Add unstable --test-builder to rustdoc

This allows overriding the rustc binary used to build tests; it should
not generally be necessary as we fallback to the sysroot.

src/librustdoc/config.rs

@@ -86,6 +86,10 @@ pub struct Options {
     /// contains "foo" as a substring
     pub enable_per_target_ignores: bool,
 
+    /// The path to a rustc-like binary to build tests with. If not set, we
+    /// default to loading from $sysroot/bin/rustc.
+    pub test_builder: Option<PathBuf>,
+
     // Options that affect the documentation process
 
     /// The selected default set of passes to use.
@@ -476,6 +480,7 @@ impl Options {
         let generate_search_filter = !matches.opt_present("disable-per-crate-search");
         let persist_doctests = matches.opt_str("persist-doctests").map(PathBuf::from);
         let generate_redirect_pages = matches.opt_present("generate-redirect-pages");
+        let test_builder = matches.opt_str("test-builder").map(PathBuf::from);
         let codegen_options_strs = matches.opt_strs("C");
         let lib_strs = matches.opt_strs("L");
         let extern_strs = matches.opt_strs("extern");
@@ -515,6 +520,7 @@ impl Options {
             runtool,
             runtool_args,
             enable_per_target_ignores,
+            test_builder,
             render_options: RenderOptions {
                 output,
                 external_html,

src/librustdoc/lib.rs

@@ -373,6 +373,11 @@ fn opts() -> Vec<RustcOptGroup> {
                        "",
                        "One (of possibly many) arguments to pass to the runtool")
         }),
+        unstable("test-builder", |o| {
+            o.optflag("",
+                      "test-builder",
+                      "specified the rustc-like binary to use as the test builder")
+        }),
     ]
 }
 

src/librustdoc/test.rs

@@ -248,7 +248,10 @@ fn run_test(
     };
     let output_file = outdir.path().join("rust_out");
 
-    let mut compiler = Command::new(rustc_interface::util::rustc_path().expect("found rustc"));
+    let rustc_binary = options.test_builder.as_ref().map(|v| &**v).unwrap_or_else(|| {
+        rustc_interface::util::rustc_path().expect("found rustc")
+    });
+    let mut compiler = Command::new(&rustc_binary);
     compiler.arg("--crate-type").arg("bin");
     for cfg in &options.cfgs {
         compiler.arg("--cfg").arg(&cfg);