cargo_build_scripts can be run from custom directories (bazelbuild#2152)

While running from CARGO_MANIFEST_DIR is the simplest thing for
compatibility, there are cases where a cargo build script may be easier
to run from the exec root as most bazel actions do (e.g. where a C++
toolchain specifies in-repo include paths).

This mirrors the `rundir` attribute of `go_test`, which has similar
concerns:
https://github.com/bazelbuild/rules_go/blob/master/docs/go/core/rules.md#go_test-rundir

Author: illicitonion

bindgen/3rdparty/crates/BUILD.bindgen-0.65.1.bazel

@@ -20,7 +20,7 @@ use cargo_build_script_output_parser::{BuildScriptOutput, CompileAndLinkFlags};
 use std::collections::BTreeMap;
 use std::env;
 use std::fs::{create_dir_all, read_to_string, write};
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::process::Command;
 
 fn run_buildrs() -> Result<(), String> {
@@ -45,6 +45,7 @@ fn run_buildrs() -> Result<(), String> {
         output_dep_env_path,
         stdout_path,
         stderr_path,
+        rundir,
         input_dep_env_paths,
     } = parse_args()?;
 
@@ -77,9 +78,11 @@ fn run_buildrs() -> Result<(), String> {
     let target_env_vars =
         get_target_env_vars(&rustc_env).expect("Error getting target env vars from rustc");
 
+    let working_directory = resolve_rundir(&rundir, &exec_root, &manifest_dir)?;
+
     let mut command = Command::new(exec_root.join(progname));
     command
-        .current_dir(&manifest_dir)
+        .current_dir(&working_directory)
         .envs(target_env_vars)
         .env("OUT_DIR", out_dir_abs)
         .env("CARGO_MANIFEST_DIR", manifest_dir)
@@ -223,6 +226,23 @@ fn symlink_if_not_exists(original: &Path, link: &Path) -> Result<(), String> {
         .map_err(|err| format!("Failed to create symlink: {err}"))
 }
 
+fn resolve_rundir(rundir: &str, exec_root: &Path, manifest_dir: &Path) -> Result<PathBuf, String> {
+    if rundir.is_empty() {
+        return Ok(manifest_dir.to_owned());
+    }
+    let rundir_path = Path::new(rundir);
+    if rundir_path.is_absolute() {
+        return Err(format!("rundir must be empty (to run in manifest path) or relative path (relative to exec root), but was {:?}", rundir));
+    }
+    if rundir_path
+        .components()
+        .any(|c| c == std::path::Component::ParentDir)
+    {
+        return Err(format!("rundir must not contain .. but was {:?}", rundir));
+    }
+    Ok(exec_root.join(rundir_path))
+}
+
 fn swallow_already_exists(err: std::io::Error) -> std::io::Result<()> {
     if err.kind() == std::io::ErrorKind::AlreadyExists {
         Ok(())
@@ -243,6 +263,7 @@ struct Options {
     output_dep_env_path: String,
     stdout_path: String,
     stderr_path: String,
+    rundir: String,
     input_dep_env_paths: Vec<String>,
 }
 
@@ -251,7 +272,7 @@ fn parse_args() -> Result<Options, String> {
     let mut args = env::args().skip(1);
 
     // TODO: we should consider an alternative to positional arguments.
-    match (args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next()) {
+    match (args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next(), args.next()) {
         (
             Some(progname),
             Some(crate_links),
@@ -263,6 +284,7 @@ fn parse_args() -> Result<Options, String> {
             Some(output_dep_env_path),
             Some(stdout_path),
             Some(stderr_path),
+            Some(rundir),
         ) => {
             Ok(Options{
                 progname,
@@ -275,6 +297,7 @@ fn parse_args() -> Result<Options, String> {
                 output_dep_env_path,
                 stdout_path,
                 stderr_path,
+                rundir,
                 input_dep_env_paths: args.collect(),
             })
         }

bindgen/3rdparty/crates/defs.bzl

@@ -226,6 +226,7 @@ def _cargo_build_script_impl(ctx):
     args.add(dep_env_out)
     args.add(streams.stdout)
     args.add(streams.stderr)
+    args.add(ctx.attr.rundir)
 
     build_script_inputs = []
     for dep in ctx.attr.link_deps:
@@ -305,6 +306,16 @@ cargo_build_script = rule(
         "links": attr.string(
             doc = "The name of the native library this crate links against.",
         ),
+        "rundir": attr.string(
+            default = "",
+            doc = dedent("""\
+                A directory to cd to before the cargo_build_script is run. This should be a path relative to the exec root.
+
+                The default behaviour (and the behaviour if rundir is set to the empty string) is to change to the relative path corresponding to the cargo manifest directory, which replicates the normal behaviour of cargo so it is easy to write compatible build scripts.
+
+                If set to `.`, the cargo build script will run in the exec root.
+            """),
+        ),
         "rustc_flags": attr.string_list(
             doc = dedent("""\
                 List of compiler flags passed to `rustc`.

cargo/cargo_build_script_runner/bin.rs

@@ -18,6 +18,7 @@ def cargo_build_script(
         data = [],
         tools = [],
         links = None,
+        rundir = None,
         rustc_env = {},
         rustc_flags = [],
         visibility = None,
@@ -93,6 +94,11 @@ def cargo_build_script(
         data (list, optional): Files needed by the build script.
         tools (list, optional): Tools (executables) needed by the build script.
         links (str, optional): Name of the native library this crate links against.
+        rundir (str, optional): A directory to `cd` to before the cargo_build_script is run. This should be a path relative to the exec root.
+
+            The default behaviour (and the behaviour if rundir is set to the empty string) is to change to the relative path corresponding to the cargo manifest directory, which replicates the normal behaviour of cargo so it is easy to write compatible build scripts.
+
+            If set to `.`, the cargo build script will run in the exec root.
         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.
@@ -150,6 +156,7 @@ def cargo_build_script(
         link_deps = link_deps,
         data = data,
         tools = tools,
+        rundir = rundir,
         rustc_flags = rustc_flags,
         visibility = visibility,
         tags = tags,