Support no_std in Prost toolchain

extensions/prost/private/prost.bzl

@@ -86,6 +86,9 @@ def _compile_proto(
     additional_args.add("--additional_srcs={}".format(",".join([f.path for f in all_additional_srcs.to_list()])))
     additional_args.add_all(prost_toolchain.prost_opts + prost_opts, format_each = "--prost_opt=%s")
 
+    if prost_toolchain.is_no_std:
+        additional_args.add("--is_no_std")
+
     if prost_toolchain.tonic_plugin:
         tonic_plugin = prost_toolchain.tonic_plugin[DefaultInfo].files_to_run
         additional_args.add(prost_toolchain.tonic_plugin_flag % tonic_plugin.executable.path)
@@ -442,6 +445,7 @@ def _rust_prost_toolchain_impl(ctx):
         tonic_plugin_flag = ctx.attr.tonic_plugin_flag,
         tonic_runtime = ctx.attr.tonic_runtime,
         include_transitive_deps = ctx.attr.include_transitive_deps,
+        is_no_std = ctx.attr.is_no_std,
     )]
 
 rust_prost_toolchain = rule(
@@ -453,6 +457,10 @@ rust_prost_toolchain = rule(
             doc = "Whether to include transitive dependencies. If set to True, all transitive dependencies will directly accessible by the dependent crate.",
             default = False,
         ),
+        "is_no_std": attr.bool(
+            doc = "If a no_std tag should be put into the generated code.",
+            default = False,
+        ),
         "prost_opts": attr.string_list(
             doc = "Additional options to add to Prost.",
         ),

extensions/prost/private/protoc_wrapper.rs

@@ -156,8 +156,17 @@ fn generate_lib_rs(
     is_tonic: bool,
     direct_dep_crate_names: Vec<String>,
     additional_content: String,
+    is_no_std: bool,
 ) -> String {
-    let mut contents = vec!["// @generated".to_string(), "".to_string()];
+    let mut contents = vec![
+        if is_no_std {
+            "#![no_std]".to_string()
+        } else {
+            "".to_string()
+        },
+        "// @generated".to_string(),
+        "".to_string(),
+    ];
     for crate_name in direct_dep_crate_names {
         contents.push(format!("pub use {crate_name};"));
     }
@@ -457,6 +466,9 @@ struct Args {
     /// Whether to generate tonic code.
     is_tonic: bool,
 
+    // Whether to put a no_std tag into the generated code.
+    is_no_std: bool,
+
     /// Extra arguments to pass to protoc.
     extra_args: Vec<String>,
 }
@@ -479,6 +491,7 @@ impl Args {
         let mut tonic_or_prost_opts = Vec::new();
         let mut direct_dep_crate_names = Vec::new();
         let mut is_tonic = false;
+        let mut is_no_std = false;
 
         let mut extra_args = Vec::new();
 
@@ -501,6 +514,10 @@ impl Args {
                 is_tonic = true;
                 return;
             }
+            if arg == "--is_no_std" {
+                is_no_std = true;
+                return;
+            }
 
             if !arg.contains('=') {
                 extra_args.push(arg);
@@ -644,6 +661,7 @@ impl Args {
             proto_paths,
             direct_dep_crate_names,
             is_tonic,
+            is_no_std,
             label: label.unwrap(),
             extra_args,
         })
@@ -748,6 +766,7 @@ fn main() {
         proto_paths,
         direct_dep_crate_names,
         is_tonic,
+        is_no_std,
         extra_args,
     } = Args::parse().expect("Failed to parse args");
 
@@ -917,6 +936,7 @@ fn main() {
             is_tonic,
             direct_dep_crate_names,
             additional_content,
+            is_no_std,
         ),
     )
     .expect("Failed to write file.");
@@ -972,7 +992,6 @@ fn escape_keyword(s: String) -> String {
 
 #[cfg(test)]
 mod test {
-
     use super::*;
 
     use prost_types::{FieldDescriptorProto, ServiceDescriptorProto};