Introduce a ruff_diagnostics crate (#3409)

## Summary

This PR moves `Diagnostic`, `DiagnosticKind`, and `Fix` into their own crate, which will enable us to further split up Ruff, since sub-linter crates (which need to implement functions that return `Diagnostic`) can now depend on `ruff_diagnostics` rather than Ruff.

crates/ruff_diagnostics/src/violation.rs

@@ -0,0 +1,80 @@
+use std::fmt::Debug;
+
+pub enum Availability {
+    Sometimes,
+    Always,
+}
+
+pub struct AutofixKind {
+    pub available: Availability,
+}
+
+impl AutofixKind {
+    pub const fn new(available: Availability) -> Self {
+        Self { available }
+    }
+}
+
+pub trait Violation: Debug + PartialEq + Eq {
+    /// `None` in the case an autofix is never available or otherwise Some
+    /// [`AutofixKind`] describing the available autofix.
+    const AUTOFIX: Option<AutofixKind> = None;
+
+    /// The message used to describe the violation.
+    fn message(&self) -> String;
+
+    /// The explanation used in documentation and elsewhere.
+    fn explanation() -> Option<&'static str> {
+        None
+    }
+
+    /// If autofix is (potentially) available for this violation returns another
+    /// function that in turn can be used to obtain a string describing the
+    /// autofix.
+    fn autofix_title_formatter(&self) -> Option<fn(&Self) -> String> {
+        None
+    }
+
+    /// Returns the format strings used by [`message`](Violation::message).
+    fn message_formats() -> &'static [&'static str];
+}
+
+/// This trait exists just to make implementing the [`Violation`] trait more
+/// convenient for violations that can always be autofixed.
+pub trait AlwaysAutofixableViolation: Debug + PartialEq + Eq {
+    /// The message used to describe the violation.
+    fn message(&self) -> String;
+
+    /// The explanation used in documentation and elsewhere.
+    fn explanation() -> Option<&'static str> {
+        None
+    }
+
+    /// The title displayed for the available autofix.
+    fn autofix_title(&self) -> String;
+
+    /// Returns the format strings used by
+    /// [`message`](AlwaysAutofixableViolation::message).
+    fn message_formats() -> &'static [&'static str];
+}
+
+/// A blanket implementation.
+impl<VA: AlwaysAutofixableViolation> Violation for VA {
+    const AUTOFIX: Option<AutofixKind> = Some(AutofixKind::new(Availability::Always));
+
+    fn message(&self) -> String {
+        <Self as AlwaysAutofixableViolation>::message(self)
+    }
+
+    fn explanation() -> Option<&'static str> {
+        <Self as AlwaysAutofixableViolation>::explanation()
+    }
+
+    fn autofix_title_formatter(&self) -> Option<fn(&Self) -> String> {
+        Some(Self::autofix_title)
+    }
+
+    fn message_formats() -> &'static [&'static str] {
+        <Self as AlwaysAutofixableViolation>::message_formats()
+    }
+}