rustbuild: Fixup calling rustdoc in various stages

The stage0 rustdoc comes from the snapshot, and we need a shim like with `rustc`
to pass `--cfg` for now.

src/bootstrap/build/compile.rs

@@ -325,4 +325,3 @@ pub fn tool(build: &Build, stage: u32, host: &str, tool: &str) {
          .arg(build.src.join(format!("src/tools/{}/Cargo.toml", tool)));
     build.run(&mut cargo);
 }
-

src/bootstrap/build/doc.rs

@@ -8,9 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::path::Path;
 use std::fs::{self, File};
 use std::io::prelude::*;
+use std::path::Path;
+use std::process::Command;
 
 use build::{Build, Compiler, Mode};
 use build::util::{up_to_date, cp_r};
@@ -69,7 +70,7 @@ pub fn standalone(build: &Build, stage: u32, host: &str, out: &Path) {
         }
 
         let html = out.join(filename).with_extension("html");
-        let rustdoc = build.tool(&compiler, "rustdoc");
+        let rustdoc = build.rustdoc(&compiler);
         if up_to_date(&path, &html) &&
            up_to_date(&footer, &html) &&
            up_to_date(&favicon, &html) &&
@@ -79,7 +80,7 @@ pub fn standalone(build: &Build, stage: u32, host: &str, out: &Path) {
             continue
         }
 
-        let mut cmd = build.tool_cmd(&compiler, "rustdoc");
+        let mut cmd = Command::new(&rustdoc);
         cmd.arg("--html-after-content").arg(&footer)
            .arg("--html-before-content").arg(&version_info)
            .arg("--html-in-header").arg(&favicon)
@@ -108,10 +109,9 @@ pub fn std(build: &Build, stage: u32, host: &str, out: &Path) {
     let compiler = Compiler::new(stage, host);
     let out_dir = build.stage_out(stage, host, Mode::Libstd)
                        .join(host).join("doc");
-    let rustdoc = build.tool(&compiler, "rustdoc");
-    if !up_to_date(&rustdoc, &out_dir.join("std/index.html")) {
-        t!(fs::remove_dir_all(&out_dir));
-    }
+    let rustdoc = build.rustdoc(&compiler);
+
+    build.clear_if_dirty(&out_dir, &rustdoc);
 
     let mut cargo = build.cargo(stage, &compiler, Mode::Libstd, Some(host),
                                 "doc");
@@ -127,7 +127,7 @@ pub fn rustc(build: &Build, stage: u32, host: &str, out: &Path) {
     let compiler = Compiler::new(stage, host);
     let out_dir = build.stage_out(stage, host, Mode::Librustc)
                        .join(host).join("doc");
-    let rustdoc = build.tool(&compiler, "rustdoc");
+    let rustdoc = build.rustdoc(&compiler);
     if !up_to_date(&rustdoc, &out_dir.join("rustc/index.html")) {
         t!(fs::remove_dir_all(&out_dir));
     }

src/bootstrap/build/mod.rs

@@ -275,7 +275,8 @@ impl Build {
              .env("RUSTC_SYSROOT", self.sysroot(stage, host))
              .env("RUSTC_SNAPSHOT_LIBDIR", self.rustc_snapshot_libdir())
              .env("RUSTC_RPATH", self.config.rust_rpath.to_string())
-             .env("RUSTDOC", self.tool(compiler, "rustdoc"));
+             .env("RUSTDOC", self.out.join("bootstrap/debug/rustdoc"))
+             .env("RUSTDOC_REAL", self.rustdoc(compiler));
 
         if let Some(target) = target {
              cargo.env("RUSTC_FLAGS", self.rustc_flags(target).join(" "));
@@ -317,13 +318,26 @@ impl Build {
         }
     }
 
-    /// Get the specified tool next to the specified compiler
+    /// Get the specified tool built by the specified compiler
     fn tool(&self, compiler: &Compiler, tool: &str) -> PathBuf {
         self.stage_out(compiler.stage, compiler.host, Mode::Tool)
             .join(self.cargo_dir())
             .join(exe(tool, compiler.host))
     }
 
+    /// Get the `rustdoc` executable next to the specified compiler
+    fn rustdoc(&self, compiler: &Compiler) -> PathBuf {
+        let root = if compiler.is_snapshot(self) {
+            let mut rustdoc = self.rustc.clone();
+            rustdoc.pop();
+            rustdoc
+        } else {
+            let (stage, host) = (compiler.stage, compiler.host);
+            self.cargo_out(stage - 1, host, Mode::Librustc, host)
+        };
+        root.join(exe("rustdoc", compiler.host))
+    }
+
     /// Get a `Command` which is ready to run `tool` in `stage` built for
     /// `host`.
     #[allow(dead_code)] // this will be used soon