Rollup merge of #148171 - GuillaumeGomez:line-number-highlight, r=yotamofek,lolbinarycat

Simplify code to generate line numbers in highlight

Follow-up of https://github.com/rust-lang/rust/pull/146992.

cc `````@yotamofek`````
r? `````@lolbinarycat`````

src/librustdoc/html/highlight.rs

@@ -343,7 +343,7 @@ struct TokenHandler<'a, 'tcx, F: Write> {
     /// We need to keep the `Class` for each element because it could contain a `Span` which is
     /// used to generate links.
     href_context: Option<HrefContext<'a, 'tcx>>,
-    write_line_number: fn(u32) -> String,
+    line_number_kind: LineNumberKind,
     line: u32,
     max_lines: u32,
 }
@@ -355,10 +355,10 @@ fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 }
 
 impl<'a, F: Write> TokenHandler<'a, '_, F> {
-    fn handle_backline(&mut self) -> Option<String> {
+    fn handle_backline(&mut self) -> Option<impl Display + use<F>> {
         self.line += 1;
         if self.line < self.max_lines {
-            return Some((self.write_line_number)(self.line));
+            return Some(self.line_number_kind.render(self.line));
         }
         None
     }
@@ -376,8 +376,7 @@ fn push_token(&mut self, class: Option<Class>, text: Cow<'a, str>) {
         if text == "\n"
             && let Some(backline) = self.handle_backline()
         {
-            self.out.write_str(&text).unwrap();
-            self.out.write_str(&backline).unwrap();
+            write!(self.out, "{text}{backline}").unwrap();
         } else {
             self.push_token_without_backline_check(class, text, true);
         }
@@ -437,20 +436,29 @@ fn drop(&mut self) {
     }
 }
 
-fn scraped_line_number(line: u32) -> String {
-    // https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag#data-nosnippet-attr
-    // Do not show "1 2 3 4 5 ..." in web search results.
-    format!("<span data-nosnippet>{line}</span>")
+/// Represents the type of line number to be generated as HTML.
+#[derive(Clone, Copy)]
+enum LineNumberKind {
+    /// Used for scraped code examples.
+    Scraped,
+    /// Used for source code pages.
+    Normal,
+    /// Code examples in documentation don't have line number generated by rustdoc.
+    Empty,
 }
 
-fn line_number(line: u32) -> String {
-    // https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag#data-nosnippet-attr
-    // Do not show "1 2 3 4 5 ..." in web search results.
-    format!("<a href=#{line} id={line} data-nosnippet>{line}</a>")
-}
-
-fn empty_line_number(_: u32) -> String {
-    String::new()
+impl LineNumberKind {
+    fn render(self, line: u32) -> impl Display {
+        fmt::from_fn(move |f| {
+            match self {
+                // https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag#data-nosnippet-attr
+                // Do not show "1 2 3 4 5 ..." in web search results.
+                Self::Scraped => write!(f, "<span data-nosnippet>{line}</span>"),
+                Self::Normal => write!(f, "<a href=#{line} id={line} data-nosnippet>{line}</a>"),
+                Self::Empty => Ok(()),
+            }
+        })
+    }
 }
 
 fn get_next_expansion(
@@ -534,15 +542,15 @@ pub(super) fn write_code(
     let mut token_handler = TokenHandler {
         out,
         href_context,
-        write_line_number: match line_info {
+        line_number_kind: match line_info {
             Some(line_info) => {
                 if line_info.is_scraped_example {
-                    scraped_line_number
+                    LineNumberKind::Scraped
                 } else {
-                    line_number
+                    LineNumberKind::Normal
                 }
             }
-            None => empty_line_number,
+            None => LineNumberKind::Empty,
         },
         line: 0,
         max_lines: u32::MAX,
@@ -552,8 +560,12 @@ pub(super) fn write_code(
     if let Some(line_info) = line_info {
         token_handler.line = line_info.start_line - 1;
         token_handler.max_lines = line_info.max_lines;
-        if let Some(text) = token_handler.handle_backline() {
-            token_handler.push_token_without_backline_check(None, Cow::Owned(text), false);
+        if let Some(backline) = token_handler.handle_backline() {
+            token_handler.push_token_without_backline_check(
+                None,
+                Cow::Owned(backline.to_string()),
+                false,
+            );
         }
     }