Accept additional custom classes in fenced code blocks

This allow users to write the following:

```
/// Some code block with `rust,class:test:name` as the language string:
///
/// ```rust,class:test:name
/// int main(void) {
///     return 0;
/// }
/// ```
fn main() {}
```

This block will be rendered without any highlighting and will have a class
`test:name` in the resulting CSS.

compiler/rustc_feature/src/active.rs

@@ -126,7 +126,7 @@ declare_features! (
     /// macros disappear).
     (active, allow_internal_unsafe, "1.0.0", None, None),
 
-    /// no-tracking-issue-end
+    // no-tracking-issue-end
 
     /// Allows using `#[link_name="llvm.*"]`.
     (active, link_llvm_intrinsics, "1.0.0", Some(29602), None),
@@ -644,6 +644,9 @@ declare_features! (
     /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
     (active, c_unwind, "1.52.0", Some(74990), None),
 
+    /// Allows users to provide classes for fenced code block using `class:classname`.
+    (active, custom_code_classes_in_docs, "1.52.0", Some(79483), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/borrow_check/region_infer/mod.rs

@@ -1241,7 +1241,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// it. However, it works pretty well in practice. In particular,
     /// this is needed to deal with projection outlives bounds like
     ///
-    /// ```ignore (internal compiler representation so lifetime syntax is invalid)
+    /// ```text (internal compiler representation so lifetime syntax is invalid)
     /// <T as Foo<'0>>::Item: '1
     /// ```
     ///

compiler/rustc_span/src/symbol.rs

@@ -434,6 +434,7 @@ symbols! {
         cttz,
         cttz_nonzero,
         custom_attribute,
+        custom_code_classes_in_docs,
         custom_derive,
         custom_inner_attributes,
         custom_test_frameworks,

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -46,6 +46,7 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// type Foo = impl Baz;
     /// fn bar() -> Foo {
     /// //          ^^^ This is the span we are looking for!
+    /// }
     /// ```
     ///
     /// In cases where the fn returns `(impl Trait, impl Trait)` or

src/doc/rustdoc/src/unstable-features.md

@@ -38,6 +38,20 @@ future.
 Attempting to use these error numbers on stable will result in the code sample being interpreted as
 plain text.
 
+### Custom CSS classes for code blocks
+
+```rust
+#![feature(custom_code_classes_in_docs)]
+
+/// ```class:language-c
+/// int main(void) { return 0; }
+/// ```
+fn main() {}
+```
+
+The text `int main(void) { return 0; }` is rendered without highlighting in a code block with the class
+`language-c`. This can be used to highlight other languages through JavaScript libraries for example.
+
 ## Extensions to the `#[doc]` attribute
 
 These features operate by extending the `#[doc]` attribute, and thus can be caught by the compiler

src/librustdoc/html/highlight.rs

@@ -27,6 +27,30 @@ crate fn render_with_highlighting(
     edition: Edition,
 ) {
     debug!("highlighting: ================\n{}\n==============", src);
+
+    write_tooltip(out, tooltip);
+    write_header(out, class);
+    write_highlighted_code(out, src, edition);
+    write_footer(out, playground_button);
+}
+
+/// Does not highlight `src` because additional classes have been provided with
+/// the `class:` modifier.
+crate fn render_with_added_classes(
+    src: &str,
+    out: &mut Buffer,
+    classes: String,
+    tooltip: Option<(Option<Edition>, &str)>,
+) {
+    debug!("*not* highlighting: ================\n{}\n==============", src);
+
+    write_tooltip(out, tooltip);
+    write_header(out, Some(&classes));
+    write_raw_code(out, src);
+    write_footer(out, None);
+}
+
+fn write_tooltip(out: &mut Buffer, tooltip: Option<(Option<Edition>, &str)>) {
     if let Some((edition_info, class)) = tooltip {
         write!(
             out,
@@ -39,17 +63,13 @@ crate fn render_with_highlighting(
             },
         );
     }
-
-    write_header(out, class);
-    write_code(out, &src, edition);
-    write_footer(out, playground_button);
 }
 
 fn write_header(out: &mut Buffer, class: Option<&str>) {
     write!(out, "<div class=\"example-wrap\"><pre class=\"rust {}\">\n", class.unwrap_or_default());
 }
 
-fn write_code(out: &mut Buffer, src: &str, edition: Edition) {
+fn write_highlighted_code(out: &mut Buffer, src: &str, edition: Edition) {
     // This replace allows to fix how the code source with DOS backline characters is displayed.
     let src = src.replace("\r\n", "\n");
     Classifier::new(&src, edition).highlight(&mut |highlight| {
@@ -65,6 +85,10 @@ fn write_footer(out: &mut Buffer, playground_button: Option<&str>) {
     write!(out, "</pre>{}</div>\n", playground_button.unwrap_or_default());
 }
 
