Rollup merge of #101873 - WaffleLapkin:x-build-proc-macro-srv, r=jyn514

Allow building `rust-analyzer-proc-macro-srv` as a standalone tool

This PR allows building `rust-analyzer-proc-macro-srv` as a standalone tool via `x b proc-macro-srv-cli` (I thought that `x b rust-analyzer-proc-macro-srv` should work, but it doesn't for some reason...). Also this PR adds a copy of `rust-analyzer-proc-macro-srv` binary to `build/{triple}/{stage}/libexec/` when building `rust-analyzer-proc-macro-srv`, so that r-a can pick it up.

This is useful to make r-a (and I assume Intellij IDEA) to expand macros when using a custom, build from source toolchain.

r? ``@jyn514``
[_zulip thread_](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/How.20to.20fix.20.60UnsupportedABI.60.20for.20custom.20toolchains.3F/near/299040175)

src/bootstrap/tool.rs

@@ -746,14 +746,18 @@ impl Step for RustAnalyzerProcMacroSrv {
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;
-        run.path("src/tools/rust-analyzer").default_condition(
-            builder.config.extended
-                && builder
-                    .config
-                    .tools
-                    .as_ref()
-                    .map_or(true, |tools| tools.iter().any(|tool| tool == "rust-analyzer")),
-        )
+
+        // Allow building `rust-analyzer-proc-macro-srv` both as part of the `rust-analyzer` and as a stand-alone tool.
+        run.path("src/tools/rust-analyzer")
+            .path("src/tools/rust-analyzer/crates/proc-macro-srv-cli")
+            .default_condition(
+                builder.config.extended
+                    && builder.config.tools.as_ref().map_or(true, |tools| {
+                        tools.iter().any(|tool| {
+                            tool == "rust-analyzer" || tool == "rust-analyzer-proc-macro-srv"
+                        })
+                    }),
+            )
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -764,7 +768,7 @@ impl Step for RustAnalyzerProcMacroSrv {
     }
 
     fn run(self, builder: &Builder<'_>) -> Option<PathBuf> {
-        builder.ensure(ToolBuild {
+        let path = builder.ensure(ToolBuild {
             compiler: self.compiler,
             target: self.target,
             tool: "rust-analyzer-proc-macro-srv",
@@ -773,7 +777,15 @@ impl Step for RustAnalyzerProcMacroSrv {
             extra_features: vec!["proc-macro-srv/sysroot-abi".to_owned()],
             is_optional_tool: false,
             source_type: SourceType::InTree,
-        })
+        })?;
+
+        // Copy `rust-analyzer-proc-macro-srv` to `<sysroot>/libexec/`
+        // so that r-a can use it.
+        let libexec_path = builder.sysroot(self.compiler).join("libexec");
+        t!(fs::create_dir_all(&libexec_path));
+        builder.copy(&path, &libexec_path.join("rust-analyzer-proc-macro-srv"));
+
+        Some(path)
     }
 }