Ignore end-of-line file exemption comments (#6160)

## Summary

This PR protects against code like:

```python
from typing import Optional

import bar  # ruff: noqa
import baz

class Foo:
    x: Optional[str] = None
```

In which the user wrote `# ruff: noqa` to ignore a specific error, not
realizing that it was a file-level exemption that thus turned off all
lint rules.

Specifically, if a `# ruff: noqa` directive is not at the start of a
line, we now ignore it and warn, since this is almost certainly a
mistake.

crates/ruff/resources/test/fixtures/ruff/ruff_noqa_invalid.py

@@ -0,0 +1,5 @@
+import os  # ruff: noqa: F401
+
+
+def f():
+    x = 1

crates/ruff/src/noqa.rs

@@ -12,6 +12,7 @@ use ruff_python_ast::Ranged;
 use ruff_text_size::{TextLen, TextRange, TextSize};
 
 use ruff_diagnostics::Diagnostic;
+use ruff_python_trivia::indentation_at_offset;
 use ruff_source_file::{LineEnding, Locator};
 
 use crate::codes::NoqaCode;
@@ -248,10 +249,20 @@ impl FileExemption {
                     let path_display = relativize_path(path);
                     warn!("Invalid `# ruff: noqa` directive at {path_display}:{line}: {err}");
                 }
-                Ok(Some(ParsedFileExemption::All)) => {
+                Ok(Some(exemption)) => {
+                    if indentation_at_offset(range.start(), locator).is_none() {
+                        #[allow(deprecated)]
+                        let line = locator.compute_line_index(range.start());
+                        let path_display = relativize_path(path);
+                        warn!("Unexpected `# ruff: noqa` directive at {path_display}:{line}. File-level suppression comments must appear on their own line.");
+                        continue;
+                    }
+
+                    match exemption {
+                        ParsedFileExemption::All => {
                             return Some(Self::All);
                         }
-                Ok(Some(ParsedFileExemption::Codes(codes))) => {
+                        ParsedFileExemption::Codes(codes) => {
                             exempt_codes.extend(codes.into_iter().filter_map(|code| {
                                 if let Ok(rule) = Rule::from_code(get_redirect_target(code).unwrap_or(code))
                                 {
@@ -260,11 +271,13 @@ impl FileExemption {
                                     #[allow(deprecated)]
                                     let line = locator.compute_line_index(range.start());
                                     let path_display = relativize_path(path);
-                            warn!("Invalid code provided to `# ruff: noqa` at {path_display}:{line}: {code}");
+                                    warn!("Invalid rule code provided to `# ruff: noqa` at {path_display}:{line}: {code}");
                                     None
                                 }
                             }));
                         }
+                    }
+                }
                 Ok(None) => {}
             }
         }

crates/ruff/src/rules/ruff/mod.rs

@@ -167,9 +167,9 @@ mod tests {
     }
 
     #[test]
-    fn ruff_noqa() -> Result<()> {
+    fn ruff_noqa_all() -> Result<()> {
         let diagnostics = test_path(
-            Path::new("ruff/ruff_noqa.py"),
+            Path::new("ruff/ruff_noqa_all.py"),
             &settings::Settings::for_rules(vec![Rule::UnusedImport, Rule::UnusedVariable]),
         )?;
         assert_messages!(diagnostics);
@@ -177,9 +177,19 @@ mod tests {
     }
 
     #[test]
-    fn ruff_targeted_noqa() -> Result<()> {
+    fn ruff_noqa_codes() -> Result<()> {
         let diagnostics = test_path(
-            Path::new("ruff/ruff_targeted_noqa.py"),
+            Path::new("ruff/ruff_noqa_codes.py"),
+            &settings::Settings::for_rules(vec![Rule::UnusedImport, Rule::UnusedVariable]),
+        )?;
+        assert_messages!(diagnostics);
+        Ok(())
+    }
+
+    #[test]
+    fn ruff_noqa_invalid() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("ruff/ruff_noqa_invalid.py"),
             &settings::Settings::for_rules(vec![Rule::UnusedImport, Rule::UnusedVariable]),
         )?;
         assert_messages!(diagnostics);

crates/ruff/src/rules/ruff/snapshots/ruff__rules__ruff__tests__ruff_noqa_codes.snap