+fn write_raw_code(out: &mut Buffer, src: &str) {
+    write!(out, "{}", src.replace("\r\n", "\n"));
+}
+
 /// How a span of text is classified. Mostly corresponds to token kinds.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 enum Class {

src/librustdoc/html/highlight/tests.rs

@@ -1,5 +1,7 @@
-use super::write_code;
+use super::write_highlighted_code;
+
 use crate::html::format::Buffer;
+
 use expect_test::expect_file;
 use rustc_span::edition::Edition;
 
@@ -20,7 +22,7 @@ fn test_html_highlighting() {
     let src = include_str!("fixtures/sample.rs");
     let html = {
         let mut out = Buffer::new();
-        write_code(&mut out, src, Edition::Edition2018);
+        write_highlighted_code(&mut out, src, Edition::Edition2018);
         format!("{}<pre><code>{}</code></pre>\n", STYLE, out.into_inner())
     };
     expect_file!["fixtures/sample.html"].assert_eq(&html);
@@ -31,7 +33,8 @@ fn test_dos_backline() {
     let src = "pub fn foo() {\r\n\
     println!(\"foo\");\r\n\
 }\r\n";
+
     let mut html = Buffer::new();
-    write_code(&mut html, src, Edition::Edition2018);
+    write_highlighted_code(&mut html, src, Edition::Edition2018);
     expect_file!["fixtures/dos_line.html"].assert_eq(&html.into_inner());
 }

src/librustdoc/html/markdown.rs

@@ -207,6 +207,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
         let should_panic;
         let ignore;
         let edition;
+        let added_classes;
         if let Some(Event::Start(Tag::CodeBlock(kind))) = event {
             let parse_result = match kind {
                 CodeBlockKind::Fenced(ref lang) => {
@@ -221,6 +222,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             should_panic = parse_result.should_panic;
             ignore = parse_result.ignore;
             edition = parse_result.edition;
+            added_classes = parse_result.added_classes;
         } else {
             return event;
         }
@@ -289,33 +291,42 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             ))
         });
 
-        let tooltip = if ignore != Ignore::None {
-            Some((None, "ignore"))
+        let (tooltip, special_class) = if ignore != Ignore::None {
+            (Some((None, "ignore")), " ignore")
         } else if compile_fail {
-            Some((None, "compile_fail"))
+            (Some((None, "compile_fail")), " compile_fail")
         } else if should_panic {
-            Some((None, "should_panic"))
+            (Some((None, "should_panic")), " should_panic")
         } else if explicit_edition {
-            Some((Some(edition), "edition"))
+            (Some((Some(edition), "edition")), " edition")
         } else {
-            None
+            (None, "")
         };
 
         // insert newline to clearly separate it from the
         // previous block so we can shorten the html output
         let mut s = Buffer::new();
         s.push_str("\n");
-        highlight::render_with_highlighting(
-            &text,
-            &mut s,
-            Some(&format!(
-                "rust-example-rendered{}",
-                if let Some((_, class)) = tooltip { format!(" {}", class) } else { String::new() }
-            )),
-            playground_button.as_deref(),
-            tooltip,
-            edition,
-        );
+
+        if added_classes.is_empty() {
+            highlight::render_with_highlighting(
+                &text,
+                &mut s,
+                Some(&format!("rust-example-rendered{}", special_class)),
+                playground_button.as_deref(),
+                tooltip,
+                edition,
+            );
+        } else {
+            let classes = if special_class.is_empty() {
+                added_classes.join(" ")
+            } else {
+                format!("{} {}", special_class.trim(), added_classes.join(" "))
+            };
+
+            highlight::render_with_added_classes(&text, &mut s, classes, tooltip);
+        }
+
         Some(Event::Html(s.into_inner().into()))
     }
 }
@@ -744,6 +755,7 @@ crate struct LangString {
     crate error_codes: Vec<String>,
     crate allow_fail: bool,
     crate edition: Option<Edition>,
+    crate added_classes: Vec<String>,
 }
 
 #[derive(Eq, PartialEq, Clone, Debug)]
@@ -766,6 +778,7 @@ impl Default for LangString {
             error_codes: Vec::new(),
             allow_fail: false,
             edition: None,
+            added_classes: Vec::new(),
         }
     }
 }
@@ -775,7 +788,7 @@ impl LangString {
         string: &str,
         allow_error_code_check: ErrorCodes,
         enable_per_target_ignores: bool,
-    ) -> LangString {
+    ) -> Self {
         Self::parse(string, allow_error_code_check, enable_per_target_ignores, None)
     }
 
