Auto merge of #126111 - Zalathar:fulldeps-hotplug, r=jieyouxu

Port `tests/run-make-fulldeps/hotplug_codegen_backend` to ui-fulldeps

This is the last remaining run-make-fulldeps test, which means I actually had to leave behind a dummy README file to prevent compiletest from complaining about a missing directory.

(Removing the run-make-fulldeps suite entirely is non-trivial, so I intend to do so in a separate PR after this one.)

---

I wasn't sure about adding a new kind of aux build just for this one test, so I also tried to just port this test from Makefile to [rmake](https://github.com/rust-lang/rust/issues/121876) instead.

But I found that I couldn't get rmake to fully work for a run-make-fulldeps test, which convinced me that getting rid of run-make-fulldeps is worthwhile.

r? `@jieyouxu`

src/tools/compiletest/src/header.rs

@@ -107,6 +107,9 @@ pub struct TestProps {
     // Similar to `aux_builds`, but a list of NAME=somelib.rs of dependencies
     // to build and pass with the `--extern` flag.
     pub aux_crates: Vec<(String, String)>,
+    /// Similar to `aux_builds`, but also passes the resulting dylib path to
+    /// `-Zcodegen-backend`.
+    pub aux_codegen_backend: Option<String>,
     // Environment settings to use for compiling
     pub rustc_env: Vec<(String, String)>,
     // Environment variables to unset prior to compiling.
@@ -231,6 +234,7 @@ mod directives {
     pub const AUX_BIN: &'static str = "aux-bin";
     pub const AUX_BUILD: &'static str = "aux-build";
     pub const AUX_CRATE: &'static str = "aux-crate";
+    pub const AUX_CODEGEN_BACKEND: &'static str = "aux-codegen-backend";
     pub const EXEC_ENV: &'static str = "exec-env";
     pub const RUSTC_ENV: &'static str = "rustc-env";
     pub const UNSET_EXEC_ENV: &'static str = "unset-exec-env";
@@ -267,6 +271,7 @@ impl TestProps {
             aux_builds: vec![],
             aux_bins: vec![],
             aux_crates: vec![],
+            aux_codegen_backend: None,
             revisions: vec![],
             rustc_env: vec![
                 ("RUSTC_ICE".to_string(), "0".to_string()),
@@ -446,6 +451,9 @@ impl TestProps {
                         &mut self.aux_crates,
                         Config::parse_aux_crate,
                     );
+                    if let Some(r) = config.parse_name_value_directive(ln, AUX_CODEGEN_BACKEND) {
+                        self.aux_codegen_backend = Some(r.trim().to_owned());
+                    }
                     config.push_name_value_directive(
                         ln,
                         EXEC_ENV,
@@ -722,6 +730,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "assembly-output",
     "aux-bin",
     "aux-build",
+    "aux-codegen-backend",
     "aux-crate",
     "build-aux-docs",
     "build-fail",

src/tools/compiletest/src/runtest.rs

@@ -1833,6 +1833,16 @@ impl<'test> TestCx<'test> {
                 ));
             }
         }
+
+        // Build any `//@ aux-codegen-backend`, and pass the resulting library
+        // to `-Zcodegen-backend` when compiling the test file.
+        if let Some(aux_file) = &self.props.aux_codegen_backend {
+            let aux_type = self.build_auxiliary(of, aux_file, aux_dir, false);
+            if let Some(lib_name) = get_lib_name(aux_file.trim_end_matches(".rs"), aux_type) {
+                let lib_path = aux_dir.join(&lib_name);
+                rustc.arg(format!("-Zcodegen-backend={}", lib_path.display()));
+            }
+        }
     }
 
     fn compose_and_run_compiler(&self, mut rustc: Command, input: Option<String>) -> ProcRes {
@@ -2254,6 +2264,9 @@ impl<'test> TestCx<'test> {
         }
 
         match output_file {
+            // If the test's compile flags specify an output path with `-o`,
+            // avoid a compiler warning about `--out-dir` being ignored.
+            _ if self.props.compile_flags.iter().any(|flag| flag == "-o") => {}
             TargetLocation::ThisFile(path) => {
                 rustc.arg("-o").arg(path);
             }