errors: simplify referring to fluent attributes

compiler/rustc_error_messages/src/lib.rs

@@ -234,6 +234,48 @@ pub fn fallback_fluent_bundle(
 /// Identifier for the Fluent message/attribute corresponding to a diagnostic message.
 type FluentId = Cow<'static, str>;
 
+/// Abstraction over a message in a subdiagnostic (i.e. label, note, help, etc) to support both
+/// translatable and non-translatable diagnostic messages.
+///
+/// Translatable messages for subdiagnostics are typically attributes attached to a larger Fluent
+/// message so messages of this type must be combined with a `DiagnosticMessage` (using
+/// `DiagnosticMessage::with_subdiagnostic_message`) before rendering. However, subdiagnostics from
+/// the `SessionSubdiagnostic` derive refer to Fluent identifiers directly.
+pub enum SubdiagnosticMessage {
+    /// Non-translatable diagnostic message.
+    // FIXME(davidtwco): can a `Cow<'static, str>` be used here?
+    Str(String),
+    /// Identifier of a Fluent message. Instances of this variant are generated by the
+    /// `SessionSubdiagnostic` derive.
+    FluentIdentifier(FluentId),
+    /// Attribute of a Fluent message. Needs to be combined with a Fluent identifier to produce an
+    /// actual translated message. Instances of this variant are generated by the `fluent_messages`
+    /// macro.
+    ///
+    /// <https://projectfluent.org/fluent/guide/attributes.html>
+    FluentAttr(FluentId),
+}
+
+impl SubdiagnosticMessage {
+    /// Create a `SubdiagnosticMessage` for the provided Fluent attribute.
+    pub fn attr(id: impl Into<FluentId>) -> Self {
+        SubdiagnosticMessage::FluentAttr(id.into())
+    }
+
+    /// Create a `SubdiagnosticMessage` for the provided Fluent identifier.
+    pub fn message(id: impl Into<FluentId>) -> Self {
+        SubdiagnosticMessage::FluentIdentifier(id.into())
+    }
+}
+
+/// `From` impl that enables existing diagnostic calls to functions which now take
+/// `impl Into<SubdiagnosticMessage>` to continue to work as before.
+impl<S: Into<String>> From<S> for SubdiagnosticMessage {
+    fn from(s: S) -> Self {
+        SubdiagnosticMessage::Str(s.into())
+    }
+}
+
 /// Abstraction over a message in a diagnostic to support both translatable and non-translatable
 /// diagnostic messages.
 ///
@@ -252,6 +294,29 @@ pub enum DiagnosticMessage {
 }
 
 impl DiagnosticMessage {
+    /// Given a `SubdiagnosticMessage` which may contain a Fluent attribute, create a new
+    /// `DiagnosticMessage` that combines that attribute with the Fluent identifier of `self`.
+    ///
+    /// - If the `SubdiagnosticMessage` is non-translatable then return the message as a
+    /// `DiagnosticMessage`.
+    /// - If `self` is non-translatable then return `self`'s message.
+    pub fn with_subdiagnostic_message(&self, sub: SubdiagnosticMessage) -> Self {
+        let attr = match sub {
+            SubdiagnosticMessage::Str(s) => return DiagnosticMessage::Str(s.clone()),
+            SubdiagnosticMessage::FluentIdentifier(id) => {
+                return DiagnosticMessage::FluentIdentifier(id, None);
+            }
+            SubdiagnosticMessage::FluentAttr(attr) => attr,
+        };
+
+        match self {
+            DiagnosticMessage::Str(s) => DiagnosticMessage::Str(s.clone()),
+            DiagnosticMessage::FluentIdentifier(id, _) => {
+                DiagnosticMessage::FluentIdentifier(id.clone(), Some(attr))
+            }
+        }
+    }
+
     /// Returns the `String` contained within the `DiagnosticMessage::Str` variant, assuming that
     /// this diagnostic message is of the legacy, non-translatable variety. Panics if this
     /// assumption does not hold.
@@ -266,14 +331,9 @@ impl DiagnosticMessage {
     }
 
     /// Create a `DiagnosticMessage` for the provided Fluent identifier.
-    pub fn fluent(id: impl Into<FluentId>) -> Self {
+    pub fn new(id: impl Into<FluentId>) -> Self {
         DiagnosticMessage::FluentIdentifier(id.into(), None)
     }
-
-    /// Create a `DiagnosticMessage` for the provided Fluent identifier and attribute.
-    pub fn fluent_attr(id: impl Into<FluentId>, attr: impl Into<FluentId>) -> Self {
-        DiagnosticMessage::FluentIdentifier(id.into(), Some(attr.into()))
-    }
 }
 
 /// `From` impl that enables existing diagnostic calls to functions which now take