Struct Message 

pub struct Message {
    pub message: Cow<'static, str>,
    pub class: Option<Class>,
    pub values: Metadata,
}

A diagnostic message with an optional semantic class and named diagnostic values.

Use this instead of chaining message, classification, and scalar-context errors when they describe a single failure. Self::new() starts without a class or values; Self::with_class() and Self::with() add them. Class-based constructors such as crate::not_found() combine the message and class in one step.

Unlike ClassificationMarker, this is a visible diagnostic: it participates in error iteration, downcasting, reports, and cause selection. A marker only adds a classification to an existing error without a diagnostic of its own, preserving that error’s concrete type. Both are inspected by crate::classify(). The class itself isn’t displayed, and crate::types::Classification::error() refers to this error, not a synthetic source.

Preserve real callee errors with ResultExt::or_raise() or Exn::raise(). Keep concrete error types when recovery requires their specific payloads; use classification predicates to recognize categories, and document diagnostic keys on the function returning them. Exn::metadata() and crate::Error::metadata() yield each message’s non-empty value dictionary. Dictionaries from separate contexts aren’t merged. To identify a specific failure without inspecting its values, use Class::Tagged; see matching a specific failure.

Debug formatting omits absent classes and empty values. Present classes omit their Some wrapper, and the class and values stay on single lines, even in pretty output.

Fields

message: Cow<'static, str>

The operation or situation described by these values.

class: Option<Class>

The semantic class of this diagnostic, if known.

values: Metadata

Diagnostic values, ordered by key. Functions returning metadata document their keys.

Implementations

impl Message

pub fn new(message: impl Into<Cow<'static, str>>) -> Self

Create a diagnostic with message, no classification, and no values.

pub fn with_class(self, class: Class) -> Self

Set class, replacing any previous classification without adding a cause.

pub fn with( self, key: impl Into<Cow<'static, str>>, value: impl Into<MetadataValue>, ) -> Self

Add value under key, replacing any previous value in this context. Inspect values through crate::Exn::metadata() after raising, or crate::Error::metadata() after wrapping.

Trait Implementations

impl Debug for Message

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

Formats the value using the given formatter.

impl Display for Message

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

Formats the value using the given formatter.

impl Error for Message

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

impl From<&'static str> for Message

fn from(message: &'static str) -> Self

Converts to this type from the input type.

impl From<Cow<'static, str>> for Message

fn from(message: Cow<'static, str>) -> Self

Converts to this type from the input type.

impl From<String> for Message

fn from(message: String) -> Self

Converts to this type from the input type.