Diagnostic Internals: Messages and Arguments

from Fluent resources while building the compiler, preventing invalid Fluent resources from causing panics in the compiler. Compile-time validation also emits an error if multiple Fluent messages have the same identifier. ## Internals Various parts of rustc's diagnostic internals are modified in order to support translation. ### Messages All of rustc's traditional diagnostic APIs (e.g. `struct_span_err` or `note`) take any message that can be converted into a `DiagMessage` (or `SubdiagMessage`). [`rustc_error_messages::DiagMessage`] can represent legacy non-translatable diagnostic messages and translatable messages. Non-translatable messages are just `String`s. Translatable messages are just a `&'static str` with the identifier of the Fluent message (sometimes with an additional `&'static str` with an attribute). `DiagMessage` never needs to be interacted with directly: `DiagMessage` constants are created for each diagnostic message in a Fluent resource (described in more detail below), or `DiagMessage`s will either be created in the macro-generated code of a diagnostic derive. `rustc_error_messages::SubdiagMessage` is similar, it can correspond to a legacy non-translatable diagnostic message or the name of an attribute to a Fluent message. Translatable `SubdiagMessage`s must be combined with a `DiagMessage` (using `DiagMessage::with_subdiagnostic_message`) to be emitted (an attribute name on its own is meaningless without a corresponding message identifier, which is what `DiagMessage` provides). Both `DiagMessage` and `SubdiagMessage` implement `Into` for any type that can be converted into a string, and converts these into non-translatable diagnostics - this keeps all existing diagnostic calls working. ### Arguments Additional context for Fluent messages which are interpolated into message contents needs to be provided to translatable diagnostics. Diagnostics have a `set_arg` function that can be used to provide this additional context to a diagnostic. Arguments have both a name (e.g. "what" in the earlier example) and a value. Argument values are represented using the `DiagArgValue` type, which is just a string or a number. rustc types can implement `IntoDiagArg` with conversion into a string or a number, and common types like `Ty<'tcx>` already have such implementations.

This section delves into the internal modifications within rustc to support translation. It discusses how `DiagMessage` and `SubdiagMessage` represent translatable and non-translatable diagnostic messages, and how they interact. It also explains how additional context for Fluent messages is provided through arguments using the `set_arg` function and the `DiagArgValue` type, allowing for interpolation of values like strings or numbers into the message.