Allow cargo_build_script to forward rustc args to build scripts

cargo/cargo_build_script.bzl

@@ -7,7 +7,7 @@ load("//rust:defs.bzl", "rust_binary", "rust_common")
 load("//rust/private:rustc.bzl", "BuildInfo", "get_compilation_mode_opts", "get_linker_and_args")
 
 # buildifier: disable=bzl-visibility
-load("//rust/private:utils.bzl", "expand_dict_value_locations", "find_cc_toolchain", "find_toolchain", "name_to_crate_name")
+load("//rust/private:utils.bzl", "dedent", "expand_dict_value_locations", "find_cc_toolchain", "find_toolchain", "name_to_crate_name")
 
 def get_cc_compile_env(cc_toolchain, feature_configuration):
     """Gather cc environment variables from the given `cc_toolchain`
@@ -56,11 +56,7 @@ def _build_script_impl(ctx):
 
     pkg_name = _name_to_pkg_name(ctx.label.name)
 
-    toolchain_tools = [
-        # Needed for rustc to function.
-        toolchain.rustc_lib,
-        toolchain.rust_std,
-    ]
+    toolchain_tools = [toolchain.all_files]
 
     cc_toolchain = find_cpp_toolchain(ctx)
 
@@ -121,6 +117,13 @@ def _build_script_impl(ctx):
         if cc_toolchain.sysroot:
             env["SYSROOT"] = cc_toolchain.sysroot
 
+    # Inform build scripts of rustc flags
+    # https://github.com/rust-lang/cargo/issues/9600
+    env["CARGO_ENCODED_RUSTFLAGS"] = "\\x1f".join([
+        # Allow build scripts to locate the generated sysroot
+        "--sysroot=${{pwd}}/{}".format(toolchain.sysroot),
+    ] + ctx.attr.rustc_flags)
+
     for f in ctx.attr.crate_features:
         env["CARGO_FEATURE_" + f.upper().replace("-", "_")] = "1"
 
@@ -136,7 +139,6 @@ def _build_script_impl(ctx):
         direct = [
             script,
             ctx.executable._cargo_build_script_runner,
-            toolchain.rustc,
         ] + ctx.files.data + ctx.files.tools + ([toolchain.target_json] if toolchain.target_json else []),
         transitive = toolchain_tools,
     )
@@ -217,6 +219,16 @@ _build_script_run = rule(
         "links": attr.string(
             doc = "The name of the native library this crate links against.",
         ),
+        "rustc_flags": attr.string_list(
+            doc = dedent("""\
+                List of compiler flags passed to `rustc`.
+
+                These strings are subject to Make variable expansion for predefined
+                source/output path variables like `$location`, `$execpath`, and 
+                `$rootpath`. This expansion is useful if you wish to pass a generated
+                file of arguments to rustc: `@$(location //package:target)`.
+            """),
+        ),
         # The source of truth will be the `cargo_build_script` macro until stardoc
         # implements documentation inheritence. See https://github.com/bazelbuild/stardoc/issues/27
         "script": attr.label(
@@ -262,6 +274,7 @@ def cargo_build_script(
         tools = [],
         links = None,
         rustc_env = {},
+        rustc_flags = [],
         visibility = None,
         tags = None,
         **kwargs):
@@ -333,6 +346,7 @@ def cargo_build_script(
         tools (list, optional): Tools (executables) needed by the build script.
         links (str, optional): Name of the native library this crate links against.
         rustc_env (dict, optional): Environment variables to set in rustc when compiling the build script.
+        rustc_flags (list, optional): List of compiler flags passed to `rustc`.
         visibility (list of label, optional): Visibility to apply to the generated build script output.
         tags: (list of str, optional): Tags to apply to the generated build script output.
         **kwargs: Forwards to the underlying `rust_binary` rule.
@@ -357,6 +371,7 @@ def cargo_build_script(
         deps = deps,
         data = data,
         rustc_env = rustc_env,
+        rustc_flags = rustc_flags,
         tags = binary_tags,
         **kwargs
     )
@@ -370,6 +385,7 @@ def cargo_build_script(
         deps = deps,
         data = data,
         tools = tools,
+        rustc_flags = rustc_flags,
         visibility = visibility,
         tags = tags,
     )

cargo/cargo_build_script_runner/bin.rs

@@ -100,6 +100,15 @@ fn run_buildrs() -> Result<(), String> {
         }
     }
 
+    // Bazel does not support byte strings so in order to correctly represent `CARGO_ENCODED_RUSTFLAGS`
+    // the escaped `\x1f` sequences need to be unescaped
+    if let Ok(encoded_rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
+        command.env(
+            "CARGO_ENCODED_RUSTFLAGS",
+            encoded_rustflags.replace("\\x1f", "\x1f"),
+        );
+    }
+
     if let Some(ar_path) = env::var_os("AR") {
         // The default OSX toolchain uses libtool as ar_executable not ar.
         // This doesn't work when used as $AR, so simply don't set it - tools will probably fall back to

docs/cargo.md

@@ -46,7 +46,7 @@ A rule for bootstrapping a Rust binary using [Cargo](https://doc.rust-lang.org/c
 
