Auto merge of #110800 - GuillaumeGomez:custom_code_classes_in_docs, r=t-rustdoc

Accept additional user-defined syntax classes in fenced code blocks

Part of #79483.

This is a re-opening of https://github.com/rust-lang/rust/pull/79454 after a big update/cleanup. I also converted the syntax to pandoc as suggested by `@notriddle:` the idea is to be as compatible as possible with the existing instead of having our own syntax.

## Motivation

From the original issue: https://github.com/rust-lang/rust/issues/78917

> The technique used by `inline-c-rs` can be ported to other languages. It's just super fun to see C code inside Rust documentation that is also tested by `cargo doc`. I'm sure this technique can be used by other languages in the future.

Having custom CSS classes for syntax highlighting will allow tools like `highlight.js` to be used in order to provide highlighting for languages other than Rust while not increasing technical burden on rustdoc.

## What is the feature about?

In short, this PR changes two things, both related to codeblocks in doc comments in Rust documentation:

 * Allow to disable generation of `language-*` CSS classes with the `custom` attribute.
 * Add your own CSS classes to a code block so that you can use other tools to highlight them.

#### The `custom` attribute

Let's start with the new `custom` attribute: it will disable the generation of the `language-*` CSS class on the generated HTML code block. For example:

```rust
/// ```custom,c
/// int main(void) {
///     return 0;
/// }
/// ```
```

The generated HTML code block will not have `class="language-c"` because the `custom` attribute has been set. The `custom` attribute becomes especially useful with the other thing added by this feature: adding your own CSS classes.

#### Adding your own CSS classes

The second part of this feature is to allow users to add CSS classes themselves so that they can then add a JS library which will do it (like `highlight.js` or `prism.js`), allowing to support highlighting for other languages than Rust without increasing burden on rustdoc. To disable the automatic `language-*` CSS class generation, you need to use the `custom` attribute as well.

This allow users to write the following:

```rust
/// Some code block with `{class=language-c}` as the language string.
///
/// ```custom,{class=language-c}
/// int main(void) {
///     return 0;
/// }
/// ```
fn main() {}
```

This will notably produce the following HTML:

```html
<pre class="language-c">
int main(void) {
    return 0;
}</pre>
```

Instead of:

```html
<pre class="rust rust-example-rendered">
<span class="ident">int</span> <span class="ident">main</span>(<span class="ident">void</span>) {
    <span class="kw">return</span> <span class="number">0</span>;
}
</pre>
```

To be noted, we could have written `{.language-c}` to achieve the same result. `.` and `class=` have the same effect.

One last syntax point: content between parens (`(like this)`) is now considered as comment and is not taken into account at all.

In addition to this, I added an `unknown` field into `LangString` (the parsed code block "attribute") because of cases like this:

```rust
/// ```custom,class:language-c
/// main;
/// ```
pub fn foo() {}
```

Without this `unknown` field, it would generate in the DOM: `<pre class="language-class:language-c language-c">`, which is quite bad. So instead, it now stores all unknown tags into the `unknown` field and use the first one as "language". So in this case, since there is no unknown tag, it'll simply generate `<pre class="language-c">`. I added tests to cover this.

Finally, I added a parser for the codeblock attributes to make it much easier to maintain. It'll be pretty easy to extend.

As to why this syntax for adding attributes was picked: it's [Pandoc's syntax](https://pandoc.org/MANUAL.html#extension-fenced_code_attributes). Even if it seems clunkier in some cases, it's extensible, and most third-party Markdown renderers are smart enough to ignore Pandoc's brace-delimited attributes (from [this comment](https://github.com/rust-lang/rust/pull/110800#issuecomment-1522044456)).

## Raised concerns

#### It's not obvious when the `language-*` attribute generation will be added or not.

It is added by default. If you want to disable it, you will need to use the `custom` attribute.

#### Why not using HTML in markdown directly then?

Code examples in most languages are likely to contain `<`, `>`, `&` and `"` characters. These characters [require escaping](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/pre) when written inside the `<pre>` element. Using the \`\`\` code blocks allows rustdoc to take care of escaping, which means doc authors can paste code samples directly without manually converting them to HTML.

cc `@poliorcetics`
r? `@notriddle`

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -88,7 +88,7 @@
 //!
 //! When generating the `expr` for the `A` impl, the `SubstructureFields` is
 //!
-//! ```{.text}
+//! ```text
 //! Struct(vec![FieldInfo {
 //!            span: <span of x>
 //!            name: Some(<ident of x>),
@@ -99,7 +99,7 @@
 //!
 //! For the `B` impl, called with `B(a)` and `B(b)`,
 //!
-//! ```{.text}
+//! ```text
 //! Struct(vec![FieldInfo {
 //!           span: <span of `i32`>,
 //!           name: None,
@@ -113,7 +113,7 @@
 //! When generating the `expr` for a call with `self == C0(a)` and `other
 //! == C0(b)`, the SubstructureFields is
 //!
-//! ```{.text}
+//! ```text
 //! EnumMatching(0, <ast::Variant for C0>,
 //!              vec![FieldInfo {
 //!                 span: <span of i32>
@@ -125,7 +125,7 @@
 //!
 //! For `C1 {x}` and `C1 {x}`,
 //!
-//! ```{.text}
+//! ```text
 //! EnumMatching(1, <ast::Variant for C1>,
 //!              vec![FieldInfo {
 //!                 span: <span of x>
@@ -137,7 +137,7 @@
 //!
 //! For the tags,
 //!
-//! ```{.text}
+//! ```text
 //! EnumTag(
 //!     &[<ident of self tag>, <ident of other tag>], <expr to combine with>)
 //! ```
@@ -149,7 +149,7 @@
 //!
 //! A static method on the types above would result in,
 //!
-//! ```{.text}
+//! ```text
 //! StaticStruct(<ast::VariantData of A>, Named(vec![(<ident of x>, <span of x>)]))
 //!
 //! StaticStruct(<ast::VariantData of B>, Unnamed(vec![<span of x>]))

compiler/rustc_feature/src/active.rs

@@ -401,6 +401,8 @@ declare_features! (
     /// Allows function attribute `#[coverage(on/off)]`, to control coverage
     /// instrumentation of that function.
     (active, coverage_attribute, "CURRENT_RUSTC_VERSION", Some(84605), None),
+    /// Allows users to provide classes for fenced code block using `class:classname`.
+    (active, custom_code_classes_in_docs, "CURRENT_RUSTC_VERSION", Some(79483), None),
     /// Allows non-builtin attributes in inner attribute position.
     (active, custom_inner_attributes, "1.30.0", Some(54726), None),
     /// Allows custom test frameworks with `#![test_runner]` and `#[test_case]`.

