Struct Error 

pub struct Error {
    pub kind: ErrorKind,
    pub span: NodeSpan,
    pub declaration: Name,
    /* private fields */
}

A type error: what went wrong, where, and in which declaration.

Where the positions come from

The typer does not check the canonical AST directly — it translates it into its own Term language — but that translation now carries the canonical node’s [NodeSpan] along, and each constraint keeps the span of the term that produced it. So a unification failure knows the sub-expression it is about, and labels draws the caret there.

The declaration’s own span is still kept as span, for the one case that has no finer answer: an error whose origin came from a hand-built term, or from a canonical node the parser never spanned. Then the label degrades to the whole declaration rather than disappearing.

Fields

kind: ErrorKind

span: NodeSpan

Where the declaration the error was found in was written. Used only as the fallback described above.

declaration: Name

That declaration’s name, which is what the note names: in a module of a hundred declarations, a caret is not much use without it when the diagnostic is rendered without a file (see compile_package).

Trait Implementations

impl Debug for Error

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

Formats the value using the given formatter.

impl PhaseError for Error

Type errors are about types, and Type’s Display writes them the way the source does — so the message can name both sides of a mismatch instead of dumping the typer’s internal representation.

fn labels(&self) -> Vec<SpanLabel>

A caret under the text that disagrees, and — when inference can say where the type it disagrees with came from — a second, secondary one under that.

Falls back to underlining the whole declaration when the failing constraint has no position, which is what a term built by hand, or one translated from a canonical node with no span, produces.

fn message(&self) -> String

One line naming what went wrong, in the vocabulary of the user’s source.

fn notes(&self) -> Vec<String>

Supporting detail, one string per rendered note. Empty by default.

fn message_and_notes(&self) -> Vec<String>

This error’s message followed by its notes.