 <pre>
 cargo_build_script(<a href="#cargo_build_script-name">name</a>, <a href="#cargo_build_script-crate_features">crate_features</a>, <a href="#cargo_build_script-version">version</a>, <a href="#cargo_build_script-deps">deps</a>, <a href="#cargo_build_script-build_script_env">build_script_env</a>, <a href="#cargo_build_script-data">data</a>, <a href="#cargo_build_script-tools">tools</a>, <a href="#cargo_build_script-links">links</a>,
-                   <a href="#cargo_build_script-rustc_env">rustc_env</a>, <a href="#cargo_build_script-visibility">visibility</a>, <a href="#cargo_build_script-tags">tags</a>, <a href="#cargo_build_script-kwargs">kwargs</a>)
+                   <a href="#cargo_build_script-rustc_env">rustc_env</a>, <a href="#cargo_build_script-rustc_flags">rustc_flags</a>, <a href="#cargo_build_script-visibility">visibility</a>, <a href="#cargo_build_script-tags">tags</a>, <a href="#cargo_build_script-kwargs">kwargs</a>)
 </pre>
 
 Compile and execute a rust build script to generate build attributes
@@ -121,6 +121,7 @@ The `hello_lib` target will be build with the flags and the environment variable
 | <a id="cargo_build_script-tools"></a>tools |  Tools (executables) needed by the build script.   |  <code>[]</code> |
 | <a id="cargo_build_script-links"></a>links |  Name of the native library this crate links against.   |  <code>None</code> |
 | <a id="cargo_build_script-rustc_env"></a>rustc_env |  Environment variables to set in rustc when compiling the build script.   |  <code>{}</code> |
+| <a id="cargo_build_script-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   |  <code>[]</code> |
 | <a id="cargo_build_script-visibility"></a>visibility |  Visibility to apply to the generated build script output.   |  <code>None</code> |
 | <a id="cargo_build_script-tags"></a>tags |  (list of str, optional): Tags to apply to the generated build script output.   |  <code>None</code> |
 | <a id="cargo_build_script-kwargs"></a>kwargs |  Forwards to the underlying <code>rust_binary</code> rule.   |  none |

docs/flatten.md

@@ -1389,7 +1389,7 @@ A collection of files either found within the `rust-stdlib` artifact or generate
 
 <pre>
 cargo_build_script(<a href="#cargo_build_script-name">name</a>, <a href="#cargo_build_script-crate_features">crate_features</a>, <a href="#cargo_build_script-version">version</a>, <a href="#cargo_build_script-deps">deps</a>, <a href="#cargo_build_script-build_script_env">build_script_env</a>, <a href="#cargo_build_script-data">data</a>, <a href="#cargo_build_script-tools">tools</a>, <a href="#cargo_build_script-links">links</a>,
-                   <a href="#cargo_build_script-rustc_env">rustc_env</a>, <a href="#cargo_build_script-visibility">visibility</a>, <a href="#cargo_build_script-tags">tags</a>, <a href="#cargo_build_script-kwargs">kwargs</a>)
+                   <a href="#cargo_build_script-rustc_env">rustc_env</a>, <a href="#cargo_build_script-rustc_flags">rustc_flags</a>, <a href="#cargo_build_script-visibility">visibility</a>, <a href="#cargo_build_script-tags">tags</a>, <a href="#cargo_build_script-kwargs">kwargs</a>)
 </pre>
 
 Compile and execute a rust build script to generate build attributes
@@ -1464,6 +1464,7 @@ The `hello_lib` target will be build with the flags and the environment variable
 | <a id="cargo_build_script-tools"></a>tools |  Tools (executables) needed by the build script.   |  <code>[]</code> |
 | <a id="cargo_build_script-links"></a>links |  Name of the native library this crate links against.   |  <code>None</code> |
 | <a id="cargo_build_script-rustc_env"></a>rustc_env |  Environment variables to set in rustc when compiling the build script.   |  <code>{}</code> |
+| <a id="cargo_build_script-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   |  <code>[]</code> |
 | <a id="cargo_build_script-visibility"></a>visibility |  Visibility to apply to the generated build script output.   |  <code>None</code> |
 | <a id="cargo_build_script-tags"></a>tags |  (list of str, optional): Tags to apply to the generated build script output.   |  <code>None</code> |
 | <a id="cargo_build_script-kwargs"></a>kwargs |  Forwards to the underlying <code>rust_binary</code> rule.   |  none |

test/cargo_build_script/BUILD.bazel

@@ -8,6 +8,8 @@ cargo_build_script(
     name = "tools_exec_build_rs",
     srcs = ["build.rs"],
     build_script_env = {"TOOL": "$(execpath :tool)"},
+    # Add a flag to test that they're exposed to the build script
+    rustc_flags = ["--verbose"],
     tools = [":tool"],
 )
 

test/cargo_build_script/build.rs

@@ -1,4 +1,24 @@
+//! A Cargo build script binary used in unit tests for the Bazel `cargo_build_script` rule
+
+/// `cargo_build_script` should always set `CARGO_ENCODED_RUSTFLAGS`
+fn test_encoded_rustflags() {
+    let encoded_rustflags = std::env::var("CARGO_ENCODED_RUSTFLAGS").unwrap();
+
+    let flags: Vec<String> = encoded_rustflags
+        .split('\x1f')
+        .map(str::to_string)
+        .collect();
+    assert_eq!(flags.len(), 2);
+
+    assert!(flags[0].starts_with("--sysroot"));
+    assert!(flags[1].starts_with("--verbose"));
+}
+
 fn main() {
+
+    // Perform some unit testing
+    test_encoded_rustflags();
+
     // Pass the TOOL_PATH along to the rust_test so we can assert on it.
     println!(
         "cargo:rustc-env=TOOL_PATH={}",