Suppress noisy build script warnings

cargo/private/cargo_build_script.bzl

@@ -627,6 +627,16 @@ def _cargo_build_script_impl(ctx):
     if out_dir_volatile_basenames:
         env["RULES_RUST_OUT_DIR_VOLATILE_BASENAMES"] = ":".join(out_dir_volatile_basenames)
 
+    emit_warnings_setting = ctx.attr._emit_build_script_warnings[BuildSettingInfo].value
+    if emit_warnings_setting == "on":
+        emit_warnings = True
+    elif emit_warnings_setting == "off":
+        emit_warnings = False
+    else:
+        emit_warnings = ctx.attr.emit_warnings
+    if not emit_warnings:
+        env["RULES_RUST_SUPPRESS_BUILD_SCRIPT_WARNINGS"] = "1"
+
     ctx.actions.run(
         executable = ctx.executable._cargo_build_script_runner,
         arguments = [args, runfiles_args],
@@ -704,6 +714,16 @@ cargo_build_script = rule(
             providers = [[DepInfo], [CrateGroupInfo]],
             cfg = "exec",
         ),
+        "emit_warnings": attr.bool(
+            doc = dedent("""\
+                Whether to forward `cargo::warning=` lines from the build script to stderr.
+
+                Honored only when `--@rules_rust//cargo/settings:emit_build_script_warnings`
+                is `auto` (the default). Setting the flag to `on` or `off` overrides
+                this attribute for every target.
+            """),
+            default = True,
+        ),
         "link_deps": attr.label_list(
             doc = dedent("""\
                 The subset of the Rust (normal) dependencies of the crate that
@@ -776,6 +796,9 @@ cargo_build_script = rule(
         "_debug_std_streams_output_group": attr.label(
             default = Label("//cargo/settings:debug_std_streams_output_group"),
         ),
+        "_emit_build_script_warnings": attr.label(
+            default = Label("//cargo/settings:emit_build_script_warnings"),
+        ),
         "_default_use_default_shell_env": attr.label(
             default = Label("//cargo/settings:use_default_shell_env"),
         ),

cargo/private/cargo_build_script_runner/bin.rs

@@ -22,7 +22,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 
 use cargo_build_script_runner::cargo_manifest_dir::{remove_symlink, symlink, RunfilesMaker};
-use cargo_build_script_runner::{BuildScriptOutput, CompileAndLinkFlags};
+use cargo_build_script_runner::{BuildScriptOutput, CompileAndLinkFlags, SUPPRESS_WARNINGS_ENV};
 
 fn run_buildrs() -> Result<(), String> {
     // We use exec_root.join rather than std::fs::canonicalize, to avoid resolving symlinks, as
@@ -158,7 +158,12 @@ fn run_buildrs() -> Result<(), String> {
         );
     }
 
-    let (buildrs_outputs, process_output) = BuildScriptOutput::outputs_from_command(&mut command)
+    let emit_warnings = env::var_os(SUPPRESS_WARNINGS_ENV).is_none_or(|v| v != "1");
+
+    let (buildrs_outputs, process_output) = BuildScriptOutput::outputs_from_command(
+        &mut command,
+        emit_warnings,
+    )
         .map_err(|process_output| {
         format!(
             "Build script process failed{}\n--stdout:\n{}\n--stderr:\n{}",

cargo/private/cargo_build_script_runner/lib.rs

@@ -17,6 +17,8 @@
 use std::io::{BufRead, BufReader, Read};
 use std::process::{Command, Output};
 
+pub const SUPPRESS_WARNINGS_ENV: &str = "RULES_RUST_SUPPRESS_BUILD_SCRIPT_WARNINGS";
+
 pub mod cargo_manifest_dir;
 
 #[derive(Debug, PartialEq, Eq)]
@@ -46,13 +48,7 @@ pub enum BuildScriptOutput {
 }
 
 impl BuildScriptOutput {
-    /// Converts a line into a [BuildScriptOutput] enum.
-    ///
-    /// Examples
-    /// ```rust
-    /// assert_eq!(BuildScriptOutput::new("cargo::rustc-link-lib=lib"), Some(BuildScriptOutput::LinkLib("lib".to_owned())));
-    /// ```
-    fn new(line: &str) -> Option<BuildScriptOutput> {
+    fn new(line: &str, emit_warnings: bool) -> Option<BuildScriptOutput> {
         let split = line.splitn(2, '=').collect::<Vec<_>>();
         if split.len() <= 1 {
             // Not a cargo directive.
@@ -83,7 +79,9 @@ impl BuildScriptOutput {
                 None
             }
             "warning" => {
-                eprint!("Build Script Warning: {}", split[1]);
+                if emit_warnings {
+                    eprint!("Build Script Warning: {}", split[1]);
+                }
                 None
             }
             "metadata" => {
@@ -122,7 +120,10 @@ impl BuildScriptOutput {
     }
 
