Rust Compile Time Invariants

Rust Compile-Time Invariants

Core rule: put the compiler in charge of enforcing invariants. Every pattern below converts a runtime check or convention into a compile error when the type evolves.

Construction & invariants

_private: () enforces intra-crate; #[non_exhaustive] enforces cross-crate. Pick by scope.

Pattern matching that survives refactors

// Slice patterns — no indexing, length and access in one step
match users.as_slice() {
    []            => /* empty */,
    [only]        => /* exactly one */,
    [first, rest @ ..] => /* head/tail */,
}

// Destructure in trait impls so adding a field is a compile error
impl PartialEq for Order {
    fn eq(&self, other: &Self) -> bool {
        let Self { size, toppings, ordered_at: _ } = self;   // _ is intentional
        let Self { size: s2, toppings: t2, ordered_at: _ } = other;
        size == s2 && toppings == t2
    }
}

// Exhaustive enum match — no `_` arm
match state {
    State::Init | State::Ready => start(),
    State::Running             => tick(),
    State::Done                => cleanup(),
}   // adding State::Failed forces a compile error here

// Default fill that breaks loudly when fields are added
let Foo { a, b, c, d } = Foo::default();
let foo = Foo { a: my_a, b: my_b, c, d };   // not `..Default::default()`

API shape

• Replace boolean parameters with enums or a parameter struct. process(data, true, false, true) is unreadable; process(data, Params::production()) is.
• Scoped temporary mutability — bind once, mutate inside a block, return immutable:

  let data = { let mut v = fetch(); v.sort(); v };
  

• Name ignored fields (has_fuel: _, not bare _) so reviewers can see what was deliberately skipped.

Clippy enforcement

Add to Cargo.toml:

[lints.clippy]
indexing_slicing            = "deny"   # forces slice patterns / .get()
fallible_impl_from          = "deny"   # forces TryFrom when conversion can fail
wildcard_enum_match_arm     = "deny"   # bans `_ =>` on enums
wildcard_in_or_patterns     = "deny"
unneeded_field_pattern      = "deny"   # bans `..` when fields could be named
fn_params_excessive_bools   = "deny"
must_use_candidate          = "warn"

Pitfalls

• #[non_exhaustive] blocks struct-literal construction only across crates. Inside the defining crate, anyone can still build the struct directly — combine with _private: () if you also need intra-crate enforcement.
• #[must_use] on a function applies to the return value, not the function. To force callers to use a type everywhere it appears, put #[must_use] on the type definition.
• wildcard_enum_match_arm fires on _ => unreachable!() too; use explicit Variant::A | Variant::B => unreachable!() so future variants surface.
• Self { ordered_at: _, .. } defeats the destructure-forces-update trick — .. silently absorbs new fields. Use named _ for each ignored field, never ...
• Sealed types via private field break #[derive(Default)] and serde's default deserialization — provide an explicit Default impl that goes through the constructor, or accept that defaults are unavailable.
• TryFrom is auto-implemented for any From, so adding TryFrom next to an existing From causes a conflicting-impl error; remove the From first.