Allow to pass "compiler" arguments to doc subcommand

src/bootstrap/doc.rs

@@ -537,7 +537,7 @@ impl Step for Rustc {
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;
-        run.krate("rustc-main").default_condition(builder.config.docs)
+        run.krate("rustc-main").path("compiler").default_condition(builder.config.docs)
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -553,9 +553,24 @@ fn make_run(run: RunConfig<'_>) {
     fn run(self, builder: &Builder<'_>) {
         let stage = self.stage;
         let target = self.target;
+        let mut is_explicit_request = false;
         builder.info(&format!("Documenting stage{} compiler ({})", stage, target));
 
-        if !builder.config.compiler_docs {
+        let paths = builder
+            .paths
+            .iter()
+            .map(components_simplified)
+            .filter_map(|path| {
+                if path.get(0) == Some(&"compiler") {
+                    is_explicit_request = true;
+                    path.get(1).map(|p| p.to_owned())
+                } else {
+                    None
+                }
+            })
+            .collect::<Vec<_>>();
+
+        if !builder.config.compiler_docs && !is_explicit_request {
             builder.info("\tskipping - compiler/librustdoc docs disabled");
             return;
         }
@@ -603,15 +618,34 @@ fn run(self, builder: &Builder<'_>) {
         cargo.rustdocflag("--extern-html-root-url");
         cargo.rustdocflag("ena=https://docs.rs/ena/latest/");
 
-        // Find dependencies for top level crates.
         let mut compiler_crates = HashSet::new();
-        for root_crate in &["rustc_driver", "rustc_codegen_llvm", "rustc_codegen_ssa"] {
-            compiler_crates.extend(
-                builder
-                    .in_tree_crates(root_crate, Some(target))
-                    .into_iter()
-                    .map(|krate| krate.name),
-            );
+
+        if paths.is_empty() {
+            // Find dependencies for top level crates.
+            for root_crate in &["rustc_driver", "rustc_codegen_llvm", "rustc_codegen_ssa"] {
+                compiler_crates.extend(
+                    builder
+                        .in_tree_crates(root_crate, Some(target))
+                        .into_iter()
+                        .map(|krate| krate.name),
+                );
+            }
+        } else {
+            for root_crate in paths {
+                if !builder.src.join("compiler").join(&root_crate).exists() {
+                    builder.info(&format!(
+                        "\tskipping - compiler/{} (unknown compiler crate)",
+                        root_crate
+                    ));
+                } else {
+                    compiler_crates.extend(
+                        builder
+                            .in_tree_crates(root_crate, Some(target))
+                            .into_iter()
+                            .map(|krate| krate.name),
+                    );
+                }
+            }
         }
 
         for krate in &compiler_crates {