Add expect test for rustdoc html highlighting

It's a unit-test in a sense that it only checks syntax highlighting.
However, the resulting HTML is written to disk and can be easily
inspected in the browser.

To update the test, run with `--bless` argument or set
`UPDATE_EXPEC=1` env var

Author: matklad

Diff for: Cargo.lock

@@ -4122,6 +4122,7 @@ dependencies = [
 name = "rustdoc"
 version = "0.0.0"
 dependencies = [
+ "expect-test",
  "itertools 0.8.2",
  "minifier",
  "pulldown-cmark",

Diff for: src/librustdoc/Cargo.toml

@@ -17,3 +17,6 @@ serde_json = "1.0"
 smallvec = "1.0"
 tempfile = "3"
 itertools = "0.8"
+
+[dev-dependencies]
+expect-test = "0.1"

Diff for: src/librustdoc/html/highlight.rs

@@ -45,7 +45,7 @@ fn write_header(out: &mut String, class: Option<&str>) {
         .unwrap()
 }
 
-fn write_code(out: &mut String, src: &str) {
+pub(crate) fn write_code(out: &mut String, src: &str) {
     Classifier::new(src).highlight(&mut |highlight| {
         match highlight {
             Highlight::Token { text, class } => string(out, Escape(text), class),
@@ -343,6 +343,3 @@ fn string<T: Display>(out: &mut String, text: T, klass: Class) {
         klass => write!(out, "<span class=\"{}\">{}</span>", klass.as_html(), text).unwrap(),
     }
 }
-
-#[cfg(test)]
-mod tests;

Diff for: src/librustdoc/html/highlight/tests.rs

@@ -0,0 +1,20 @@
+
+<style>
+.kw { color: #8959A8; }
+.kw-2, .prelude-ty { color: #4271AE; }
+.number, .string { color: #718C00; }
+.self, .bool-val, .prelude-val, .attribute, .attribute .ident { color: #C82829; }
+.macro, .macro-nonterminal { color: #3E999F; }
+.lifetime { color: #B76514; }
+.question-mark { color: #ff9011; }
+</style>
+<pre><code><span class="attribute">#![<span class="ident">crate_type</span> <span class="op">=</span> <span class="string">&quot;lib&quot;</span>]</span>
+
+<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">target_os</span> <span class="op">=</span> <span class="string">&quot;linux&quot;</span>)]</span>
+<span class="kw">fn</span> <span class="ident">main</span>() {
+    <span class="kw">let</span> <span class="ident">foo</span> <span class="op">=</span> <span class="bool-val">true</span> <span class="op">&amp;&amp;</span> <span class="bool-val">false</span> <span class="op">|</span><span class="op">|</span> <span class="bool-val">true</span>;
+    <span class="kw">let</span> <span class="kw">_</span>: <span class="kw-2">*</span><span class="kw">const</span> () <span class="op">=</span> <span class="number">0</span>;
+    <span class="macro">mac</span><span class="macro">!</span>(<span class="ident">foo</span>, <span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="ident">bar</span>);
+    <span class="macro">assert</span><span class="macro">!</span>(<span class="self">self</span>.<span class="ident">length</span> <span class="op">&lt;</span> <span class="ident">N</span> <span class="op">&amp;&amp;</span> <span class="ident">index</span> <span class="op">&lt;</span><span class="op">=</span> <span class="self">self</span>.<span class="ident">length</span>);
+}
+</code></pre>

Diff for: src/librustdoc/test/fixtures/sample.html

@@ -0,0 +1,9 @@
+#![crate_type = "lib"]
+
+#[cfg(target_os = "linux")]
+fn main() {
+    let foo = true && false || true;
+    let _: *const () = 0;
+    mac!(foo, &mut bar);
+    assert!(self.length < N && index <= self.length);
+}

Diff for: src/librustdoc/test/fixtures/sample.rs

@@ -1,4 +1,5 @@
 use super::{make_test, TestOptions};
+use expect_test::expect_file;
 use rustc_span::edition::DEFAULT_EDITION;
 
 #[test]
@@ -277,3 +278,26 @@ test_wrapper! {
     let output = make_test(input, Some("my_crate"), false, &opts, DEFAULT_EDITION);
     assert_eq!(output, (expected, 1));
 }
+
+#[test]
+fn test_html_highlighting() {
+    let src = include_str!("fixtures/sample.rs");
+    let html = {
+        let mut out = String::new();
+        crate::html::highlight::write_code(&mut out, src);
+        format!("{}<pre><code>{}</code></pre>\n", STYLE, out)
+    };
+    expect_file!["src/librustdoc/test/fixtures/sample.html"].assert_eq(&html);
+
+    const STYLE: &str = r#"
+<style>
+.kw { color: #8959A8; }
+.kw-2, .prelude-ty { color: #4271AE; }
+.number, .string { color: #718C00; }
+.self, .bool-val, .prelude-val, .attribute, .attribute .ident { color: #C82829; }
+.macro, .macro-nonterminal { color: #3E999F; }
+.lifetime { color: #B76514; }
+.question-mark { color: #ff9011; }
+</style>
+"#;
+}