@@ -1,7 +1,7 @@
 ---
 source: crates/ruff/src/rules/ruff/mod.rs
 ---
-ruff_targeted_noqa.py:8:5: F841 [*] Local variable `x` is assigned to but never used
+ruff_noqa_codes.py:8:5: F841 [*] Local variable `x` is assigned to but never used
   |
 7 | def f():
 8 |     x = 1

crates/ruff/src/rules/ruff/snapshots/ruff__rules__ruff__tests__ruff_noqa_invalid.snap

@@ -0,0 +1,32 @@
+---
+source: crates/ruff/src/rules/ruff/mod.rs
+---
+ruff_noqa_invalid.py:1:8: F401 [*] `os` imported but unused
+  |
+1 | import os  # ruff: noqa: F401
+  |        ^^ F401
+  |
+  = help: Remove unused import: `os`
+
+ℹ Fix
+1   |-import os  # ruff: noqa: F401
+2 1 | 
+3 2 | 
+4 3 | def f():
+
+ruff_noqa_invalid.py:5:5: F841 [*] Local variable `x` is assigned to but never used
+  |
+4 | def f():
+5 |     x = 1
+  |     ^ F841
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Suggested fix
+2 2 | 
+3 3 | 
+4 4 | def f():
+5   |-    x = 1
+  5 |+    pass
+
+

crates/ruff_python_ast/src/whitespace.rs

@@ -12,7 +12,7 @@ pub fn indentation<'a, T>(locator: &'a Locator, located: &T) -> Option<&'a str>
 where
     T: Ranged,
 {
-    indentation_at_offset(locator, located.start())
+    indentation_at_offset(located.start(), locator)
 }
 
 /// Return the end offset at which the empty lines following a statement.

crates/ruff_python_formatter/src/comments/placement.rs

@@ -297,7 +297,7 @@ fn handle_own_line_comment_between_branches<'a>(
 
     // It depends on the indentation level of the comment if it is a leading comment for the
     // following branch or if it a trailing comment of the previous body's last statement.
-    let comment_indentation = indentation_at_offset(locator, comment.slice().range().start())
+    let comment_indentation = indentation_at_offset(comment.slice().range().start(), locator)
         .unwrap_or_default()
         .len();
 
@@ -402,7 +402,7 @@ fn handle_match_comment<'a>(
 
     let next_case = match_stmt.cases.get(current_case_index + 1);
 
-    let comment_indentation = indentation_at_offset(locator, comment.slice().range().start())
+    let comment_indentation = indentation_at_offset(comment.slice().range().start(), locator)
         .unwrap_or_default()
         .len();
     let match_case_indentation = indentation(locator, match_case).unwrap().len();
@@ -480,7 +480,7 @@ fn handle_own_line_comment_after_branch<'a>(
 
     // We only care about the length because indentations with mixed spaces and tabs are only valid if
     // the indent-level doesn't depend on the tab width (the indent level must be the same if the tab width is 1 or 8).
-    let comment_indentation = indentation_at_offset(locator, comment.slice().range().start())
+    let comment_indentation = indentation_at_offset(comment.slice().range().start(), locator)
         .unwrap_or_default()
         .len();
 
@@ -493,7 +493,7 @@ fn handle_own_line_comment_after_branch<'a>(
     // # Trailing if comment
     // ```
     // Here we keep the comment a trailing comment of the `if`
-    let preceding_indentation = indentation_at_offset(locator, preceding_node.start())
+    let preceding_indentation = indentation_at_offset(preceding_node.start(), locator)
         .unwrap_or_default()
         .len();
     if comment_indentation == preceding_indentation {

crates/ruff_python_trivia/src/whitespace.rs

@@ -2,7 +2,7 @@ use ruff_source_file::Locator;
 use ruff_text_size::{TextRange, TextSize};
 
 /// Extract the leading indentation from a line.
-pub fn indentation_at_offset<'a>(locator: &'a Locator, offset: TextSize) -> Option<&'a str> {
+pub fn indentation_at_offset<'a>(offset: TextSize, locator: &'a Locator) -> Option<&'a str> {
     let line_start = locator.line_start(offset);
     let indentation = &locator.contents()[TextRange::new(line_start, offset)];