Make it possible to write doctests for bootstrap

This probably isn't super useful in practice, but it was easy to fix
and avoids confusing errors about mismatched versions between beta and the default toolchain.

src/bootstrap/test.rs

@@ -2362,6 +2362,7 @@ fn run(self, builder: &Builder<'_>) {
             .env("RUSTFLAGS", "-Cdebuginfo=2")
             .env("CARGO_TARGET_DIR", builder.out.join("bootstrap"))
             .env("RUSTC_BOOTSTRAP", "1")
+            .env("RUSTDOC", builder.rustdoc(builder.compiler(0, builder.build.build)))
             .env("RUSTC", &builder.initial_rustc);
         if let Some(flags) = option_env!("RUSTFLAGS") {
             // Use the same rustc flags for testing as for "normal" compilation,
@@ -2372,6 +2373,16 @@ fn run(self, builder: &Builder<'_>) {
         if !builder.fail_fast {
             cmd.arg("--no-fail-fast");
         }
+        match builder.doc_tests {
+            DocTests::Only => {
+                cmd.arg("--doc");
+            }
+            DocTests::No => {
+                cmd.args(&["--lib", "--bins", "--examples", "--tests", "--benches"]);
+            }
+            DocTests::Yes => {}
+        }
+
         cmd.arg("--").args(&builder.config.cmd.test_args());
         // rustbuild tests are racy on directory creation so just run them one at a time.
         // Since there's not many this shouldn't be a problem.