Enum Unknown 

pub enum Unknown {
    Recovery,
    DynamicBoundary,
    Unconstrained,
    Var(u32),
    Placeholder,
}

Why the checker does not know a type.

This is the classification the module documentation describes, carried by the type rather than implied by which constructor built it. Every kind compares equal to every type — telling them apart decides what a reader is told, never what type-checks — so the only thing this changes about the checking rules is that a form can ask whether the unknown standing in for its context has already been accounted for.

Variants

Recovery

An error was already reported about this place, or about the value this one was derived from. Silent.

DynamicBoundary

A host module no schema describes — neither a shipped one nor one an embedder handed over. The remedy is at the use that names the module, not here.

Unconstrained

Nothing that has been read states this type: a shipped schema’s HostType::Any, or a type parameter no argument, annotation, or expected type settles.

Var(u32)

An unconstrained unknown carrying an identity, so that a later use can say what it is.

This is not a fifth kind of not-knowing. It is the same fact as Unknown::Unconstrained — a type parameter nothing read so far settles — with a number attached, and the number is what lets the uses of a local binding after its initializer settle the type its initializer left open. See the module documentation under “Inference variables”: one never leaves the body that minted it, and what is left in Facts is either the type its uses settled or a plain unconstrained unknown.

Placeholder

A position no reachable program observes.

This is not a classification of a program’s type. It marks the internal positions the surrounding form settles before anything reads them and the ones no reachable program produces at all, and Checker::expr and Checker::declare assert in debug builds that one never reaches a type a program can observe. If one ever does, the assertion names the site rather than leaving the unknown to validate whatever came after it.

Trait Implementations

impl Clone for Unknown

fn clone(&self) -> Unknown

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Unknown

impl Debug for Unknown

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for Unknown

impl PartialEq for Unknown

fn eq(&self, other: &Unknown) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for Unknown