Append the fix title to a Diagnostic on drop

Summary
--

As noted in #22972 (comment),
attaching sub-diagnostics to Ruff's diagnostics can interact a bit awkwardly
with how we attach the `Violation::fix_title` and then render a fix diff. In
particular, the fix title is currently attached as a `help` sub-diagnostic when
the diagnostic is originally created, meaning that any additional
sub-diagnostics appear between the fix title and the rendered fix:

```
RUF064 [*] Non-octal mode
 --> RUF064.py:6:17
  |
4 | from pathlib import Path
5 |
6 | os.chmod("foo", 444)  # Error
  |                 ^^^
7 | os.chmod("foo", 0o444)  # OK
8 | os.chmod("foo", 7777)  # Error
  |
help: Replace with octal literal
info: Current value of mode 444 (0o674) sets permissions: u=rw-, g=rwx, o=r--)
info: Suggested value of 292 sets permissions: u=r--, g=r--, o=r--
3 | import os
4 | from pathlib import Path
5 |
  - os.chmod("foo", 444)  # Error
6 + os.chmod("foo", 0o444)  # Error
7 | os.chmod("foo", 0o444)  # OK
8 | os.chmod("foo", 7777)  # Error
9 | os.chmod("foo", 10000)  # Error
note: This is an unsafe fix and may change runtime behavior
```

Instead of adding the fix title immediately, this PR stores it on our
`DiagnosticGuard` type to be added just before the guard is dropped and the
diagnostic is stored.

I think a better long-term fix would be to attach the diff to the sub-diagnostic
with the `help` message/fix title somehow and render these `info`
sub-diagnostics _after_ the diff (or let the diagnostic author choose the
order), but this seemed like an easy improvement over the current approach, at
least.

Test Plan
--

Existing tests for ISC004 and from #22972 showing the `help` message at the end.

Author: ntBre

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

@@ -3365,6 +3365,7 @@ impl<'a> LintContext<'a> {
     ) -> DiagnosticGuard<'chk, 'a> {
         DiagnosticGuard {
             context: self,
+            fix_title: kind.fix_title(),
             diagnostic: Some(kind.into_diagnostic(range, &self.source_file)),
             rule: T::rule(),
         }
@@ -3384,6 +3385,7 @@ impl<'a> LintContext<'a> {
         if self.is_rule_enabled(rule) {
             Some(DiagnosticGuard {
                 context: self,
+                fix_title: kind.fix_title(),
                 diagnostic: Some(kind.into_diagnostic(range, &self.source_file)),
                 rule,
             })
@@ -3447,6 +3449,11 @@ pub(crate) struct DiagnosticGuard<'a, 'b> {
     ///
     /// This is always `Some` until the `Drop` (or `defuse`) call.
     diagnostic: Option<Diagnostic>,
+    /// The optional help message to display.
+    ///
+    /// This is stored here so that it can be added to the `Diagnostic` in the `Drop` impl and
+    /// render just above the diff, even if other sub-diagnostics have been added.
+    fix_title: Option<String>,
     rule: Rule,
 }
 
@@ -3592,7 +3599,10 @@ impl Drop for DiagnosticGuard<'_, '_> {
             return;
         }
 
-        if let Some(diagnostic) = self.diagnostic.take() {
+        if let Some(mut diagnostic) = self.diagnostic.take() {
+            if let Some(fix_title) = self.fix_title.take() {
+                diagnostic.help(fix_title);
+            }
             self.context.diagnostics.borrow_mut().push(diagnostic);
         }
     }

crates/ruff_linter/src/message/mod.rs

@@ -75,10 +75,8 @@ pub fn create_panic_diagnostic(error: &PanicError, path: Option<&Path>) -> Diagn
     diagnostic
 }
 
