types: bare Ok/Err/Some constructors as sugar for Result::Ok/Err and Option::Some

[rfunix]

Summary

Closes #52.

Implements bare Ok(v), Err(e), and Some(v) constructors as idiomatic sugar for the qualified forms Result::Ok(v), Result::Err(e), and Option::Some(v).

Before this PR, the following code failed with undefined type \Ok``:

fn find_user(id: Int) -> Result<String, AppError> {
    if id == 1 { return Ok("alice") }
    return Err(AppError::NotFound)
}

After this PR, both forms are accepted:

// qualified (already worked)
return Result::Ok("alice")
return Result::Err(AppError::NotFound)

// bare (new — matches the issue #52 target)
return Ok("alice")
return Err(AppError::NotFound)

Changes

• crates/kodo_types/src/expr.rs: In check_call, intercept bare Ok, Err, and Some identifiers before the generic-function lookup and delegate to check_enum_variant_expr("Result"/"Option", ...). This reuses all existing type-inference, monomorphisation, and ownership-tracking logic.

• crates/kodo_mir/src/lowering/expr.rs: In lower_call, mirror the type-checker interception: redirect Ok/Err/Some to lower_enum_variant so codegen emits the correct discriminant-based EnumVariant instruction.

• tests/ui/types/result_bare_constructors.ko + .stdout: New run-pass UI test covering all three match patterns (Ok(name), Err(AppError::Variant), Err(_)).

• examples/result_bare_constructors.ko: Runnable example demonstrating the feature end-to-end.

Test plan

• cargo fmt --all -- --check — passes
• cargo clippy --workspace -- -D warnings — zero warnings
• cargo test --workspace — all 1,063 tests pass
• make ui-test — 84 UI tests pass (1 new)
• Example builds and produces correct output (alice, unauthorized, not found)

🤖 Generated with Claude Code

[rfunix]

Code Review — REVIEWER agent

Grade: B (Bom, com um bug menor a corrigir antes do merge)

✅ Pontos positivos

• Correção: A abordagem de interceptar Ok/Err/Some no type checker e delegar para check_enum_variant_expr é cirúrgica e reutiliza toda a lógica existente de inferência, monomorphização e ownership tracking.
• CI: Todos os 12 checks passando (clippy, fmt, tests, fuzz, docs, coverage, MSRV).
• Testes: UI test run-pass com output verificado + exemplo completo cobrindo todos os casos de AppError.
• Comentários: Doc comments explicativos no diff, com referência a [TAPL] Ch.22 — adequado.
• Zero unwrap/expect introduzidos no diff do PR.
• Zero unsafe no diff.

---

🐛 Bug: None implementado no type checker mas ausente no MIR lowering

Arquivo: crates/kodo_mir/src/lowering/expr.rs

O type checker em kodo_types/src/expr.rs intercepta None (linha adicionada):

if name == "None" && args.is_empty() {
    return self.check_enum_variant_expr("Option", "None", args, span);
}

Mas o MIR lowering em crates/kodo_mir/src/lowering/expr.rs não trata None:

match callee_name.as_str() {
    "Ok"   => return self.lower_enum_variant("Result", "Ok", args),
    "Err"  => return self.lower_enum_variant("Result", "Err", args),
    "Some" => return self.lower_enum_variant("Option", "Some", args),
    // None está faltando aqui!
    _ => {}
}

Consequência: código com return None ou let x: Option<Int> = None passa type checking mas falha no MIR lowering / codegen — bug silencioso em runtime.

Fix necessário — adicionar ao match em lowering/expr.rs:

"None" => return self.lower_enum_variant("Option", "None", &[]),

Também recomendado: adicionar um UI test para None bare constructor:

fn maybe_value(flag: Bool) -> Option<Int> {
    if flag { return Some(42) }
    return None
}

---

Minor

• O UI test cobre Ok/Err/Some mas não None — adicionar após o fix acima.
• Os comentários no diff mencionam Option::None em dois lugares distintos — ligeiramente redundante mas não problemático.

Ação: corrigir o None no MIR lowering + adicionar teste antes do merge.