Document private items by default for binaries

Summary: This matches Cargo's behavior. See [rust-lang/cargo#7593](rust-lang/cargo#7593). Documentation for `rust_binary` targets is otherwise useless, since usually there is nothing in them declared `pub`.

Reviewed By: zertosh

Differential Revision: D36563393

fbshipit-source-id: 9ee64fb3f77caf7376a93eabfb309dc0adc1b45d

prelude/rust/build.bzl

@@ -92,7 +92,8 @@ def generate_rustdoc(
         # link style doesn't matter, but caller should pass in build params
         # with static-pic (to get best cache hits for deps)
         params: BuildParams.type,
-        default_roots: [str.type]) -> "artifact":
+        default_roots: [str.type],
+        document_private_items: bool.type) -> "artifact":
     toolchain_info = ctx_toolchain_info(ctx)
 
     common_args = _compute_common_args(
@@ -122,6 +123,9 @@ def generate_rustdoc(
         common_args.args,
     )
 
+    if document_private_items:
+        rustdoc_cmd.add("--document-private-items")
+
     url_prefix = toolchain_info.extern_html_root_url_prefix
     for rust_dependency in resolve_deps(ctx):
         dep = rust_dependency.dep

prelude/rust/rust_binary.bzl

@@ -109,7 +109,14 @@ def _rust_binary_common(
     )
 
     extra_targets += [
-        ("doc", generate_rustdoc(ctx, compile_ctx, crate, style_param[LinkStyle("static_pic")], default_roots)),
+        ("doc", generate_rustdoc(
+            ctx = ctx,
+            compile_ctx = compile_ctx,
+            crate = crate,
+            params = style_param[LinkStyle("static_pic")],
+            default_roots = default_roots,
+            document_private_items = True,
+        )),
         ("expand", expand.outputs[Emit("expand")]),
     ]
     sub_targets = {k: [DefaultInfo(default_outputs = [v])] for k, v in extra_targets}

prelude/rust/rust_library.bzl

@@ -160,11 +160,12 @@ def rust_library_impl(ctx: "context") -> ["provider"]:
             fail("Unhandled lang {}".format(lang))
 
     rustdoc = generate_rustdoc(
-        ctx,
-        compile_ctx,
-        crate,
-        lang_style_param[(LinkageLang("rust"), LinkStyle("static_pic"))],
-        ["lib.rs"],
+        ctx = ctx,
+        compile_ctx = compile_ctx,
+        crate = crate,
+        params = lang_style_param[(LinkageLang("rust"), LinkStyle("static_pic"))],
+        default_roots = ["lib.rs"],
+        document_private_items = False,
     )
 
     expand = rust_compile(