     /// Converts a [BufReader] into a vector of [BuildScriptOutput] enums.
-    fn outputs_from_reader<T: Read>(mut reader: BufReader<T>) -> Vec<BuildScriptOutput> {
+    fn outputs_from_reader<T: Read>(
+        mut reader: BufReader<T>,
+        emit_warnings: bool,
+    ) -> Vec<BuildScriptOutput> {
         let mut result = Vec::<BuildScriptOutput>::new();
         let mut buf = Vec::new();
         while reader
@@ -132,7 +133,7 @@ impl BuildScriptOutput {
         {
             // like cargo, ignore any lines that are not valid utf8
             if let Ok(line) = String::from_utf8(buf.clone()) {
-                if let Some(bso) = BuildScriptOutput::new(&line) {
+                if let Some(bso) = BuildScriptOutput::new(&line, emit_warnings) {
                     result.push(bso);
                 }
             }
@@ -144,13 +145,14 @@ impl BuildScriptOutput {
     /// Take a [Command], execute it and converts its input into a vector of [BuildScriptOutput]
     pub fn outputs_from_command(
         cmd: &mut Command,
+        emit_warnings: bool,
     ) -> Result<(Vec<BuildScriptOutput>, Output), Output> {
         let child_output = cmd
             .output()
             .unwrap_or_else(|e| panic!("Unable to start command:\n{:#?}\n{:?}", cmd, e));
         if child_output.status.success() {
             let reader = BufReader::new(child_output.stdout.as_slice());
-            let output = Self::outputs_from_reader(reader);
+            let output = Self::outputs_from_reader(reader, emit_warnings);
             Ok((output, child_output))
         } else {
             Err(child_output)
@@ -277,7 +279,7 @@ mod tests {
 
     fn from_read_buffer_to_env_and_flags_test_impl(buff: Cursor<&str>) {
         let reader = BufReader::new(buff);
-        let result = BuildScriptOutput::outputs_from_reader(reader);
+        let result = BuildScriptOutput::outputs_from_reader(reader, true);
         assert_eq!(result.len(), 13);
         assert_eq!(result[0], BuildScriptOutput::LinkLib("sdfsdf".to_owned()));
         assert_eq!(result[1], BuildScriptOutput::Env("FOO=BAR".to_owned()));
@@ -393,7 +395,7 @@ cargo::rustc-env=valid2=2
 ",
         );
         let reader = BufReader::new(buff);
-        let result = BuildScriptOutput::outputs_from_reader(reader);
+        let result = BuildScriptOutput::outputs_from_reader(reader, true);
         assert_eq!(result.len(), 2);
         assert_eq!(
             &BuildScriptOutput::outputs_to_env(&result, "/some/absolute/path", ""),
@@ -412,18 +414,30 @@ cargo:rustc-env=valid2=2
 ",
         );
         let reader = BufReader::new(buff);
-        let result = BuildScriptOutput::outputs_from_reader(reader);
+        let result = BuildScriptOutput::outputs_from_reader(reader, true);
         assert_eq!(result.len(), 2);
         assert_eq!(
             &BuildScriptOutput::outputs_to_env(&result, "/some/absolute/path", ""),
             "valid1=1\nvalid2=2"
         );
     }
 
+    #[test]
+    fn warning_lines_never_appear_in_outputs() {
+        let lines = "cargo::warning=hello\ncargo::rustc-env=A=1\n";
+        for emit_warnings in [true, false] {
+            let result = BuildScriptOutput::outputs_from_reader(
+                BufReader::new(Cursor::new(lines)),
+                emit_warnings,
+            );
+            assert_eq!(result, vec![BuildScriptOutput::Env("A=1".to_owned())]);
+        }
+    }
+
     #[test]
     fn metadata_directive_maps_to_dep_env_key_value() {
         let reader = BufReader::new(Cursor::new("cargo::metadata=version_1_10_0=1\n"));
-        let result = BuildScriptOutput::outputs_from_reader(reader);
+        let result = BuildScriptOutput::outputs_from_reader(reader, true);
         assert_eq!(
             result,
             vec![BuildScriptOutput::DepEnv("VERSION_1_10_0=1".to_owned())]
@@ -447,7 +461,7 @@ cargo::rustc-env=BAR=/abs/exec_root/elsewhere/file.rs
 ",
         );
         let reader = BufReader::new(buff);
-        let result = BuildScriptOutput::outputs_from_reader(reader);
+        let result = BuildScriptOutput::outputs_from_reader(reader, true);
         assert_eq!(
             BuildScriptOutput::outputs_to_env(
                 &result,

cargo/private/cargo_build_script_wrapper.bzl

@@ -23,6 +23,7 @@ def cargo_build_script(
         proc_macro_deps = [],
         build_script_env = {},
         build_script_env_files = [],
+        emit_warnings = True,
         use_default_shell_env = None,
         data = [],
         compile_data = [],
@@ -112,6 +113,10 @@ def cargo_build_script(
         build_script_env (dict, optional): Environment variables for build scripts.
         build_script_env_files (list of label, optional): Files containing additional environment variables to set
             when running the build script.
+        emit_warnings (bool, optional): Whether to forward `cargo::warning=` lines from the build script
+            to stderr. Honored only when
+            `@rules_rust//cargo/settings:emit_build_script_warnings` is `auto` (the
+            default); set the flag to `on` or `off` to override every target.
         use_default_shell_env (bool, optional): Whether or not to include the default shell environment for the build script action. If unset the global
             setting `@rules_rust//cargo/settings:use_default_shell_env` will be used to determine this value.
         data (list, optional): Files needed by the build script.
@@ -221,6 +226,7 @@ def cargo_build_script(
         version = version,
         build_script_env = build_script_env,
         build_script_env_files = build_script_env_files,
+        emit_warnings = emit_warnings,
         use_default_shell_env = sanitized_use_default_shell_env,
         links = links,
         deps = deps,