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -165,7 +165,7 @@ pub struct TypeckResults<'tcx> {
     /// reading places that are mentioned in a closure (because of _ patterns). However,
     /// to ensure the places are initialized, we introduce fake reads.
     /// Consider these two examples:
-    /// ``` (discriminant matching with only wildcard arm)
+    /// ```ignore (discriminant matching with only wildcard arm)
     /// let x: u8;
     /// let c = || match x { _ => () };
     /// ```
@@ -173,7 +173,7 @@ pub struct TypeckResults<'tcx> {
     /// want to capture it. However, we do still want an error here, because `x` should have
     /// to be initialized at the point where c is created. Therefore, we add a "fake read"
     /// instead.
-    /// ``` (destructured assignments)
+    /// ```ignore (destructured assignments)
     /// let c = || {
     ///     let (t1, t2) = t;
     /// }

compiler/rustc_span/src/symbol.rs

@@ -592,6 +592,7 @@ symbols! {
         cttz,
         cttz_nonzero,
         custom_attribute,
+        custom_code_classes_in_docs,
         custom_derive,
         custom_inner_attributes,
         custom_mir,

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

@@ -625,3 +625,47 @@ and check the values of `feature`: `foo` and `bar`.
 
 This flag enables the generation of links in the source code pages which allow the reader
 to jump to a type definition.
+
+### Custom CSS classes for code blocks
+
+```rust
+#![feature(custom_code_classes_in_docs)]
+
+/// ```custom,{class=language-c}
+/// int main(void) { return 0; }
+/// ```
+pub struct Bar;
+```
+
+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.
+
+Without the `custom` attribute, it would be generated as a Rust code example with an additional
+`language-C` CSS class. Therefore, if you specifically don't want it to be a Rust code example,
+don't forget to add the `custom` attribute.
+
+To be noted that you can replace `class=` with `.` to achieve the same result:
+
+```rust
+#![feature(custom_code_classes_in_docs)]
+
+/// ```custom,{.language-c}
+/// int main(void) { return 0; }
+/// ```
+pub struct Bar;
+```
+
+To be noted, `rust` and `.rust`/`class=rust` have different effects: `rust` indicates that this is
+a Rust code block whereas the two others add a "rust" CSS class on the code block.
+
+You can also use double quotes:
+
+```rust
+#![feature(custom_code_classes_in_docs)]
+
+/// ```"not rust" {."hello everyone"}
+/// int main(void) { return 0; }
+/// ```
+pub struct Bar;
+```

src/librustdoc/html/highlight.rs

@@ -52,8 +52,9 @@ pub(crate) fn render_example_with_highlighting(
     out: &mut Buffer,
     tooltip: Tooltip,
     playground_button: Option<&str>,
+    extra_classes: &[String],
 ) {
-    write_header(out, "rust-example-rendered", None, tooltip);
+    write_header(out, "rust-example-rendered", None, tooltip, extra_classes);
     write_code(out, src, None, None);
     write_footer(out, playground_button);
 }
@@ -65,7 +66,13 @@ pub(crate) fn render_item_decl_with_highlighting(src: &str, out: &mut Buffer) {
     write!(out, "</pre>");
 }
 
-fn write_header(out: &mut Buffer, class: &str, extra_content: Option<Buffer>, tooltip: Tooltip) {
+fn write_header(
+    out: &mut Buffer,
+    class: &str,
+    extra_content: Option<Buffer>,
+    tooltip: Tooltip,
+    extra_classes: &[String],
+) {
     write!(
         out,
         "<div class=\"example-wrap{}\">",
@@ -100,9 +107,19 @@ fn write_header(out: &mut Buffer, class: &str, extra_content: Option<Buffer>, to
         out.push_buffer(extra);
     }
     if class.is_empty() {
-        write!(out, "<pre class=\"rust\">");
+        write!(
+            out,
+            "<pre class=\"rust{}{}\">",
+            if extra_classes.is_empty() { "" } else { " " },
+            extra_classes.join(" "),
+        );
     } else {
-        write!(out, "<pre class=\"rust {class}\">");
+        write!(
+            out,
+            "<pre class=\"rust {class}{}{}\">",
+            if extra_classes.is_empty() { "" } else { " " },
+            extra_classes.join(" "),
+        );
     }
     write!(out, "<code>");
 }

src/librustdoc/html/markdown.rs

@@ -26,6 +26,7 @@
 //! ```
 
 use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::TyCtxt;
 pub(crate) use rustc_resolve::rustdoc::main_body_opts;
@@ -37,8 +38,9 @@ use once_cell::sync::Lazy;
 use std::borrow::Cow;
 use std::collections::VecDeque;
 use std::fmt::Write;
+use std::iter::Peekable;
 use std::ops::{ControlFlow, Range};
-use std::str;
+use std::str::{self, CharIndices};
 
 use crate::clean::RenderedLink;
 use crate::doctest;
@@ -243,11 +245,21 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
                 let parse_result =
                     LangString::parse_without_check(lang, self.check_error_codes, false);
                 if !parse_result.rust {
+                    let added_classes = parse_result.added_classes;
+                    let lang_string = if let Some(lang) = parse_result.unknown.first() {
+                        format!("language-{}", lang)
+                    } else {
+                        String::new()
+                    };
+                    let whitespace = if added_classes.is_empty() { "" } else { " " };
                     return Some(Event::Html(
                         format!(
                             "<div class=\"example-wrap\">\
-                                 <pre class=\"language-{lang}\"><code>{text}</code></pre>\
+                                 <pre class=\"{lang_string}{whitespace}{added_classes}\">\
+                                     <code>{text}</code>\
+                                 </pre>\
                              </div>",
+                            added_classes = added_classes.join(" "),
                             text = Escape(&original_text),
                         )
                         .into(),
@@ -258,6 +270,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             CodeBlockKind::Indented => Default::default(),
         };
 
