Apply fix availability and applicability when adding to DiagnosticGuard and remove NoqaCode::rule (#18834)

## Summary

This PR removes the last two places we were using `NoqaCode::rule` in
`linter.rs` (see
https://github.com/astral-sh/ruff/pull/18391#discussion_r2154637329 and
https://github.com/astral-sh/ruff/pull/18391#discussion_r2154649726) by
checking whether fixes are actually desired before adding them to a
`DiagnosticGuard`. I implemented this by storing a `Violation`'s `Rule`
on the `DiagnosticGuard` so that we could check if it was enabled in the
embedded `LinterSettings` when trying to set a fix.

All of the corresponding `set_fix` methods on `OldDiagnostic` were now
unused (except in tests where I just set `.fix` directly), so I moved
these to the guard instead of keeping both sets.

The very last place where we were using `NoqaCode::rule` was in the
cache. I just reverted this to parsing the `Rule` from the name. I had
forgotten to update the comment there anyway. Hopefully this doesn't
cause too much of a perf hit.

In terms of binary size, we're back down almost to where `main` was two
days ago
(https://github.com/astral-sh/ruff/pull/18391#discussion_r2155034320):

```
41,559,344 bytes for main 2 days ago
41,669,840 bytes for #18391
41,653,760 bytes for main now (after #18391 merged)
41,602,224 bytes for this branch
```

Only 43 kb up, but that shouldn't all be me this time :)

## Test Plan

Existing tests and benchmarks on this PR

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -28,7 +28,7 @@ use itertools::Itertools;
 use log::debug;
 use rustc_hash::{FxHashMap, FxHashSet};
 
-use ruff_diagnostics::IsolationLevel;
+use ruff_diagnostics::{Applicability, Fix, IsolationLevel};
 use ruff_notebook::{CellOffsets, NotebookIndex};
 use ruff_python_ast::helpers::{collect_import_from_member, is_docstring_stmt, to_module_path};
 use ruff_python_ast::identifier::Identifier;
@@ -3154,6 +3154,7 @@ impl<'a> LintContext<'a> {
         DiagnosticGuard {
             context: self,
             diagnostic: Some(OldDiagnostic::new(kind, range, &self.source_file)),
+            rule: T::rule(),
         }
     }
 
@@ -3167,10 +3168,12 @@ impl<'a> LintContext<'a> {
         kind: T,
         range: TextRange,
     ) -> Option<DiagnosticGuard<'chk, 'a>> {
-        if self.is_rule_enabled(T::rule()) {
+        let rule = T::rule();
+        if self.is_rule_enabled(rule) {
             Some(DiagnosticGuard {
                 context: self,
                 diagnostic: Some(OldDiagnostic::new(kind, range, &self.source_file)),
+                rule,
             })
         } else {
             None
@@ -3222,6 +3225,7 @@ pub(crate) struct DiagnosticGuard<'a, 'b> {
     ///
     /// This is always `Some` until the `Drop` (or `defuse`) call.
     diagnostic: Option<OldDiagnostic>,
+    rule: Rule,
 }
 
 impl DiagnosticGuard<'_, '_> {
@@ -3234,6 +3238,50 @@ impl DiagnosticGuard<'_, '_> {
     }
 }
 
+impl DiagnosticGuard<'_, '_> {
+    fn resolve_applicability(&self, fix: &Fix) -> Applicability {
+        self.context
+            .settings
+            .fix_safety
+            .resolve_applicability(self.rule, fix.applicability())
+    }
+
+    /// Set the [`Fix`] used to fix the diagnostic.
+    #[inline]
+    pub(crate) fn set_fix(&mut self, fix: Fix) {
+        if !self.context.rules.should_fix(self.rule) {
+            self.fix = None;
+            return;
+        }
+        let applicability = self.resolve_applicability(&fix);
+        self.fix = Some(fix.with_applicability(applicability));
+    }
+
+    /// Set the [`Fix`] used to fix the diagnostic, if the provided function returns `Ok`.
+    /// Otherwise, log the error.
+    #[inline]
+    pub(crate) fn try_set_fix(&mut self, func: impl FnOnce() -> anyhow::Result<Fix>) {
+        match func() {
+            Ok(fix) => self.set_fix(fix),
+            Err(err) => log::debug!("Failed to create fix for {}: {}", self.name(), err),
+        }
+    }
+
+    /// Set the [`Fix`] used to fix the diagnostic, if the provided function returns `Ok`.
+    /// Otherwise, log the error.
+    #[inline]
+    pub(crate) fn try_set_optional_fix(
+        &mut self,
+        func: impl FnOnce() -> anyhow::Result<Option<Fix>>,
+    ) {
+        match func() {
+            Ok(None) => {}
+            Ok(Some(fix)) => self.set_fix(fix),
+            Err(err) => log::debug!("Failed to create fix for {}: {}", self.name(), err),
+        }
+    }
+}
+
 impl std::ops::Deref for DiagnosticGuard<'_, '_> {
     type Target = OldDiagnostic;
 

