Fix LSP issues found by the new tests

[Rigidity]

Stacked on #105. This fixes the behaviour that #105's snapshots recorded but left alone. Each fix is a separate commit, and the snapshot diffs show exactly what changed.

Changes

UTF-16 positions (2f76b93)

• LSP positions count characters in UTF-16 code units. The server treated them as character counts and then used the result as a byte offset, so hover, definitions and diagnostics were off after any non-ASCII text on the line.
• Added position.rs, which converts between byte offsets and LSP positions in both directions. It's used for incoming cursor positions, locations and diagnostics.
• LineCol::new now treats its index as a byte offset, which is what callers pass. This also corrects CLI error columns after non-ASCII text.

Declaration keywords and builtin signatures (4e504af)

• Hover now shows fn name<T>(params) -> Ret, const NAME: T and name: T for parameters, and adds inline where it applies. Previously everything was shown as let.
• Builtins have no function type, so completions listed them as {unresolved}. Builtin::signatures() in rue-hir now gives a hand-written signature for each way a builtin can be called, matching the rules in compile_builtin. Hover lists all of them; the completion detail shows the first.

Generic arguments (f5957af)

• substitute copied aliases and structs but kept their declared generic parameters, so List<Int> was printed as List. The generics are now substituted along with the inner type, and stringify includes them for named aliases and structs.
• Arguments of an Apply type are now printed in the order of their parameters. Previously they followed HashMap order.
• Hover on a declaration shows its generic parameters, e.g. type List<T> = nil | (T, List<T>) and struct Wrapper<T> { ... }.

Completion in empty type positions (b644e2c)

• Completion previously suggested nothing until at least one character of the type had been typed. It now checks the token before the cursor in the syntax tree. Colons in parameters, let, const and struct fields count as type positions, as do ->, as, is, and the = of a type alias.
• A colon in a struct initializer is not a type position, so no types are suggested there. It still suggests nothing at all, and expressions should really be offered (marked with a TODO).

Test plan

• cargo test --workspace --all-features: 75 LSP tests, up from 53, plus new position and LineCol unit tests. The compiler's YAML snapshots are unchanged.
• cargo clippy --workspace --all-features --all-targets (no warnings), cargo fmt --check, cargo machete, rue fmt --check

---

Note

Medium Risk
Touches position math used by all LSP features and type substitution/stringify shared beyond the server; regressions could misalign diagnostics or change displayed types.

Overview
Fixes several Rue LSP behaviors that broke on non-ASCII text, empty type positions, and richer type/symbol display.

UTF-16 positions: Adds position.rs to convert between byte offsets and LSP positions (UTF-16 code units). Hover, go-to-definition, references, diagnostics, and cursor mapping now use it. LineCol treats indices as byte offsets and counts columns in Unicode characters, aligning CLI columns with the same model.

Hover and completions: Hover distinguishes bindings (let), parameters, const/inline const, and fn/inline fn with full signatures (generics, spread params). Builtin::signatures() supplies completion details and multi-signature hover for builtins instead of {unresolved}.

Generic types: substitute now substitutes generic parameters on structs/aliases; stringify prints Name<Args> in parameter order for Apply types. Hover shows generic declarations (e.g. List<T>, Wrapper<T>).

Completions: When no type AST exists yet, syntax-aware expects_type offers types after ->, as, is, annotated colons, and type-alias =—but not struct initializer values.

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[cursor]

Inline let hover drops keyword

Low Severity

SymbolHoverKind::Binding has no inline flag, and symbol_hover_kind ignores BindingSymbol.inline. Hover therefore always renders let, even when the binding was declared inline let. Constants and functions in this change do surface inline.

Additional Locations (1)

• crates/rue-lsp/src/lib.rs#L260-L263

 

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