-#[expect(clippy::too_many_arguments)]
-pub fn create_lint_diagnostic<B, S>(
+pub fn create_lint_diagnostic<B>(
     body: B,
-    suggestion: Option<S>,
     range: TextRange,
     fix: Option<Fix>,
     parent: Option<TextSize>,
@@ -88,7 +86,6 @@ pub fn create_lint_diagnostic<B, S>(
 ) -> Diagnostic
 where
     B: Display,
-    S: Display,
 {
     let mut diagnostic = Diagnostic::new(
         DiagnosticId::Lint(LintName::of(rule.into())),
@@ -108,10 +105,6 @@ where
     }
     diagnostic.annotate(annotation);
 
-    if let Some(suggestion) = suggestion {
-        diagnostic.help(suggestion);
-    }
-
     if let Some(fix) = fix {
         diagnostic.set_fix(fix);
     }
@@ -278,9 +271,8 @@ def fibonacci(n):
         let fib_source = SourceFileBuilder::new("fib.py", fib).finish();
 
         let unused_import_start = TextSize::from(7);
-        let unused_import = create_lint_diagnostic(
+        let mut unused_import = create_lint_diagnostic(
             "`os` imported but unused",
-            Some("Remove unused import: `os`"),
             TextRange::new(unused_import_start, TextSize::from(9)),
             Some(Fix::unsafe_edit(Edit::range_deletion(TextRange::new(
                 TextSize::from(0),
@@ -291,11 +283,11 @@ def fibonacci(n):
             Some(unused_import_start),
             Rule::UnusedImport,
         );
+        unused_import.help("Remove unused import: `os`");
 
         let unused_variable_start = TextSize::from(94);
-        let unused_variable = create_lint_diagnostic(
+        let mut unused_variable = create_lint_diagnostic(
             "Local variable `x` is assigned to but never used",
-            Some("Remove assignment to unused variable `x`"),
             TextRange::new(unused_variable_start, TextSize::from(95)),
             Some(Fix::unsafe_edit(Edit::deletion(
                 TextSize::from(94),
@@ -306,13 +298,13 @@ def fibonacci(n):
             Some(unused_variable_start),
             Rule::UnusedVariable,
         );
+        unused_variable.help("Remove assignment to unused variable `x`");
 
         let file_2 = r"if a == 1: pass";
 
         let undefined_name_start = TextSize::from(3);
         let undefined_name = create_lint_diagnostic(
             "Undefined name `a`",
-            Option::<&'static str>::None,
             TextRange::new(undefined_name_start, TextSize::from(4)),
             None,
             None,

crates/ruff_linter/src/rules/flake8_implicit_str_concat/snapshots/ruff_linter__rules__flake8_implicit_str_concat__tests__ISC004_ISC004.py.snap

@@ -11,8 +11,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
   | |______________________________________________________________^
 6 |   )
   |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 1 | facts = (
 2 |     "Lobsters have blue blood.",
 3 |     "The liver is the only human organ that can fully regenerate itself.",
@@ -35,8 +35,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
    | |______________________________________________________________^
 13 |   ]
    |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 8  | facts = [
 9  |     "Lobsters have blue blood.",
 10 |     "The liver is the only human organ that can fully regenerate itself.",
@@ -59,8 +59,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
    | |______________________________________________________________^
 20 |   }
    |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 15 | facts = {
 16 |     "Lobsters have blue blood.",
 17 |     "The liver is the only human organ that can fully regenerate itself.",
@@ -83,8 +83,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
    | |_________________________^
 33 |   )
    |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 27 | }
 28 | 
 29 | facts = (
@@ -108,8 +108,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
    | |_________________________^
 39 |   ]
    |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 33 | )
 34 | 
 35 | facts = [
@@ -133,8 +133,8 @@ ISC004 [*] Unparenthesized implicit string concatenation in collection
    | |_________________________^
 45 |   }
    |
-help: Wrap implicitly concatenated strings in parentheses
 help: Did you forget a comma?
+help: Wrap implicitly concatenated strings in parentheses
 39 | ]
 40 | 
 41 | facts = {