THRIFT-6059: add crate_prefix option to Rust codegen for cross-file imports

[santiagomed]

Problem

The Rust code generator hardcodes use crate:: for cross-file imports (e.g., fileA.thrift includes fileB.thrift generates use crate::fileB;). This assumes the generated files are placed at the crate root (src/). When files are placed in a submodule (e.g., src/thrift/), the correct prefix is super::, and users must post-process generated files with sed.

Fix

Add a crate_prefix generator option that controls the path prefix. Default remains crate for backward compatibility.

# Files at crate root — existing behavior, unchanged
thrift --gen rs -out src/ schema.thrift

# Files in a submodule
thrift --gen rs:crate_prefix=super -out src/thrift/ schema.thrift

# Nested module paths also work
thrift --gen rs:crate_prefix=crate::generated -out src/generated/ schema.thrift

Changes

• t_rs_generator constructor reads crate_prefix from parsed_options
• crate_prefix_ member stores the value (default: "crate")
• Import generation in render_attributes_and_includes uses crate_prefix_ instead of hardcoded "crate"
• THRIFT_REGISTER_GENERATOR updated to document the option
• An empty value (rs:crate_prefix=) is rejected: it would silently emit use ::module;, which is an external-crate path in Rust 2018+
• Unknown options are reported as unknown option rs:<name>, matching the convention used by other generators

Testing

Verified manually with a two-file schema (event.thrift including trees.thrift):

Invocation	Result
--gen rs	use crate::trees; (unchanged)
--gen rs:crate_prefix=super	use super::trees;
--gen rs:crate_prefix=crate::generated	use crate::generated::trees;
--gen rs:crate_prefix=	error: crate_prefix requires a non-empty value
--gen rs:bogus=1	error: unknown option rs:bogus

There is no existing test harness for rs generator options (lib/rs/test_recursive exercises cross-file imports with the default prefix only), so no automated test is included; happy to add one if reviewers have a preferred location.

---

Portions of this change were AI-assisted (Grok 4.3, xAI) and reviewed by the author.

[santiagomed]

@mhlakhani @Jens-G could I get a review here please? Also open to hearing other solutions to this issue with submodules!

[Jens-G]

Code review

Found 1 issue:

1. The new option-parsing loop throws "unknown option: " + iter->first, but every other generator in the codebase uses a language-prefixed form (e.g., "unknown option py:", "unknown option cpp:"). The RS generator should throw "unknown option rs:" + iter->first to follow the established convention and help users identify which generator rejected their flag.

thrift/compiler/cpp/src/thrift/generate/t_rs_generator.cc

Lines 61 to 67 in 4ce4898

	std::map<std::string, std::string>::const_iterator iter;
	for (iter = parsed_options.begin(); iter != parsed_options.end(); ++iter) {
	if (iter->first.compare("crate_prefix") == 0) {
	crate_prefix_ = iter->second;
	} else {
	throw "unknown option: " + iter->first;
	}

🤖 Generated with Claude Code

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[santiagomed]

Code review

Found 1 issue:

1. The new option-parsing loop throws "unknown option: " + iter->first, but every other generator in the codebase uses a language-prefixed form (e.g., "unknown option py:", "unknown option cpp:"). The RS generator should throw "unknown option rs:" + iter->first to follow the established convention and help users identify which generator rejected their flag.

thrift/compiler/cpp/src/thrift/generate/t_rs_generator.cc

Lines 61 to 67 in 4ce4898

	std::map<std::string, std::string>::const_iterator iter;
	for (iter = parsed_options.begin(); iter != parsed_options.end(); ++iter) {
	if (iter->first.compare("crate_prefix") == 0) {
	crate_prefix_ = iter->second;
	} else {
	throw "unknown option: " + iter->first;
	}

🤖 Generated with Claude Code

• If this code review was useful, please react with 👍. Otherwise, react with 👎.

Thanks for the review @Jens-G. Addressed this + added some additional fixes specified in the summary.