@@ -809,7 +822,7 @@ impl LangString {
         allow_error_code_check: ErrorCodes,
         enable_per_target_ignores: bool,
         extra: Option<&ExtraInfo<'_>>,
-    ) -> LangString {
+    ) -> Self {
         let allow_error_code_check = allow_error_code_check.as_bool();
         let mut seen_rust_tags = false;
         let mut seen_other_tags = false;
@@ -868,6 +881,14 @@ impl LangString {
                         seen_other_tags = true;
                     }
                 }
+                x if x.starts_with("class:") => {
+                    let class = x.trim_start_matches("class:");
+                    if class.is_empty() {
+                        seen_other_tags = true;
+                    } else {
+                        data.added_classes.push(class.to_owned());
+                    }
+                }
                 x if extra.is_some() => {
                     let s = x.to_lowercase();
                     match if s == "compile-fail" || s == "compile_fail" || s == "compilefail" {

src/librustdoc/html/markdown/tests.rs

@@ -118,6 +118,26 @@ fn test_lang_string_parse() {
         edition: Some(Edition::Edition2018),
         ..Default::default()
     });
+    t(LangString {
+        original: "class:test".into(),
+        added_classes: vec!["test".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "rust,class:test".into(),
+        added_classes: vec!["test".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "class:test:with:colon".into(),
+        added_classes: vec!["test:with:colon".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "class:first,class:second".into(),
+        added_classes: vec!["first".into(), "second".into()],
+        ..Default::default()
+    });
 }
 
 #[test]

src/librustdoc/passes/check_custom_code_classes.rs

@@ -0,0 +1,83 @@
+//! NIGHTLY & UNSTABLE CHECK: custom_code_classes_in_docs
+//!
+//! This pass will produce errors when finding custom classes outside of
+//! nightly + relevant feature active.
+
+use super::{span_of_attrs, Pass};
+use crate::clean::{Crate, Item};
+use crate::core::DocContext;
+use crate::fold::DocFolder;
+use crate::html::markdown::{find_testable_code, ErrorCodes, LangString};
+
+use rustc_session::parse::feature_err;
+use rustc_span::symbol::sym;
+
+crate const CHECK_CUSTOM_CODE_CLASSES: Pass = Pass {
+    name: "check-custom-code-classes",
+    run: check_custom_code_classes,
+    description: "check for custom code classes while not in nightly",
+};
+
+crate fn check_custom_code_classes(krate: Crate, cx: &mut DocContext<'_>) -> Crate {
+    let mut coll = CustomCodeClassLinter { cx };
+
+    coll.fold_crate(krate)
+}
+
+struct CustomCodeClassLinter<'a, 'tcx> {
+    cx: &'a DocContext<'tcx>,
+}
+
+impl<'a, 'tcx> DocFolder for CustomCodeClassLinter<'a, 'tcx> {
+    fn fold_item(&mut self, item: Item) -> Option<Item> {
+        let dox = item.attrs.collapsed_doc_value().unwrap_or_default();
+
+        look_for_custom_classes(&self.cx, &dox, &item);
+
+        Some(self.fold_item_recur(item))
+    }
+}
+
+struct TestsWithCustomClasses {
+    custom_classes_found: Vec<String>,
+}
+
+impl crate::doctest::Tester for TestsWithCustomClasses {
+    fn add_test(&mut self, _: String, config: LangString, _: usize) {
+        self.custom_classes_found.extend(config.added_classes.into_iter());
+    }
+}
+
+crate fn look_for_custom_classes<'tcx>(cx: &DocContext<'tcx>, dox: &str, item: &Item) {
+    let hir_id = match DocContext::as_local_hir_id(cx.tcx, item.def_id) {
+        Some(hir_id) => hir_id,
+        None => {
+            // If non-local, no need to check anything.
+            return;
+        }
+    };
+
+    let mut tests = TestsWithCustomClasses { custom_classes_found: vec![] };
+
+    find_testable_code(&dox, &mut tests, ErrorCodes::No, false, None);
+
+    if !tests.custom_classes_found.is_empty() && !cx.tcx.features().custom_code_classes_in_docs {
+        debug!("reporting error for {:?} (hid_id={:?})", item, hir_id);
+        let sp = span_of_attrs(&item.attrs).unwrap_or(item.source.span());
+        feature_err(
+            &cx.tcx.sess.parse_sess,
+            sym::custom_code_classes_in_docs,
+            sp,
+            "custom classes in code blocks are unstable",
+        )
+        .note(
+            // This will list the wrong items to make them more easily searchable.
+            // To ensure the most correct hits, it adds back the 'class:' that was stripped.
+            &format!(
+                "found these custom classes: class:{}",
+                tests.custom_classes_found.join(", class:")
+            ),
+        )
+        .emit();
+    }
+}