Fix for D403 to split on first whitespace character when extracting f…

…irst word of docstring.

crates/ruff_linter/resources/test/fixtures/pydocstyle/D403.py

@@ -41,3 +41,13 @@ def first_word_lots_of_whitespace():
 
     What do you think?    
     """
+
+def single_word_then_newline():
+    """singleword
+
+    """
+
+def single_word_on_second_line():
+    """
+    singleword
+    """

crates/ruff_linter/src/rules/pydocstyle/rules/capitalized.rs

@@ -60,14 +60,10 @@ pub(crate) fn capitalized(checker: &mut Checker, docstring: &Docstring) {
 
     let body = docstring.body();
     let trim_start_body = body.trim_start();
-    let first_word = trim_start_body.split_once(' ').map_or_else(
-        || {
-            // If the docstring is a single word, trim the punctuation marks because
-            // it makes the ASCII test below fail.
-            body.trim_end_matches(['.', '!', '?'])
-        },
-        |(first_word, _)| first_word,
-    );
+    let first_word = trim_start_body
+        .find(char::is_whitespace)
+        .map_or(trim_start_body, |idx| &trim_start_body[..idx])
+        .trim_end_matches(['.', '!', '?']);
 
     let mut first_word_chars = first_word.chars();
     let Some(first_char) = first_word_chars.next() else {

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D403_D403.py.snap

@@ -1,5 +1,6 @@
 ---
 source: crates/ruff_linter/src/rules/pydocstyle/mod.rs
+snapshot_kind: text
 ---
 D403.py:2:5: D403 [*] First word of the docstring should be capitalized: `this` -> `This`
   |
@@ -72,6 +73,8 @@ D403.py:36:5: D403 [*] First word of the docstring should be capitalized: `here`
 42 | |     What do you think?    
 43 | |     """
    | |_______^ D403
+44 |   
+45 |   def single_word_then_newline():
    |
    = help: Capitalize `here` to `Here`
 
@@ -84,3 +87,45 @@ D403.py:36:5: D403 [*] First word of the docstring should be capitalized: `here`
 41 41 | 
 42 42 |     What do you think?    
 43 43 |     """
+
+D403.py:46:5: D403 [*] First word of the docstring should be capitalized: `singleword` -> `Singleword`
+   |
+45 |   def single_word_then_newline():
+46 |       """singleword
+   |  _____^
+47 | | 
+48 | |     """
+   | |_______^ D403
+49 |   
+50 |   def single_word_on_second_line():
+   |
+   = help: Capitalize `singleword` to `Singleword`
+
+ℹ Safe fix
+43 43 |     """
+44 44 | 
+45 45 | def single_word_then_newline():
+46    |-    """singleword
+   46 |+    """Singleword
+47 47 | 
+48 48 |     """
+49 49 | 
+
+D403.py:51:5: D403 [*] First word of the docstring should be capitalized: `singleword` -> `Singleword`
+   |
+50 |   def single_word_on_second_line():
+51 |       """
+   |  _____^
+52 | |     singleword
+53 | |     """
+   | |_______^ D403
+   |
+   = help: Capitalize `singleword` to `Singleword`
+
+ℹ Safe fix
+49 49 | 
+50 50 | def single_word_on_second_line():
+51 51 |     """
+52    |-    singleword
+   52 |+    Singleword
+53 53 |     """