crates/ruff_linter/src/fix/edits.rs

@@ -749,15 +749,16 @@ x = 1 \
         let diag = {
             use crate::rules::pycodestyle::rules::MissingNewlineAtEndOfFile;
             let mut iter = edits.into_iter();
-            OldDiagnostic::new(
+            let mut diagnostic = OldDiagnostic::new(
                 MissingNewlineAtEndOfFile, // The choice of rule here is arbitrary.
                 TextRange::default(),
                 &SourceFileBuilder::new("<filename>", "<code>").finish(),
-            )
-            .with_fix(Fix::safe_edits(
+            );
+            diagnostic.fix = Some(Fix::safe_edits(
                 iter.next().ok_or(anyhow!("expected edits nonempty"))?,
                 iter,
-            ))
+            ));
+            diagnostic
         };
         assert_eq!(apply_fixes([diag].iter(), &locator).code, expect);
         Ok(())

crates/ruff_linter/src/fix/mod.rs

@@ -186,12 +186,13 @@ mod tests {
         edit.into_iter()
             .map(|edit| {
                 // The choice of rule here is arbitrary.
-                let diagnostic = OldDiagnostic::new(
+                let mut diagnostic = OldDiagnostic::new(
                     MissingNewlineAtEndOfFile,
                     edit.range(),
                     &SourceFileBuilder::new(filename, source).finish(),
                 );
-                diagnostic.with_fix(Fix::safe_edit(edit))
+                diagnostic.fix = Some(Fix::safe_edit(edit));
+                diagnostic
             })
             .collect()
     }

crates/ruff_linter/src/linter.rs

@@ -378,32 +378,7 @@ pub fn check_path(
 
     let (mut diagnostics, source_file) = context.into_parts();
 
-    if parsed.has_valid_syntax() {
-        // Remove fixes for any rules marked as unfixable.
-        for diagnostic in &mut diagnostics {
-            if diagnostic
-                .noqa_code()
-                .and_then(|code| code.rule())
-                .is_none_or(|rule| !settings.rules.should_fix(rule))
-            {
-                diagnostic.fix = None;
-            }
-        }
-
-        // Update fix applicability to account for overrides
-        if !settings.fix_safety.is_empty() {
-            for diagnostic in &mut diagnostics {
-                if let Some(fix) = diagnostic.fix.take() {
-                    if let Some(rule) = diagnostic.noqa_code().and_then(|code| code.rule()) {
-                        let fixed_applicability = settings
-                            .fix_safety
-                            .resolve_applicability(rule, fix.applicability());
-                        diagnostic.set_fix(fix.with_applicability(fixed_applicability));
-                    }
-                }
-            }
-        }
-    } else {
+    if !parsed.has_valid_syntax() {
         // Avoid fixing in case the source code contains syntax errors.
         for diagnostic in &mut diagnostics {
             diagnostic.fix = None;

crates/ruff_linter/src/message/mod.rs

@@ -186,41 +186,6 @@ impl OldDiagnostic {
         )
     }
 
-    /// Consumes `self` and returns a new `Diagnostic` with the given `fix`.
-    #[inline]
-    #[must_use]
-    pub fn with_fix(mut self, fix: Fix) -> Self {
-        self.set_fix(fix);
-        self
-    }
-
-    /// Set the [`Fix`] used to fix the diagnostic.
-    #[inline]
-    pub fn set_fix(&mut self, fix: Fix) {
-        self.fix = Some(fix);
-    }
-
-    /// Set the [`Fix`] used to fix the diagnostic, if the provided function returns `Ok`.
-    /// Otherwise, log the error.
-    #[inline]
-    pub fn try_set_fix(&mut self, func: impl FnOnce() -> anyhow::Result<Fix>) {
-        match func() {
-            Ok(fix) => self.fix = Some(fix),
-            Err(err) => log::debug!("Failed to create fix for {}: {}", self.name(), err),
-        }
-    }
-
-    /// Set the [`Fix`] used to fix the diagnostic, if the provided function returns `Ok`.
-    /// Otherwise, log the error.
-    #[inline]
-    pub fn try_set_optional_fix(&mut self, func: impl FnOnce() -> anyhow::Result<Option<Fix>>) {
-        match func() {
-            Ok(None) => {}
-            Ok(Some(fix)) => self.fix = Some(fix),
-            Err(err) => log::debug!("Failed to create fix for {}: {}", self.name(), err),
-        }
-    }
-
     /// Consumes `self` and returns a new `Diagnostic` with the given parent node.
     #[inline]
     #[must_use]

crates/ruff_linter/src/registry.rs

@@ -215,6 +215,12 @@ pub enum Linter {
 }
 
 pub trait RuleNamespace: Sized {
+    /// Returns the prefix that every single code that ruff uses to identify
+    /// rules from this linter starts with.  In the case that multiple
+    /// `#[prefix]`es are configured for the variant in the `Linter` enum
+    /// definition this is the empty string.
+    fn common_prefix(&self) -> &'static str;
+
     /// Attempts to parse the given rule code. If the prefix is recognized
     /// returns the respective variant along with the code with the common
     /// prefix stripped.

crates/ruff_linter/src/rule_selector.rs

@@ -265,6 +265,7 @@ mod schema {
     use strum::IntoEnumIterator;
 
     use crate::RuleSelector;
+    use crate::registry::RuleNamespace;
     use crate::rule_selector::{Linter, RuleCodePrefix};
 
     impl JsonSchema for RuleSelector {