fix: generator fixes for keyword identifiers, null optionals and Default

[lsjostro]

Three varlink_generator fixes/improvements, found while generating client code for systemd's io.systemd.* interfaces (which use type and friends as field names liberally). They are bundled because they touch the same emission sites and share one regenerated golden file — happy to split if preferred.

1. Keywords that cannot be raw identifiers

The generator blindly prefixes every identifier with r#, but self, Self, crate and super cannot be raw identifiers — syn::parse_str("r#self") fails, so the generator panics on any interface using them as a type, field, variant or parameter name.

New safe_ident() suffixes those four with _ and pairs them with #[serde(rename = "...")] so the wire name is unchanged. All other names keep the existing r# treatment (no rename needed — serde already strips the r# prefix).

2. Null optionals in custom types

Method argument and reply structs already annotate optional fields with #[serde(skip_serializing_if = "Option::is_none")], but typedef structs don't. A minimally-populated custom type therefore serializes every absent field as null, which servers may reject — systemd, for example, treats an explicit null as an attempt to set a property rather than omitting it. Typedef structs now get the same annotation.

3. derive(Default)

All generated structs and enums now derive Default (enums mark the first variant #[default] — this only affects Enum::default(), never the wire encoding). This makes sparsely-populated argument structs ergonomic:

let ctx = UnitContext { description: Some("test".into()), ..Default::default() };

which matters for interfaces whose types carry dozens of optional fields.

Tests

tests/org.example.complex.varlink gains ReservedEnum, a Reserved type and a Baz method exercising the non-raw-able keywords as type, field, variant and method-parameter names; the golden output is regenerated (via varlink-rust-generator + rustfmt, minus the standalone-binary header lines).

Verified beyond the golden diff: generated code for a reserved-keywords interface compiles, self/Self fields round-trip through serde with the correct wire names, null optionals are omitted, and Enum::default() serializes as the first variant. (The org.example.complex golden itself doesn't compile as Rust — that's pre-existing on master, e.g. duplicate anonymous-struct emission for ErrorFoo_Args_enum — so the compile checks were done with a separate clean interface.)

cargo test -p varlink_generator, cargo test -p varlink, cargo clippy -p varlink_generator --all-targets -- -D warnings and cargo fmt --check pass.

[coveralls]

Coverage Report for CI Build 31330034567

Coverage increased (+0.2%) to 56.477%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: 35 of 35 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	5427
Covered Lines:	3065
Line Coverage:	56.48%
Coverage Strength:	13.94 hits per line

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💛 - Coveralls

[lsjostro]

ping @haraldh 😄

[haraldh]

Thank you!

[lsjostro]

now it's easy to consume this crate with bazel. https://registry.bazel.build/modules/rules_varlink . Thanks @haraldh!