+        let added_classes = parse_result.added_classes;
         let lines = original_text.lines().filter_map(|l| map_line(l).for_html());
         let text = lines.intersperse("\n".into()).collect::<String>();
 
@@ -315,6 +328,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             &mut s,
             tooltip,
             playground_button.as_deref(),
+            &added_classes,
         );
         Some(Event::Html(s.into_inner().into()))
     }
@@ -711,6 +725,17 @@ pub(crate) fn find_testable_code<T: doctest::Tester>(
     error_codes: ErrorCodes,
     enable_per_target_ignores: bool,
     extra_info: Option<&ExtraInfo<'_>>,
+) {
+    find_codes(doc, tests, error_codes, enable_per_target_ignores, extra_info, false)
+}
+
+pub(crate) fn find_codes<T: doctest::Tester>(
+    doc: &str,
+    tests: &mut T,
+    error_codes: ErrorCodes,
+    enable_per_target_ignores: bool,
+    extra_info: Option<&ExtraInfo<'_>>,
+    include_non_rust: bool,
 ) {
     let mut parser = Parser::new(doc).into_offset_iter();
     let mut prev_offset = 0;
@@ -734,7 +759,7 @@ pub(crate) fn find_testable_code<T: doctest::Tester>(
                     }
                     CodeBlockKind::Indented => Default::default(),
                 };
-                if !block_info.rust {
+                if !include_non_rust && !block_info.rust {
                     continue;
                 }
 
@@ -784,7 +809,23 @@ impl<'tcx> ExtraInfo<'tcx> {
         ExtraInfo { def_id, sp, tcx }
     }
 
-    fn error_invalid_codeblock_attr(&self, msg: String, help: &'static str) {
+    fn error_invalid_codeblock_attr(&self, msg: impl Into<DiagnosticMessage>) {
+        if let Some(def_id) = self.def_id.as_local() {
+            self.tcx.struct_span_lint_hir(
+                crate::lint::INVALID_CODEBLOCK_ATTRIBUTES,
+                self.tcx.hir().local_def_id_to_hir_id(def_id),
+                self.sp,
+                msg,
+                |l| l,
+            );
+        }
+    }
+
+    fn error_invalid_codeblock_attr_with_help(
+        &self,
+        msg: impl Into<DiagnosticMessage>,
+        help: impl Into<SubdiagnosticMessage>,
+    ) {
         if let Some(def_id) = self.def_id.as_local() {
             self.tcx.struct_span_lint_hir(
                 crate::lint::INVALID_CODEBLOCK_ATTRIBUTES,
@@ -808,6 +849,8 @@ pub(crate) struct LangString {
     pub(crate) compile_fail: bool,
     pub(crate) error_codes: Vec<String>,
     pub(crate) edition: Option<Edition>,
+    pub(crate) added_classes: Vec<String>,
+    pub(crate) unknown: Vec<String>,
 }
 
 #[derive(Eq, PartialEq, Clone, Debug)]
@@ -817,6 +860,276 @@ pub(crate) enum Ignore {
     Some(Vec<String>),
 }
 
+/// This is the parser for fenced codeblocks attributes. It implements the following eBNF:
+///
+/// ```eBNF
+/// lang-string = *(token-list / delimited-attribute-list / comment)
+///
+/// bareword = CHAR *(CHAR)
+/// quoted-string = QUOTE *(NONQUOTE) QUOTE
+/// token = bareword / quoted-string
+/// sep = COMMA/WS *(COMMA/WS)
+/// attribute = (DOT token)/(token EQUAL token)
+/// attribute-list = [sep] attribute *(sep attribute) [sep]
+/// delimited-attribute-list = OPEN-CURLY-BRACKET attribute-list CLOSE-CURLY-BRACKET
+/// token-list = [sep] token *(sep token) [sep]
+/// comment = OPEN_PAREN *(all characters) CLOSE_PAREN
+///
+/// OPEN_PAREN = "("
+/// CLOSE_PARENT = ")"
+/// OPEN-CURLY-BRACKET = "{"
+/// CLOSE-CURLY-BRACKET = "}"
+/// CHAR = ALPHA / DIGIT / "_" / "-" / ":"
+/// QUOTE = %x22
+/// NONQUOTE = %x09 / %x20 / %x21 / %x23-7E ; TAB / SPACE / all printable characters except `"`
+/// COMMA = ","
+/// DOT = "."
+/// EQUAL = "="
+///
+/// ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
+/// DIGIT = %x30-39
+/// WS = %x09 / " "
+/// ```
+pub(crate) struct TagIterator<'a, 'tcx> {
+    inner: Peekable<CharIndices<'a>>,
+    data: &'a str,
+    is_in_attribute_block: bool,
+    extra: Option<&'a ExtraInfo<'tcx>>,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub(crate) enum LangStringToken<'a> {
+    LangToken(&'a str),
+    ClassAttribute(&'a str),
+    KeyValueAttribute(&'a str, &'a str),
+}
+
+fn is_bareword_char(c: char) -> bool {
+    c == '_' || c == '-' || c == ':' || c.is_ascii_alphabetic() || c.is_ascii_digit()
+}
+fn is_separator(c: char) -> bool {
+    c == ' ' || c == ',' || c == '\t'
+}
+
+struct Indices {
+    start: usize,
+    end: usize,
+}
+
+impl<'a, 'tcx> TagIterator<'a, 'tcx> {
+    pub(crate) fn new(data: &'a str, extra: Option<&'a ExtraInfo<'tcx>>) -> Self {
+        Self { inner: data.char_indices().peekable(), data, is_in_attribute_block: false, extra }
+    }
+
+    fn emit_error(&self, err: impl Into<DiagnosticMessage>) {
+        if let Some(extra) = self.extra {
+            extra.error_invalid_codeblock_attr(err);
+        }
+    }
+
+    fn skip_separators(&mut self) -> Option<usize> {
+        while let Some((pos, c)) = self.inner.peek() {
+            if !is_separator(*c) {
+                return Some(*pos);
+            }
+            self.inner.next();
+        }
+        None
+    }
+
+    fn parse_string(&mut self, start: usize) -> Option<Indices> {
+        while let Some((pos, c)) = self.inner.next() {
+            if c == '"' {
+                return Some(Indices { start: start + 1, end: pos });
+            }
+        }
+        self.emit_error("unclosed quote string `\"`");
+        None
+    }
+
+    fn parse_class(&mut self, start: usize) -> Option<LangStringToken<'a>> {
+        while let Some((pos, c)) = self.inner.peek().copied() {
+            if is_bareword_char(c) {
+                self.inner.next();
+            } else {
+                let class = &self.data[start + 1..pos];
+                if class.is_empty() {
+                    self.emit_error(format!("unexpected `{c}` character after `.`"));
+                    return None;
+                } else if self.check_after_token() {
+                    return Some(LangStringToken::ClassAttribute(class));
+                } else {
+                    return None;
+                }
+            }
+        }
+        let class = &self.data[start + 1..];
+        if class.is_empty() {
+            self.emit_error("missing character after `.`");
+            None
+        } else if self.check_after_token() {
+            Some(LangStringToken::ClassAttribute(class))
+        } else {
+            None
+        }
+    }
+
+    fn parse_token(&mut self, start: usize) -> Option<Indices> {
+        while let Some((pos, c)) = self.inner.peek() {
+            if !is_bareword_char(*c) {
+                return Some(Indices { start, end: *pos });
+            }
+            self.inner.next();
+        }
+        self.emit_error("unexpected end");
+        None
+    }
+
+    fn parse_key_value(&mut self, c: char, start: usize) -> Option<LangStringToken<'a>> {
+        let key_indices =
+            if c == '"' { self.parse_string(start)? } else { self.parse_token(start)? };
+        if key_indices.start == key_indices.end {
+            self.emit_error("unexpected empty string as key");
+            return None;
+        }
+
+        if let Some((_, c)) = self.inner.next() {
+            if c != '=' {
+                self.emit_error(format!("expected `=`, found `{}`", c));
+                return None;
+            }
+        } else {
+            self.emit_error("unexpected end");
+            return None;
+        }
+        let value_indices = match self.inner.next() {
+            Some((pos, '"')) => self.parse_string(pos)?,
+            Some((pos, c)) if is_bareword_char(c) => self.parse_token(pos)?,
+            Some((_, c)) => {
+                self.emit_error(format!("unexpected `{c}` character after `=`"));
+                return None;
+            }
+            None => {
+                self.emit_error("expected value after `=`");
+                return None;
+            }
+        };
+        if value_indices.start == value_indices.end {
+            self.emit_error("unexpected empty string as value");
+            None
+        } else if self.check_after_token() {
+            Some(LangStringToken::KeyValueAttribute(
+                &self.data[key_indices.start..key_indices.end],
+                &self.data[value_indices.start..value_indices.end],
+            ))
+        } else {
+            None
+        }
+    }
+
+    /// Returns `false` if an error was emitted.
+    fn check_after_token(&mut self) -> bool {
+        if let Some((_, c)) = self.inner.peek().copied() {
+            if c == '}' || is_separator(c) || c == '(' {
+                true
+            } else {
+                self.emit_error(format!("unexpected `{c}` character"));
+                false
+            }
+        } else {
+            // The error will be caught on the next iteration.
+            true
+        }
+    }
+
+    fn parse_in_attribute_block(&mut self) -> Option<LangStringToken<'a>> {
+        while let Some((pos, c)) = self.inner.next() {
+            if c == '}' {
+                self.is_in_attribute_block = false;
+                return self.next();
+            } else if c == '.' {
+                return self.parse_class(pos);
+            } else if c == '"' || is_bareword_char(c) {
+                return self.parse_key_value(c, pos);
+            } else {
+                self.emit_error(format!("unexpected character `{c}`"));
+                return None;
+            }
+        }
+        self.emit_error("unclosed attribute block (`{}`): missing `}` at the end");
+        None
+    }
+
+    /// Returns `false` if an error was emitted.
+    fn skip_paren_block(&mut self) -> bool {
+        while let Some((_, c)) = self.inner.next() {
+            if c == ')' {
+                return true;
+            }
+        }
+        self.emit_error("unclosed comment: missing `)` at the end");
+        false
+    }
+
+    fn parse_outside_attribute_block(&mut self, start: usize) -> Option<LangStringToken<'a>> {
+        while let Some((pos, c)) = self.inner.next() {
+            if c == '"' {
+                if pos != start {
+                    self.emit_error("expected ` `, `{` or `,` found `\"`");
+                    return None;
+                }
+                let indices = self.parse_string(pos)?;
+                if let Some((_, c)) = self.inner.peek().copied() && c != '{' && !is_separator(c) && c != '(' {
+                    self.emit_error(format!("expected ` `, `{{` or `,` after `\"`, found `{c}`"));
+                    return None;
+                }
+                return Some(LangStringToken::LangToken(&self.data[indices.start..indices.end]));
+            } else if c == '{' {
+                self.is_in_attribute_block = true;
+                return self.next();
+            } else if is_bareword_char(c) {
+                continue;
+            } else if is_separator(c) {
+                if pos != start {
+                    return Some(LangStringToken::LangToken(&self.data[start..pos]));
+                }
+                return self.next();
+            } else if c == '(' {
+                if !self.skip_paren_block() {
+                    return None;
+                }
+                if pos != start {
+                    return Some(LangStringToken::LangToken(&self.data[start..pos]));
+                }
+                return self.next();
+            } else {
+                self.emit_error(format!("unexpected character `{c}`"));
+                return None;
+            }
+        }
+        let token = &self.data[start..];
+        if token.is_empty() { None } else { Some(LangStringToken::LangToken(&self.data[start..])) }
+    }
+}
+
+impl<'a, 'tcx> Iterator for TagIterator<'a, 'tcx> {
+    type Item = LangStringToken<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let Some(start) = self.skip_separators() else {
+            if self.is_in_attribute_block {
+                self.emit_error("unclosed attribute block (`{}`): missing `}` at the end");
+            }
+            return None;
+        };
+        if self.is_in_attribute_block {
+            self.parse_in_attribute_block()
+        } else {
+            self.parse_outside_attribute_block(start)
+        }
+    }
+}
+
 impl Default for LangString {
     fn default() -> Self {
         Self {
@@ -829,6 +1142,8 @@ impl Default for LangString {
             compile_fail: false,
             error_codes: Vec::new(),
             edition: None,
+            added_classes: Vec::new(),
+            unknown: Vec::new(),
         }
     }
 }
@@ -838,86 +1153,67 @@ 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)
     }
 
-    fn tokens(string: &str) -> impl Iterator<Item = &str> {
-        // Pandoc, which Rust once used for generating documentation,
-        // expects lang strings to be surrounded by `{}` and for each token
-        // to be proceeded by a `.`. Since some of these lang strings are still
-        // loose in the wild, we strip a pair of surrounding `{}` from the lang
-        // string and a leading `.` from each token.
-
-        let string = string.trim();
-
-        let first = string.chars().next();
-        let last = string.chars().last();
-
-        let string = if first == Some('{') && last == Some('}') {
-            &string[1..string.len() - 1]
-        } else {
-            string
-        };
-
-        string
-            .split(|c| c == ',' || c == ' ' || c == '\t')
-            .map(str::trim)
-            .map(|token| token.strip_prefix('.').unwrap_or(token))
-            .filter(|token| !token.is_empty())
-    }
-
     fn parse(
         string: &str,
         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;
+        let mut seen_custom_tag = false;
         let mut data = LangString::default();
         let mut ignores = vec![];
 
         data.original = string.to_owned();
 
-        for token in Self::tokens(string) {
+        for token in TagIterator::new(string, extra) {
             match token {
-                "should_panic" => {
+                LangStringToken::LangToken("should_panic") => {
                     data.should_panic = true;
                     seen_rust_tags = !seen_other_tags;
                 }
-                "no_run" => {
+                LangStringToken::LangToken("no_run") => {
                     data.no_run = true;
                     seen_rust_tags = !seen_other_tags;
                 }
-                "ignore" => {
+                LangStringToken::LangToken("ignore") => {
                     data.ignore = Ignore::All;
                     seen_rust_tags = !seen_other_tags;
                 }
-                x if x.starts_with("ignore-") => {
+                LangStringToken::LangToken(x) if x.starts_with("ignore-") => {
                     if enable_per_target_ignores {
                         ignores.push(x.trim_start_matches("ignore-").to_owned());
                         seen_rust_tags = !seen_other_tags;
                     }
                 }
-                "rust" => {
+                LangStringToken::LangToken("rust") => {
                     data.rust = true;
                     seen_rust_tags = true;
                 }
-                "test_harness" => {
+                LangStringToken::LangToken("custom") => {
+                    seen_custom_tag = true;
+                }
+                LangStringToken::LangToken("test_harness") => {
                     data.test_harness = true;
                     seen_rust_tags = !seen_other_tags || seen_rust_tags;
                 }
-                "compile_fail" => {
+                LangStringToken::LangToken("compile_fail") => {
                     data.compile_fail = true;
                     seen_rust_tags = !seen_other_tags || seen_rust_tags;
                     data.no_run = true;
                 }
-                x if x.starts_with("edition") => {
+                LangStringToken::LangToken(x) if x.starts_with("edition") => {
                     data.edition = x[7..].parse::<Edition>().ok();
                 }
-                x if allow_error_code_check && x.starts_with('E') && x.len() == 5 => {
+                LangStringToken::LangToken(x)
+                    if allow_error_code_check && x.starts_with('E') && x.len() == 5 =>
+                {
                     if x[1..].parse::<u32>().is_ok() {
                         data.error_codes.push(x.to_owned());
                         seen_rust_tags = !seen_other_tags || seen_rust_tags;
@@ -925,7 +1221,7 @@ impl LangString {
                         seen_other_tags = true;
                     }
                 }
-                x if extra.is_some() => {
+                LangStringToken::LangToken(x) if extra.is_some() => {
                     let s = x.to_lowercase();
                     if let Some((flag, help)) = if s == "compile-fail"
                         || s == "compile_fail"
@@ -958,15 +1254,30 @@ impl LangString {
                         None
                     } {
                         if let Some(extra) = extra {
-                            extra.error_invalid_codeblock_attr(
+                            extra.error_invalid_codeblock_attr_with_help(
                                 format!("unknown attribute `{x}`. Did you mean `{flag}`?"),
                                 help,
                             );
                         }
                     }
                     seen_other_tags = true;
+                    data.unknown.push(x.to_owned());
+                }
+                LangStringToken::LangToken(x) => {
+                    seen_other_tags = true;
+                    data.unknown.push(x.to_owned());
+                }
+                LangStringToken::KeyValueAttribute(key, value) => {
+                    if key == "class" {
+                        data.added_classes.push(value.to_owned());
+                    } else if let Some(extra) = extra {
+                        extra
+                            .error_invalid_codeblock_attr(format!("unsupported attribute `{key}`"));
+                    }
+                }
+                LangStringToken::ClassAttribute(class) => {
+                    data.added_classes.push(class.to_owned());
                 }
-                _ => seen_other_tags = true,
             }
         }
 
@@ -975,7 +1286,7 @@ impl LangString {
             data.ignore = Ignore::Some(ignores);
         }
 
-        data.rust &= !seen_other_tags || seen_rust_tags;
+        data.rust &= !seen_custom_tag && (!seen_other_tags || seen_rust_tags);
 
         data
     }

src/librustdoc/html/markdown/tests.rs

@@ -1,5 +1,8 @@
 use super::{find_testable_code, plain_text_summary, short_markdown_summary};
-use super::{ErrorCodes, HeadingOffset, IdMap, Ignore, LangString, Markdown, MarkdownItemInfo};
+use super::{
+    ErrorCodes, HeadingOffset, IdMap, Ignore, LangString, LangStringToken, Markdown,
+    MarkdownItemInfo, TagIterator,
+};
 use rustc_span::edition::{Edition, DEFAULT_EDITION};
 
 #[test]
@@ -51,10 +54,32 @@ fn test_lang_string_parse() {
 
     t(Default::default());
     t(LangString { original: "rust".into(), ..Default::default() });
-    t(LangString { original: ".rust".into(), ..Default::default() });
+    t(LangString {
-    t(LangString { original: "{rust}".into(), ..Default::default() });
+        original: "rusta".into(),
-    t(LangString { original: "{.rust}".into(), ..Default::default() });
+        rust: false,
-    t(LangString { original: "sh".into(), rust: false, ..Default::default() });
+        unknown: vec!["rusta".into()],
+        ..Default::default()
+    });
+    // error
+    t(LangString { original: "{rust}".into(), rust: true, ..Default::default() });
+    t(LangString {
+        original: "{.rust}".into(),
+        rust: true,
+        added_classes: vec!["rust".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{.rust}".into(),
+        rust: false,
+        added_classes: vec!["rust".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "sh".into(),
+        rust: false,
+        unknown: vec!["sh".into()],
+        ..Default::default()
+    });
     t(LangString { original: "ignore".into(), ignore: Ignore::All, ..Default::default() });
     t(LangString {
         original: "ignore-foo".into(),
@@ -70,41 +95,56 @@ fn test_lang_string_parse() {
         compile_fail: true,
         ..Default::default()
     });
-    t(LangString { original: "no_run,example".into(), no_run: true, ..Default::default() });
+    t(LangString {
+        original: "no_run,example".into(),
+        no_run: true,
+        unknown: vec!["example".into()],
+        ..Default::default()
+    });
     t(LangString {
         original: "sh,should_panic".into(),
         should_panic: true,
         rust: false,
+        unknown: vec!["sh".into()],
         ..Default::default()
     });
-    t(LangString { original: "example,rust".into(), ..Default::default() });
     t(LangString {
-        original: "test_harness,.rust".into(),
+        original: "example,rust".into(),
+        unknown: vec!["example".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "test_harness,rusta".into(),
         test_harness: true,
+        unknown: vec!["rusta".into()],
         ..Default::default()
     });
     t(LangString {
         original: "text, no_run".into(),
         no_run: true,
         rust: false,
+        unknown: vec!["text".into()],
         ..Default::default()
     });
     t(LangString {
         original: "text,no_run".into(),
         no_run: true,
         rust: false,
+        unknown: vec!["text".into()],
         ..Default::default()
     });
     t(LangString {
         original: "text,no_run, ".into(),
         no_run: true,
         rust: false,
+        unknown: vec!["text".into()],
         ..Default::default()
     });
     t(LangString {
         original: "text,no_run,".into(),
         no_run: true,
         rust: false,
+        unknown: vec!["text".into()],
         ..Default::default()
     });
     t(LangString {
@@ -117,29 +157,125 @@ fn test_lang_string_parse() {
         edition: Some(Edition::Edition2018),
         ..Default::default()
     });
+    t(LangString {
+        original: "{class=test}".into(),
+        added_classes: vec!["test".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=test}".into(),
+        added_classes: vec!["test".into()],
+        rust: false,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "{.test}".into(),
+        added_classes: vec!["test".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{.test}".into(),
+        added_classes: vec!["test".into()],
+        rust: false,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "rust,{class=test,.test2}".into(),
+        added_classes: vec!["test".into(), "test2".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "{class=test:with:colon .test1}".into(),
+        added_classes: vec!["test:with:colon".into(), "test1".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=test:with:colon .test1}".into(),
+        added_classes: vec!["test:with:colon".into(), "test1".into()],
+        rust: false,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "{class=first,class=second}".into(),
+        added_classes: vec!["first".into(), "second".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "custom,{class=first,class=second}".into(),
+        added_classes: vec!["first".into(), "second".into()],
+        rust: false,
+        ..Default::default()
+    });
+    t(LangString {
+        original: "{class=first,.second},unknown".into(),
+        added_classes: vec!["first".into(), "second".into()],
+        rust: false,
+        unknown: vec!["unknown".into()],
+        ..Default::default()
+    });
+    t(LangString {
+        original: "{class=first .second} unknown".into(),
+        added_classes: vec!["first".into(), "second".into()],
+        rust: false,
+        unknown: vec!["unknown".into()],
+        ..Default::default()
+    });
+    // error
+    t(LangString { original: "{.first.second}".into(), rust: true, ..Default::default() });
+    // error
+    t(LangString { original: "{class=first=second}".into(), rust: true, ..Default::default() });
+    // error
+    t(LangString { original: "{class=first.second}".into(), rust: true, ..Default::default() });
+    // error
+    t(LangString { original: "{class=.first}".into(), rust: true, ..Default::default() });
+    t(LangString {
+        original: r#"{class="first"}"#.into(),
+        added_classes: vec!["first".into()],
+        rust: true,
+        ..Default::default()
+    });
+    t(LangString {
+        original: r#"custom,{class="first"}"#.into(),
+        added_classes: vec!["first".into()],
+        rust: false,
+        ..Default::default()
+    });
+    // error
+    t(LangString { original: r#"{class=f"irst"}"#.into(), rust: true, ..Default::default() });
 }
 
 #[test]
 fn test_lang_string_tokenizer() {
-    fn case(lang_string: &str, want: &[&str]) {
+    fn case(lang_string: &str, want: &[LangStringToken<'_>]) {
-        let have = LangString::tokens(lang_string).collect::<Vec<&str>>();
+        let have = TagIterator::new(lang_string, None).collect::<Vec<_>>();
         assert_eq!(have, want, "Unexpected lang string split for `{}`", lang_string);
     }
 
     case("", &[]);
-    case("foo", &["foo"]);
+    case("foo", &[LangStringToken::LangToken("foo")]);
-    case("foo,bar", &["foo", "bar"]);
+    case("foo,bar", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
-    case(".foo,.bar", &["foo", "bar"]);
+    case(".foo,.bar", &[]);
-    case("{.foo,.bar}", &["foo", "bar"]);
+    case(
-    case("  {.foo,.bar}  ", &["foo", "bar"]);
+        "{.foo,.bar}",
-    case("foo bar", &["foo", "bar"]);
+        &[LangStringToken::ClassAttribute("foo"), LangStringToken::ClassAttribute("bar")],
-    case("foo\tbar", &["foo", "bar"]);
+    );
-    case("foo\t, bar", &["foo", "bar"]);
+    case(
-    case(" foo , bar ", &["foo", "bar"]);
+        "  {.foo,.bar}  ",
-    case(",,foo,,bar,,", &["foo", "bar"]);
+        &[LangStringToken::ClassAttribute("foo"), LangStringToken::ClassAttribute("bar")],
-    case("foo=bar", &["foo=bar"]);
+    );
-    case("a-b-c", &["a-b-c"]);
+    case("foo bar", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
-    case("a_b_c", &["a_b_c"]);
+    case("foo\tbar", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
+    case("foo\t, bar", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
+    case(" foo , bar ", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
+    case(",,foo,,bar,,", &[LangStringToken::LangToken("foo"), LangStringToken::LangToken("bar")]);
+    case("foo=bar", &[]);
+    case("a-b-c", &[LangStringToken::LangToken("a-b-c")]);
+    case("a_b_c", &[LangStringToken::LangToken("a_b_c")]);
 }
 
 #[test]

src/librustdoc/passes/check_custom_code_classes.rs

@@ -0,0 +1,77 @@
+//! 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::Pass;
+use crate::clean::{Crate, Item};
+use crate::core::DocContext;
+use crate::fold::DocFolder;
+use crate::html::markdown::{find_codes, ErrorCodes, LangString};
+
+use rustc_session::parse::feature_err;
+use rustc_span::symbol::sym;
+
+pub(crate) const CHECK_CUSTOM_CODE_CLASSES: Pass = Pass {
+    name: "check-custom-code-classes",
+    run: check_custom_code_classes,
+    description: "check for custom code classes without the feature-gate enabled",
+};
+
+pub(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> {
+        look_for_custom_classes(&self.cx, &item);
+        Some(self.fold_item_recur(item))
+    }
+}
+
+#[derive(Debug)]
+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());
+    }
+}
+
+pub(crate) fn look_for_custom_classes<'tcx>(cx: &DocContext<'tcx>, item: &Item) {
+    if !item.item_id.is_local() {
+        // If non-local, no need to check anything.
+        return;
+    }
+
+    let mut tests = TestsWithCustomClasses { custom_classes_found: vec![] };
+
+    let dox = item.attrs.doc_value();
+    find_codes(&dox, &mut tests, ErrorCodes::No, false, None, true);
+
+    if !tests.custom_classes_found.is_empty() && !cx.tcx.features().custom_code_classes_in_docs {
+        feature_err(
+            &cx.tcx.sess.parse_sess,
+            sym::custom_code_classes_in_docs,
+            item.attr_span(cx.tcx),
+            "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();
+    }
+}

src/librustdoc/passes/mod.rs

@@ -35,6 +35,9 @@ pub(crate) use self::calculate_doc_coverage::CALCULATE_DOC_COVERAGE;
 mod lint;
 pub(crate) use self::lint::RUN_LINTS;
 
+mod check_custom_code_classes;
+pub(crate) use self::check_custom_code_classes::CHECK_CUSTOM_CODE_CLASSES;
+
 /// A single pass over the cleaned documentation.
 ///
 /// Runs in the compiler context, so it has access to types and traits and the like.
@@ -66,6 +69,7 @@ pub(crate) enum Condition {
 
 /// The full list of passes.
 pub(crate) const PASSES: &[Pass] = &[
+    CHECK_CUSTOM_CODE_CLASSES,
     CHECK_DOC_TEST_VISIBILITY,
     STRIP_HIDDEN,
     STRIP_PRIVATE,
@@ -79,6 +83,7 @@ pub(crate) const PASSES: &[Pass] = &[
 
 /// The list of passes run by default.
 pub(crate) const DEFAULT_PASSES: &[ConditionalPass] = &[
+    ConditionalPass::always(CHECK_CUSTOM_CODE_CLASSES),
     ConditionalPass::always(COLLECT_TRAIT_IMPLS),
     ConditionalPass::always(CHECK_DOC_TEST_VISIBILITY),
     ConditionalPass::new(STRIP_HIDDEN, WhenNotDocumentHidden),

tests/rustdoc-ui/custom_code_classes_in_docs-warning.rs

@@ -0,0 +1,85 @@
+// This test ensures that warnings are working as expected for "custom_code_classes_in_docs"
+// feature.
+
+#![feature(custom_code_classes_in_docs)]
+#![deny(warnings)]
+#![feature(no_core)]
+#![no_core]
+
+/// ```{. }
+/// main;
+/// ```
+//~^^^ ERROR unexpected ` ` character after `.`
+pub fn foo() {}
+
+/// ```{class= a}
+/// main;
+/// ```
+//~^^^ ERROR unexpected ` ` character after `=`
+pub fn foo2() {}
+
+/// ```{#id}
+/// main;
+/// ```
+//~^^^ ERROR unexpected character `#`
+pub fn foo3() {}
+
+/// ```{{
+/// main;
+/// ```
+//~^^^ ERROR unexpected character `{`
+pub fn foo4() {}
+
+/// ```}
+/// main;
+/// ```
+//~^^^ ERROR unexpected character `}`
+pub fn foo5() {}
+
+/// ```)
+/// main;
+/// ```
+//~^^^ ERROR unexpected character `)`
+pub fn foo6() {}
+
+/// ```{class=}
+/// main;
+/// ```
+//~^^^ ERROR unexpected `}` character after `=`
+pub fn foo7() {}
+
+/// ```(
+/// main;
+/// ```
+//~^^^ ERROR unclosed comment: missing `)` at the end
+pub fn foo8() {}
+
+/// ```{class=one=two}
+/// main;
+/// ```
+//~^^^ ERROR unexpected `=`
+pub fn foo9() {}
+
+/// ```{.one.two}
+/// main;
+/// ```
+//~^^^ ERROR unexpected `.` character
+pub fn foo10() {}
+
+/// ```{class=.one}
+/// main;
+/// ```
+//~^^^ ERROR unexpected `.` character after `=`
+pub fn foo11() {}
+
+/// ```{class=one.two}
+/// main;
+/// ```
+//~^^^ ERROR unexpected `.` character
+pub fn foo12() {}
+
+/// ```{(comment)}
+/// main;
+/// ```
+//~^^^ ERROR unexpected character `(`
+pub fn foo13() {}

tests/rustdoc-ui/custom_code_classes_in_docs-warning.stderr

@@ -0,0 +1,113 @@
+error: unexpected ` ` character after `.`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:9:1
+   |
+LL | / /// ```{. }
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+   |
+note: the lint level is defined here
+  --> $DIR/custom_code_classes_in_docs-warning.rs:5:9
+   |
+LL | #![deny(warnings)]
+   |         ^^^^^^^^
+   = note: `#[deny(rustdoc::invalid_codeblock_attributes)]` implied by `#[deny(warnings)]`
+
+error: unexpected ` ` character after `=`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:15:1
+   |
+LL | / /// ```{class= a}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected character `#`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:21:1
+   |
+LL | / /// ```{#id}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected character `{`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:27:1
+   |
+LL | / /// ```{{
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected character `}`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:33:1
+   |
+LL | / /// ```}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected character `)`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:39:1
+   |
+LL | / /// ```)
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected `}` character after `=`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:45:1
+   |
+LL | / /// ```{class=}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unclosed comment: missing `)` at the end
+  --> $DIR/custom_code_classes_in_docs-warning.rs:51:1
+   |
+LL | / /// ```(
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected `=` character
+  --> $DIR/custom_code_classes_in_docs-warning.rs:57:1
+   |
+LL | / /// ```{class=one=two}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected `.` character
+  --> $DIR/custom_code_classes_in_docs-warning.rs:63:1
+   |
+LL | / /// ```{.one.two}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected `.` character after `=`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:69:1
+   |
+LL | / /// ```{class=.one}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected `.` character
+  --> $DIR/custom_code_classes_in_docs-warning.rs:75:1
+   |
+LL | / /// ```{class=one.two}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: unexpected character `(`
+  --> $DIR/custom_code_classes_in_docs-warning.rs:81:1
+   |
+LL | / /// ```{(comment)}
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: aborting due to 13 previous errors
+

tests/rustdoc-ui/custom_code_classes_in_docs-warning3.rs

@@ -0,0 +1,17 @@
+// This test ensures that warnings are working as expected for "custom_code_classes_in_docs"
+// feature.
+
+#![feature(custom_code_classes_in_docs)]
+#![deny(warnings)]
+#![feature(no_core)]
+#![no_core]
+
+/// ```{class="}
+/// main;
+/// ```
+//~^^^ ERROR unclosed quote string
+//~| ERROR unclosed quote string
+/// ```"
+/// main;
+/// ```
+pub fn foo() {}

tests/rustdoc-ui/custom_code_classes_in_docs-warning3.stderr

@@ -0,0 +1,33 @@
+error: unclosed quote string `"`
+  --> $DIR/custom_code_classes_in_docs-warning3.rs:9:1
+   |
+LL | / /// ```{class="}
+LL | | /// main;
+LL | | /// ```
+LL | |
+...  |
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+   |
+note: the lint level is defined here
+  --> $DIR/custom_code_classes_in_docs-warning3.rs:5:9
+   |
+LL | #![deny(warnings)]
+   |         ^^^^^^^^
+   = note: `#[deny(rustdoc::invalid_codeblock_attributes)]` implied by `#[deny(warnings)]`
+
+error: unclosed quote string `"`
+  --> $DIR/custom_code_classes_in_docs-warning3.rs:9:1
+   |
+LL | / /// ```{class="}
+LL | | /// main;
+LL | | /// ```
+LL | |
+...  |
+LL | | /// main;
+LL | | /// ```
+   | |_______^
+
+error: aborting due to 2 previous errors
+

tests/rustdoc-ui/feature-gate-custom_code_classes_in_docs.rs

@@ -0,0 +1,5 @@
+/// ```{class=language-c}
+/// int main(void) { return 0; }
+/// ```
+//~^^^ ERROR 1:1: 3:8: custom classes in code blocks are unstable [E0658]
+pub struct Bar;

tests/rustdoc-ui/feature-gate-custom_code_classes_in_docs.stderr

@@ -0,0 +1,15 @@
+error[E0658]: custom classes in code blocks are unstable
+  --> $DIR/feature-gate-custom_code_classes_in_docs.rs:1:1
+   |
+LL | / /// ```{class=language-c}
+LL | | /// int main(void) { return 0; }
+LL | | /// ```
+   | |_______^
+   |
+   = note: see issue #79483 <https://github.com/rust-lang/rust/issues/79483> for more information
+   = help: add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
+   = note: found these custom classes: class=language-c
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/rustdoc-ui/issues/issue-91713.stdout

@@ -1,4 +1,5 @@
 Available passes for running rustdoc:
+check-custom-code-classes - check for custom code classes without the feature-gate enabled
 check_doc_test_visibility - run various visibility-related lints on doctests
         strip-hidden - strips all `#[doc(hidden)]` items from the output
        strip-private - strips all private items from a crate which cannot be seen externally, implies strip-priv-imports
@@ -10,6 +11,7 @@ calculate-doc-coverage - counts the number of items with and without documentati
            run-lints - runs some of rustdoc's lints
 
 Default passes for rustdoc:
+check-custom-code-classes
  collect-trait-impls
 check_doc_test_visibility
         strip-hidden  (when not --document-hidden-items)

tests/rustdoc/custom_code_classes.rs

@@ -0,0 +1,28 @@
+// Test for `custom_code_classes_in_docs` feature.
+
+#![feature(custom_code_classes_in_docs)]
+#![crate_name = "foo"]
+#![feature(no_core)]
+#![no_core]
+
+// @has 'foo/struct.Bar.html'
+// @has - '//*[@id="main-content"]//pre[@class="language-whatever hoho-c"]' 'main;'
+// @has - '//*[@id="main-content"]//pre[@class="language-whatever2 haha-c"]' 'main;'
+// @has - '//*[@id="main-content"]//pre[@class="language-whatever4 huhu-c"]' 'main;'
+
+/// ```{class=hoho-c},whatever
+/// main;
+/// ```
+///
+/// Testing multiple kinds of orders.
+///
+/// ```whatever2 {class=haha-c}
+/// main;
+/// ```
+///
+/// Testing with multiple "unknown". Only the first should be used.
+///
+/// ```whatever4,{.huhu-c} whatever5
+/// main;
+/// ```
+pub struct Bar;