cargo/settings/BUILD.bazel

@@ -3,6 +3,7 @@ load(
     ":settings.bzl",
     "cargo_manifest_dir_filename_suffixes_to_retain",
     "debug_std_streams_output_group",
+    "emit_build_script_warnings",
     "experimental_symlink_execroot",
     "out_dir_volatile_file_basenames",
     "use_default_shell_env",
@@ -30,3 +31,5 @@ experimental_symlink_execroot()
 use_default_shell_env()
 
 out_dir_volatile_file_basenames()
+
+emit_build_script_warnings()

cargo/settings/settings.bzl

@@ -3,7 +3,7 @@
 Definitions for all `@rules_rust//cargo` settings
 """
 
-load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_list_flag")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag", "string_flag", "string_list_flag")
 
 def experimental_symlink_execroot():
     """A flag for which causes `cargo_build_script` to symlink the execroot of the action to \
@@ -43,6 +43,25 @@ def use_default_shell_env():
         build_setting_default = True,
     )
 
+def emit_build_script_warnings():
+    """A flag which controls whether `cargo_build_script` warnings \
+    (`cargo::warning=`) are printed to stderr.
+
+    Supported values:
+
+    - `on`: emit warnings for every `cargo_build_script` target, overriding any
+      per-target `emit_warnings = False`.
+    - `auto` (default): respect the per-target `emit_warnings` attribute.
+      `crate_universe`-generated targets set it to `False`, so registry/git
+      crates stay quiet (matching Cargo); first-party targets emit by default.
+    - `off`: silence warnings build-wide.
+    """
+    string_flag(
+        name = "emit_build_script_warnings",
+        build_setting_default = "auto",
+        values = ["on", "auto", "off"],
+    )
+
 def out_dir_volatile_file_basenames():
     """A flag which determines what file basenames are removed from `OUT_DIR` by `cargo_build_script` actions to make the `_bs.out_dir` TreeArtifact deterministic.
 

cargo/tests/cargo_build_script/emit_warnings/BUILD.bazel

@@ -0,0 +1,21 @@
+load("//cargo:defs.bzl", "cargo_build_script")
+load("//rust:defs.bzl", "rust_library", "rust_test")
+
+cargo_build_script(
+    name = "build_quiet",
+    srcs = ["build.rs"],
+    edition = "2018",
+    emit_warnings = False,
+)
+
+rust_library(
+    name = "quiet_lib",
+    srcs = ["test_lib.rs"],
+    edition = "2018",
+    deps = [":build_quiet"],
+)
+
+rust_test(
+    name = "quiet_test",
+    crate = ":quiet_lib",
+)

cargo/tests/cargo_build_script/emit_warnings/build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    println!("cargo::warning=this should be suppressed when emit_warnings=False");
+    println!("cargo::rustc-env=FROM_BUILD_SCRIPT=ok");
+}

cargo/tests/cargo_build_script/emit_warnings/test_lib.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn build_script_ran() {
+        assert_eq!(env!("FROM_BUILD_SCRIPT"), "ok");
+    }
+}

crate_universe/src/rendering.rs

@@ -610,6 +610,9 @@ impl Renderer {
                 ),
                 platforms,
             ),
+            // Match Cargo's default: registry/git crates are quiet unless the
+            // build fails or the user passes `-vv`.
+            emit_warnings: false,
             edition: krate.common_attrs.edition.clone(),
             linker_script: krate.common_attrs.linker_script.clone(),
             links: attrs.and_then(|attrs| attrs.links.clone()),
@@ -1244,6 +1247,11 @@ mod test {
             "```\n{}```\n",
             build_file_content
         );
+        assert!(
+            build_file_content.contains("emit_warnings = False"),
+            "```\n{}```\n",
+            build_file_content
+        );
 
         // Ensure `cargo_build_script` requirements are met
         assert!(build_file_content.contains("name = \"_bs\""));

crate_universe/src/utils/starlark.rs

@@ -111,6 +111,9 @@ pub(crate) struct CargoBuildScript {
     pub(crate) data: Data,
     #[serde(skip_serializing_if = "SelectSet::is_empty")]
     pub(crate) deps: SelectSet<Label>,
+    // Skip emitting when value matches the rule default (`True`).
+    #[serde(skip_serializing_if = "Clone::clone")]
+    pub(crate) emit_warnings: bool,
     #[serde(skip_serializing_if = "SelectSet::is_empty")]
     pub(crate) link_deps: SelectSet<Label>,
     pub(crate) edition: String,