Rust output fails to compile (E0308) wherever a string literal is returned as, or assigned to, a `str`/`string` type, which maps to owned `String`

[matt-edmondson]

What's wrong

RustGenerator's TypeMappings maps str and string to the owned type String. A string literal is written as a bare "..." (LanguageGeneratorBase, and RustGenerator does not override it), which in Rust is a &'static str. Wherever the two meet, the generated code does not compile.

Reproduction

Found with a scratch app against Coder at aae2bb6, and the output compiled with rustc (edition 2021):

AST	Rust output	rustc
FunctionDeclaration("greet") { ReturnType = "str" } whose body is ReturnStatement(Literal.Text("hello"))	pub fn greet() -> String { return "hello"; }	error[E0308]: mismatched types (expected String, found &str)
VariableDeclaration("name", "str", Literal.Text("x"))	let mut name: String = "x";	error[E0308]: mismatched types

The same AST compiles in the other targets. The tests miss it because the Rust compile tests only use &str parameters, and RustGeneratorTests.Types_AreMappedToRustSpellings asserts -> String without ever returning a literal.

Suggested fix

When a string literal's target is an owned String, write String::from("…") (or "…".to_string()). The target can be:

• the return of a function whose return type maps to String
• the initialiser of a String-typed local, let, or field
• a field value in a construction

Keep the bare literal where the target is &str or unknown. A simpler alternative is to always write String::from("…") for text literals in Rust. That is correct everywhere an owned String is expected, and borrows to &str automatically only in some positions, so check the call-argument cases.

Acceptance criteria

• A rustc compile test that returns and assigns a string literal to str/string-typed targets builds cleanly.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. Rust output does not compile for a very common pattern: returning or assigning a string literal to a str/string type. Other targets are unaffected.
• Area / suggested assignee: RustGenerator literal emission and TypeMappings. Maintainer: @matt-edmondson.
• Duplicates / in progress: No duplicate. The open PRs Spell a mutable static C++ field inline static inside a type #88–Write whole-number doubles with a fraction so they stay floating-point #90 cover C++ static fields, enum numbering and double literals, so none of them touches this. It is part of a cluster of Rust compile failures with Rust generator emits every parameter immutable, so a body that assigns to a parameter fails to compile (E0384) #85 (immutable parameters) and Rust generator turns static/const fields inside a type into per-instance struct fields and silently drops their value #81 (static fields).
• Notes: Writing String::from("…") for every text literal is the simpler fix. Check it against call arguments that expect &str, where it would need &, and against the existing &str parameter compile tests.

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