Add space when migrating to raw string (#5358)

## Summary

We had to do this for f-strings too -- if we add a prefix to `"foo"` in
`return"foo"`, we also need to add a leading space.

crates/ruff/resources/test/fixtures/pycodestyle/W605_0.py

@@ -22,6 +22,11 @@ in the middle
 #: W605:1:38
 value = 'new line\nand invalid escape \_ here'
 
+
+def f():
+    #: W605:1:11
+    return'\.png$'
+
 #: Okay
 regex = r'\.png$'
 regex = '\\.png$'

crates/ruff/resources/test/fixtures/pycodestyle/W605_1.py

@@ -19,6 +19,11 @@ with \_ somewhere
 in the middle
 """
 
+
+def f():
+    #: W605:1:11
+    return'\.png$'
+
 #: Okay
 regex = r'\.png$'
 regex = '\\.png$'

crates/ruff/src/rules/pycodestyle/rules/invalid_escape_sequence.rs

@@ -70,10 +70,10 @@ pub(crate) fn invalid_escape_sequence(
         return diagnostics;
     };
     let quote_pos = text.find(quote).unwrap();
-    let prefix = text[..quote_pos].to_lowercase();
+    let prefix = &text[..quote_pos];
     let body = &text[quote_pos + quote.len()..text.len() - quote.len()];
 
-    if !prefix.contains('r') {
+    if !prefix.contains(['r', 'R']) {
         let start_offset =
             range.start() + TextSize::try_from(quote_pos).unwrap() + quote.text_len();
 
@@ -115,7 +115,7 @@ pub(crate) fn invalid_escape_sequence(
 
         if autofix {
             if contains_valid_escape_sequence {
-                // Escape with backslash
+                // Escape with backslash.
                 for diagnostic in &mut diagnostics {
                     diagnostic.set_fix(Fix::automatic(Edit::insertion(
                         r"\".to_string(),
@@ -123,10 +123,23 @@ pub(crate) fn invalid_escape_sequence(
                     )));
                 }
             } else {
-                // Turn into raw string
+                // Turn into raw string.
                 for diagnostic in &mut diagnostics {
+                    // If necessary, add a space between any leading keyword (`return`, `yield`,
+                    // `assert`, etc.) and the string. For example, `return"foo"` is valid, but
+                    // `returnr"foo"` is not.
+                    let requires_space = locator
+                        .slice(TextRange::up_to(range.start()))
+                        .chars()
+                        .last()
+                        .map_or(false, |char| char.is_ascii_alphabetic());
+
                     diagnostic.set_fix(Fix::automatic(Edit::insertion(
-                        "r".to_string(),
+                        if requires_space {
+                            " r".to_string()
+                        } else {
+                            "r".to_string()
+                        },
                         range.start() + TextSize::try_from(quote_pos).unwrap(),
                     )));
                 }

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__W605_W605_0.py.snap

@@ -85,8 +85,6 @@ W605_0.py:23:39: W605 [*] Invalid escape sequence: `\_`
 22 | #: W605:1:38
 23 | value = 'new line\nand invalid escape \_ here'
    |                                       ^^ W605
-24 | 
-25 | #: Okay
    |
    = help: Add backslash to escape sequence
 
@@ -97,7 +95,28 @@ W605_0.py:23:39: W605 [*] Invalid escape sequence: `\_`
 23    |-value = 'new line\nand invalid escape \_ here'
    23 |+value = 'new line\nand invalid escape \\_ here'
 24 24 | 
-25 25 | #: Okay
+25 25 | 
-26 26 | regex = r'\.png$'
+26 26 | def f():
+
+W605_0.py:28:12: W605 [*] Invalid escape sequence: `\.`
+   |
+26 | def f():
+27 |     #: W605:1:11
+28 |     return'\.png$'
+   |            ^^ W605
+29 | 
+30 | #: Okay
+   |
+   = help: Add backslash to escape sequence
+
+ℹ Fix
+25 25 | 
+26 26 | def f():
+27 27 |     #: W605:1:11
+28    |-    return'\.png$'
+   28 |+    return r'\.png$'
+29 29 | 
+30 30 | #: Okay
+31 31 | regex = r'\.png$'
 
 

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__W605_W605_1.py.snap

@@ -80,4 +80,25 @@ W605_1.py:18:6: W605 [*] Invalid escape sequence: `\_`
 17 17 | literal
 18 18 | with \_ somewhere
 
+W605_1.py:25:12: W605 [*] Invalid escape sequence: `\.`
+   |
+23 | def f():
+24 |     #: W605:1:11
+25 |     return'\.png$'
+   |            ^^ W605
+26 | 
+27 | #: Okay
+   |
+   = help: Add backslash to escape sequence
+
+ℹ Fix
+22 22 | 
+23 23 | def f():
+24 24 |     #: W605:1:11
+25    |-    return'\.png$'
+   25 |+    return r'\.png$'
+26 26 | 
+27 27 | #: Okay
+28 28 | regex = r'\.png$'
+