split steps for generating the standalone docs and the shared assets

Before this commit, the step to generate the standalone docs (which
included the index page and other HTML files at the root of the
documentation) was bundled with the code copying files needed by
multiple pieces of documentation. This means it wasn't possible to avoid
generating the standalone docs.

This commit splits the step into two, allowing the standalone docs
generation to be excluded while still building the rest of the docs.

src/bootstrap/doc.rs

@@ -227,7 +227,7 @@ impl Step for TheBook {
         }
 
         // build the version info page and CSS
-        builder.ensure(Standalone { compiler, target });
+        let shared_assets = builder.ensure(SharedAssets { target });
 
         // build the redirect pages
         builder.info(&format!("Documenting book redirect pages ({})", target));
@@ -236,7 +236,7 @@ impl Step for TheBook {
             let path = file.path();
             let path = path.to_str().unwrap();
 
-            invoke_rustdoc(builder, compiler, target, path);
+            invoke_rustdoc(builder, compiler, &shared_assets, target, path);
         }
 
         if builder.was_invoked_explicitly::<Self>(Kind::Doc) {
@@ -250,6 +250,7 @@ impl Step for TheBook {
 fn invoke_rustdoc(
     builder: &Builder<'_>,
     compiler: Compiler,
+    shared_assets: &SharedAssetsPaths,
     target: TargetSelection,
     markdown: &str,
 ) {
@@ -259,7 +260,6 @@ fn invoke_rustdoc(
 
     let header = builder.src.join("src/doc/redirect.inc");
     let footer = builder.src.join("src/doc/footer.inc");
-    let version_info = out.join("version_info.html");
 
     let mut cmd = builder.rustdoc_cmd(compiler);
 
@@ -268,7 +268,7 @@ fn invoke_rustdoc(
     cmd.arg("--html-after-content")
         .arg(&footer)
         .arg("--html-before-content")
-        .arg(&version_info)
+        .arg(&shared_assets.version_info)
         .arg("--html-in-header")
         .arg(&header)
         .arg("--markdown-no-toc")
@@ -324,21 +324,11 @@ impl Step for Standalone {
         let out = builder.doc_out(target);
         t!(fs::create_dir_all(&out));
 
+        let version_info = builder.ensure(SharedAssets { target: self.target }).version_info;
+
         let favicon = builder.src.join("src/doc/favicon.inc");
         let footer = builder.src.join("src/doc/footer.inc");
         let full_toc = builder.src.join("src/doc/full-toc.inc");
-        t!(fs::copy(builder.src.join("src/doc/rust.css"), out.join("rust.css")));
-
-        let version_input = builder.src.join("src/doc/version_info.html.template");
-        let version_info = out.join("version_info.html");
-
-        if !builder.config.dry_run && !up_to_date(&version_input, &version_info) {
-            let info = t!(fs::read_to_string(&version_input))
-                .replace("VERSION", &builder.rust_release())
-                .replace("SHORT_HASH", builder.rust_info.sha_short().unwrap_or(""))
-                .replace("STAMP", builder.rust_info.sha().unwrap_or(""));
-            t!(fs::write(&version_info, &info));
-        }
 
         for file in t!(fs::read_dir(builder.src.join("src/doc"))) {
             let file = t!(file);
@@ -400,6 +390,45 @@ impl Step for Standalone {
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct SharedAssetsPaths {
+    pub version_info: PathBuf,
+}
+
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub struct SharedAssets {
+    target: TargetSelection,
+}
+
+impl Step for SharedAssets {
+    type Output = SharedAssetsPaths;
+    const DEFAULT: bool = false;
+
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        // Other tasks depend on this, no need to execute it on its own
+        run.never()
+    }
+
+    // Generate shared resources used by other pieces of documentation.
+    fn run(self, builder: &Builder<'_>) -> Self::Output {
+        let out = builder.doc_out(self.target);
+
+        let version_input = builder.src.join("src").join("doc").join("version_info.html.template");
+        let version_info = out.join("version_info.html");
+        if !builder.config.dry_run && !up_to_date(&version_input, &version_info) {
+            let info = t!(fs::read_to_string(&version_input))
+                .replace("VERSION", &builder.rust_release())
+                .replace("SHORT_HASH", builder.rust_info.sha_short().unwrap_or(""))
+                .replace("STAMP", builder.rust_info.sha().unwrap_or(""));
+            t!(fs::write(&version_info, &info));
+        }
+
+        builder.copy(&builder.src.join("src").join("doc").join("rust.css"), &out.join("rust.css"));
+
+        SharedAssetsPaths { version_info }
+    }
+}
+
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct Std {
     pub stage: u32,