Z unstable-options \
+		--emit link,dep-info -Zbinary-dep-depinfo
+	grep -x "This has been \"compiled\" successfully." $(TMPDIR)/libsome_crate.rlib
+	# but declare a dependency on the codegen backend if -Zbinary-dep-depinfo it used.
+	grep "the_backend.dylib" $(TMPDIR)/some_crate.d