Write down a Rust field's starting value when no constructor sets it [patch]

[matt-edmondson]

Fixes #202

What changed

If a field has an InitialValue and its type declares no constructor, the Rust generator used to write only name: type,. Nothing in the output showed the declared value had been dropped. Now it writes a note above the field, the same way Go and C already handle this case:

pub struct Counter {
    // count defaults to 0: Rust has no field initialisers, and no constructor sets it
    count: i32,
    // step defaults to 1: Rust has no field initialisers, and no constructor sets it
    step: i32,
}

A type that has a constructor gets no note. The constructor already builds any field it doesn't set from its declared value (#193).

The issue offered two options: a note, or a generated impl Default / fn new(). This PR takes the note option because it matches the Go and C output and doesn't make up any API. Generating Default would be a separate design change.

Tests

• RustGeneratorTests.FieldInitialValue_WithoutAConstructor_IsANote: this test fails when the generator change is reverted and passes with it.
• RustGeneratorTests.FieldInitialValue_WithAConstructor_IsNotANote
• Full suite: 1282 passed, 0 failed (rustc and gcc were on the path).

docs/gallery/generated-rust.png shows the editor's sample Counter, so it will now be slightly out of date. I didn't regenerate it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LJsBos8vaJcor3uRqngfwE

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud