Rollup merge of #100775 - GuillaumeGomez:reduce-span-v2, r=notriddle

rustdoc: Merge source code pages HTML elements together v2

This is the follow-up of https://github.com/rust-lang/rust/pull/100429.

I strongly recommend to review it one commit at a time because otherwise it's a lot at once.

For these ones, on each page, I run this JS: `document.getElementsByTagName('*').length`. The goal is to count the number of DOM elements. I took some pages that seemed big, but don't hesitate to check some others. I also added the "starting point" because it's quite nice to see how much the page was reduced thanks to these two PRs.

| file name | before #100429 | before this PR | with this PR | diff |
|-|-|-|-|-|
| std/lib.rs.html (source link on std crate page) | 3455 | 2332 | 1772 | 24% |
| alloc/vec/mod.rs.html (source on Vec type page) | 11012 | 5982 | 5833 | 2.5% |
| alloc/string.rs.html (source on String type page) | 10800 | 6010 | 5822 | 3.2% |
| std/sync/mutex.rs.html (source on Mutex type page) | 2953 | 2041 | 2038 | 0.1% |

So unsurprisingly, the more attributes you have, the bigger the difference.

You can test it [here](https://rustdoc.crud.net/imperio/reduce-span-v2/src/std/lib.rs.html).

cc ``````@jsha``````
r? ``````@notriddle``````

7 files changed, 157 insertions, 106 deletions

diff --git a/src/librustdoc/html/highlight.rs b/src/librustdoc/html/highlight.rs
index 9d8ee52a3fa..4a12d74ddef 100644
--- a/src/librustdoc/html/highlight.rs
+++ b/src/librustdoc/html/highlight.rs
@@ -111,53 +111,6 @@ fn write_header(out: &mut Buffer, class: &str, extra_content: Option<Buffer>) {
     write!(out, "<code>");
 }
 
-/// Write all the pending elements sharing a same (or at mergeable) `Class`.
-///
-/// If there is a "parent" (if a `EnterSpan` event was encountered) and the parent can be merged
-/// with the elements' class, then we simply write the elements since the `ExitSpan` event will
-/// close the tag.
-///
-/// Otherwise, if there is only one pending element, we let the `string` function handle both
-/// opening and closing the tag, otherwise we do it into this function.
-fn write_pending_elems(
-    out: &mut Buffer,
-    href_context: &Option<HrefContext<'_, '_, '_>>,
-    pending_elems: &mut Vec<(&str, Option<Class>)>,
-    current_class: &mut Option<Class>,
-    closing_tags: &[(&str, Class)],
-) {
-    if pending_elems.is_empty() {
-        return;
-    }
-    let mut done = false;
-    if let Some((_, parent_class)) = closing_tags.last() {
-        if can_merge(*current_class, Some(*parent_class), "") {
-            for (text, class) in pending_elems.iter() {
-                string(out, Escape(text), *class, &href_context, false);
-            }
-            done = true;
-        }
-    }
-    if !done {
-        // We only want to "open" the tag ourselves if we have more than one pending and if the current
-        // parent tag is not the same as our pending content.
-        let open_tag_ourselves = pending_elems.len() > 1;
-        let close_tag = if open_tag_ourselves {
-            enter_span(out, current_class.unwrap(), &href_context)
-        } else {
-            ""
-        };
-        for (text, class) in pending_elems.iter() {
-            string(out, Escape(text), *class, &href_context, !open_tag_ourselves);
-        }
-        if open_tag_ourselves {
-            exit_span(out, close_tag);
-        }
-    }
-    pending_elems.clear();
-    *current_class = None;
-}
-
 /// Check if two `Class` can be merged together. In the following rules, "unclassified" means `None`
 /// basically (since it's `Option<Class>`). The following rules apply:
 ///
@@ -171,7 +124,88 @@ fn can_merge(class1: Option<Class>, class2: Option<Class>, text: &str) -> bool {
         (Some(c1), Some(c2)) => c1.is_equal_to(c2),
         (Some(Class::Ident(_)), None) | (None, Some(Class::Ident(_))) => true,
         (Some(_), None) | (None, Some(_)) => text.trim().is_empty(),
-        _ => false,
+        (None, None) => true,
+    }
+}
+
+/// This type is used as a conveniency to prevent having to pass all its fields as arguments into
+/// the various functions (which became its methods).
+struct TokenHandler<'a, 'b, 'c, 'd, 'e> {
+    out: &'a mut Buffer,
+    /// It contains the closing tag and the associated `Class`.
+    closing_tags: Vec<(&'static str, Class)>,
+    /// This is used because we don't automatically generate the closing tag on `ExitSpan` in
+    /// case an `EnterSpan` event with the same class follows.
+    pending_exit_span: Option<Class>,
+    /// `current_class` and `pending_elems` are used to group HTML elements with same `class`
+    /// attributes to reduce the DOM size.
+    current_class: Option<Class>,
+    /// We need to keep the `Class` for each element because it could contain a `Span` which is
+    /// used to generate links.
+    pending_elems: Vec<(&'b str, Option<Class>)>,
+    href_context: Option<HrefContext<'c, 'd, 'e>>,
+}
+
+impl<'a, 'b, 'c, 'd, 'e> TokenHandler<'a, 'b, 'c, 'd, 'e> {
+    fn handle_exit_span(&mut self) {
+        // We can't get the last `closing_tags` element using `pop()` because `closing_tags` is
+        // being used in `write_pending_elems`.
+        let class = self.closing_tags.last().expect("ExitSpan without EnterSpan").1;
+        // We flush everything just in case...
+        self.write_pending_elems(Some(class));
+
+        exit_span(self.out, self.closing_tags.pop().expect("ExitSpan without EnterSpan").0);
+        self.pending_exit_span = None;
+    }
+
+    /// Write all the pending elements sharing a same (or at mergeable) `Class`.
+    ///
+    /// If there is a "parent" (if a `EnterSpan` event was encountered) and the parent can be merged
+    /// with the elements' class, then we simply write the elements since the `ExitSpan` event will
+    /// close the tag.
+    ///
+    /// Otherwise, if there is only one pending element, we let the `string` function handle both
+    /// opening and closing the tag, otherwise we do it into this function.
+    ///
+    /// It returns `true` if `current_class` must be set to `None` afterwards.
+    fn write_pending_elems(&mut self, current_class: Option<Class>) -> bool {
+        if self.pending_elems.is_empty() {
+            return false;
+        }
+        if let Some((_, parent_class)) = self.closing_tags.last() &&
+            can_merge(current_class, Some(*parent_class), "")
+        {
+            for (text, class) in self.pending_elems.iter() {
+                string(self.out, Escape(text), *class, &self.href_context, false);
+            }
+        } else {
+            // We only want to "open" the tag ourselves if we have more than one pending and if the
+            // current parent tag is not the same as our pending content.
+            let close_tag = if self.pending_elems.len() > 1 && current_class.is_some() {
+                Some(enter_span(self.out, current_class.unwrap(), &self.href_context))
+            } else {
+                None
+            };
+            for (text, class) in self.pending_elems.iter() {
+                string(self.out, Escape(text), *class, &self.href_context, close_tag.is_none());
+            }
+            if let Some(close_tag) = close_tag {
+                exit_span(self.out, close_tag);
+            }
+        }
+        self.pending_elems.clear();
+        true
+    }
+}
+
+impl<'a, 'b, 'c, 'd, 'e> Drop for TokenHandler<'a, 'b, 'c, 'd, 'e> {
+    /// When leaving, we need to flush all pending data to not have missing content.
+    fn drop(&mut self) {
+        if self.pending_exit_span.is_some() {
+            self.handle_exit_span();
+        } else {
+            self.write_pending_elems(self.current_class);
+        }
     }
 }
 
@@ -194,64 +228,72 @@ fn write_code(
 ) {
     // This replace allows to fix how the code source with DOS backline characters is displayed.
     let src = src.replace("\r\n", "\n");
-    // It contains the closing tag and the associated `Class`.
-    let mut closing_tags: Vec<(&'static str, Class)> = Vec::new();
-    // The following two variables are used to group HTML elements with same `class` attributes
-    // to reduce the DOM size.
-    let mut current_class: Option<Class> = None;
-    // We need to keep the `Class` for each element because it could contain a `Span` which is
-    // used to generate links.
-    let mut pending_elems: Vec<(&str, Option<Class>)> = Vec::new();
+    let mut token_handler = TokenHandler {
+        out,
+        closing_tags: Vec::new(),
+        pending_exit_span: None,
+        current_class: None,
+        pending_elems: Vec::new(),
+        href_context,
+    };
 
     Classifier::new(
         &src,
-        href_context.as_ref().map(|c| c.file_span).unwrap_or(DUMMY_SP),
+        token_handler.href_context.as_ref().map(|c| c.file_span).unwrap_or(DUMMY_SP),
         decoration_info,
     )
     .highlight(&mut |highlight| {
         match highlight {
             Highlight::Token { text, class } => {
+                // If we received a `ExitSpan` event and then have a non-compatible `Class`, we
+                // need to close the `<span>`.
+                let need_current_class_update = if let Some(pending) = token_handler.pending_exit_span &&
+                    !can_merge(Some(pending), class, text) {
+                        token_handler.handle_exit_span();
+                        true
                 // If the two `Class` are different, time to flush the current content and start
                 // a new one.
-                if !can_merge(current_class, class, text) {
-                    write_pending_elems(
-                        out,
-                        &href_context,
-                        &mut pending_elems,
-                        &mut current_class,
-                        &closing_tags,
-                    );
-                    current_class = class.map(Class::dummy);
-                } else if current_class.is_none() {
-                    current_class = class.map(Class::dummy);
+                } else if !can_merge(token_handler.current_class, class, text) {
+                    token_handler.write_pending_elems(token_handler.current_class);
+                    true
+                } else {
+                    token_handler.current_class.is_none()
+                };
+
+                if need_current_class_update {
+                    token_handler.current_class = class.map(Class::dummy);
                 }
-                pending_elems.push((text, class));
+                token_handler.pending_elems.push((text, class));
             }
             Highlight::EnterSpan { class } => {
-                // We flush everything just in case...
-                write_pending_elems(
-                    out,
-                    &href_context,
-                    &mut pending_elems,
-                    &mut current_class,
-                    &closing_tags,
-                );
-                closing_tags.push((enter_span(out, class, &href_context), class))
+                let mut should_add = true;
+                if let Some(pending_exit_span) = token_handler.pending_exit_span {
+                    if class.is_equal_to(pending_exit_span) {
+                        should_add = false;
+                    } else {
+                        token_handler.handle_exit_span();
+                    }
+                } else {
+                    // We flush everything just in case...
+                    if token_handler.write_pending_elems(token_handler.current_class) {
+                        token_handler.current_class = None;
+                    }
+                }
+                if should_add {
+                    let closing_tag = enter_span(token_handler.out, class, &token_handler.href_context);
+                    token_handler.closing_tags.push((closing_tag, class));
+                }
+
+                token_handler.current_class = None;
+                token_handler.pending_exit_span = None;
             }
             Highlight::ExitSpan => {
-                // We flush everything just in case...
-                write_pending_elems(
-                    out,
-                    &href_context,
-                    &mut pending_elems,
-                    &mut current_class,
-                    &closing_tags,
-                );
-                exit_span(out, closing_tags.pop().expect("ExitSpan without EnterSpan").0)
+                token_handler.current_class = None;
+                token_handler.pending_exit_span =
+                    Some(token_handler.closing_tags.last().as_ref().expect("ExitSpan without EnterSpan").1);
             }
         };
     });
-    write_pending_elems(out, &href_context, &mut pending_elems, &mut current_class, &closing_tags);
 }
 
 fn write_footer(out: &mut Buffer, playground_button: Option<&str>) {
@@ -291,8 +333,8 @@ impl Class {
         match (self, other) {
             (Self::Self_(_), Self::Self_(_))
             | (Self::Macro(_), Self::Macro(_))
-            | (Self::Ident(_), Self::Ident(_))
-            | (Self::Decoration(_), Self::Decoration(_)) => true,
+            | (Self::Ident(_), Self::Ident(_)) => true,
+            (Self::Decoration(c1), Self::Decoration(c2)) => c1 == c2,
             (x, y) => x == y,
         }
     }
@@ -761,7 +803,7 @@ impl<'a> Classifier<'a> {
             TokenKind::CloseBracket => {
                 if self.in_attribute {
                     self.in_attribute = false;
-                    sink(Highlight::Token { text: "]", class: Some(Class::Attribute) });
+                    sink(Highlight::Token { text: "]", class: None });
                     sink(Highlight::ExitSpan);
                     return;
                 }
diff --git a/src/librustdoc/html/highlight/fixtures/decorations.html b/src/librustdoc/html/highlight/fixtures/decorations.html
index 21848978721..ebf29f9cb3a 100644
--- a/src/librustdoc/html/highlight/fixtures/decorations.html
+++ b/src/librustdoc/html/highlight/fixtures/decorations.html
@@ -1,2 +1,4 @@
-<span class="example"><span class="kw">let </span>x = <span class="number">1</span>;</span>
-<span class="kw">let </span>y = <span class="number">2</span>;
\ No newline at end of file
+<span class="example"><span class="kw">let </span>x = <span class="number">1</span>;
+<span class="kw">let </span>y = <span class="number">2</span>;
+</span><span class="example2"><span class="kw">let </span>z = <span class="number">3</span>;
+</span><span class="kw">let </span>a = <span class="number">4</span>;
\ No newline at end of file
diff --git a/src/librustdoc/html/highlight/fixtures/sample.html b/src/librustdoc/html/highlight/fixtures/sample.html
index ae2650528eb..4a5a3cf609c 100644
--- a/src/librustdoc/html/highlight/fixtures/sample.html
+++ b/src/librustdoc/html/highlight/fixtures/sample.html
@@ -8,12 +8,13 @@
 .lifetime { color: #B76514; }
 .question-mark { color: #ff9011; }
 </style>
-<pre><code><span class="attribute">#![crate_type = <span class="string">&quot;lib&quot;</span>]</span>
+<pre><code><span class="attribute">#![crate_type = <span class="string">&quot;lib&quot;</span>]
 
-<span class="kw">use </span>std::path::{Path, PathBuf};
+</span><span class="kw">use </span>std::path::{Path, PathBuf};
 
-<span class="attribute">#[cfg(target_os = <span class="string">&quot;linux&quot;</span>)]</span>
-<span class="kw">fn </span>main() -&gt; () {
+<span class="attribute">#[cfg(target_os = <span class="string">&quot;linux&quot;</span>)]
+#[cfg(target_os = <span class="string">&quot;windows&quot;</span>)]
+</span><span class="kw">fn </span>main() -&gt; () {
     <span class="kw">let </span>foo = <span class="bool-val">true </span>&amp;&amp; <span class="bool-val">false </span>|| <span class="bool-val">true</span>;
     <span class="kw">let _</span>: <span class="kw-2">*const </span>() = <span class="number">0</span>;
     <span class="kw">let _ </span>= <span class="kw-2">&amp;</span>foo;
@@ -22,8 +23,8 @@
     <span class="macro">mac!</span>(foo, <span class="kw-2">&amp;mut </span>bar);
     <span class="macro">assert!</span>(<span class="self">self</span>.length &lt; N &amp;&amp; index &lt;= <span class="self">self</span>.length);
     ::std::env::var(<span class="string">&quot;gateau&quot;</span>).is_ok();
-    <span class="attribute">#[rustfmt::skip]</span>
-    <span class="kw">let </span>s:std::path::PathBuf = std::path::PathBuf::new();
+    <span class="attribute">#[rustfmt::skip]
+    </span><span class="kw">let </span>s:std::path::PathBuf = std::path::PathBuf::new();
     <span class="kw">let </span><span class="kw-2">mut </span>s = String::new();
 
     <span class="kw">match </span><span class="kw-2">&amp;</span>s {
@@ -31,7 +32,7 @@
     }
 }
 
-<span class="macro">macro_rules!</span> bar {
+<span class="macro">macro_rules! </span>bar {
     (<span class="macro-nonterminal">$foo</span>:tt) =&gt; {};
 }
 </code></pre>
diff --git a/src/librustdoc/html/highlight/fixtures/sample.rs b/src/librustdoc/html/highlight/fixtures/sample.rs
index fbfdc676733..ef85b566cb3 100644
--- a/src/librustdoc/html/highlight/fixtures/sample.rs
+++ b/src/librustdoc/html/highlight/fixtures/sample.rs
@@ -3,6 +3,7 @@
 use std::path::{Path, PathBuf};
 
 #[cfg(target_os = "linux")]
+#[cfg(target_os = "windows")]
 fn main() -> () {
     let foo = true && false || true;
     let _: *const () = 0;
diff --git a/src/librustdoc/html/highlight/tests.rs b/src/librustdoc/html/highlight/tests.rs
index 4861a8ad32d..a5e633df434 100644
--- a/src/librustdoc/html/highlight/tests.rs
+++ b/src/librustdoc/html/highlight/tests.rs
@@ -69,9 +69,12 @@ fn test_union_highlighting() {
 fn test_decorations() {
     create_default_session_globals_then(|| {
         let src = "let x = 1;
-let y = 2;";
+let y = 2;
+let z = 3;
+let a = 4;";
         let mut decorations = FxHashMap::default();
-        decorations.insert("example", vec![(0, 10)]);
+        decorations.insert("example", vec![(0, 10), (11, 21)]);
+        decorations.insert("example2", vec![(22, 32)]);
 
         let mut html = Buffer::new();
         write_code(&mut html, src, None, Some(DecorationInfo(decorations)));
diff --git a/src/test/rustdoc/issue-41783.codeblock.html b/src/test/rustdoc/issue-41783.codeblock.html
index b919935e4b4..89987491d1b 100644
--- a/src/test/rustdoc/issue-41783.codeblock.html
+++ b/src/test/rustdoc/issue-41783.codeblock.html
@@ -1,5 +1,5 @@
 <code># single
 ## double
 ### triple
-<span class="attribute">#[outer]</span>
-<span class="attribute">#![inner]</span></code>
\ No newline at end of file
+<span class="attribute">#[outer]
+#![inner]</span></code>
diff --git a/src/test/rustdoc/issue-41783.rs b/src/test/rustdoc/issue-41783.rs
index d6771602879..87267a750c6 100644
--- a/src/test/rustdoc/issue-41783.rs
+++ b/src/test/rustdoc/issue-41783.rs
@@ -1,8 +1,10 @@
 // @has issue_41783/struct.Foo.html
 // @!hasraw - 'space'
 // @!hasraw - 'comment'
-// @hasraw - '<span class="attribute">#[outer]</span>'
-// @hasraw - '<span class="attribute">#![inner]</span>'
+// @hasraw - '<span class="attribute">#[outer]'
+// @!hasraw - '<span class="attribute">#[outer]</span>'
+// @hasraw - '#![inner]</span>'
+// @!hasraw - '<span class="attribute">#![inner]</span>'
 // @snapshot 'codeblock' - '//*[@class="rustdoc-toggle top-doc"]/*[@class="docblock"]//pre/code'
 
 /// ```no_run