Accept E<error_number> notation in doctests

```compile_fail,E0000
```

The code was stolen from rustdoc at https://github.com/rust-lang/rust/blob/392ba2ba1a7d6c542d2459fb8133bebf62a4a423/src/librustdoc/html/markdown.rs#L866-L867

Author: ChayimFriedman2

crates/ide/src/syntax_highlighting/inject.rs

@@ -90,6 +90,13 @@ const RUSTDOC_FENCE_TOKENS: &[&'static str] = &[
     "edition2021",
 ];
 
+fn is_rustdoc_fence_token(token: &str) -> bool {
+    if RUSTDOC_FENCE_TOKENS.contains(&token) {
+        return true;
+    }
+    token.starts_with('E') && token.len() == 5 && token[1..].parse::<u32>().is_ok()
+}
+
 /// Injection of syntax highlighting of doctests.
 pub(super) fn doc_comment(
     hl: &mut Highlights,
@@ -174,8 +181,7 @@ pub(super) fn doc_comment(
                     is_codeblock = !is_codeblock;
                     // Check whether code is rust by inspecting fence guards
                     let guards = &line[idx + RUSTDOC_FENCE.len()..];
-                    let is_rust =
-                        guards.split(',').all(|sub| RUSTDOC_FENCE_TOKENS.contains(&sub.trim()));
+                    let is_rust = guards.split(',').all(|sub| is_rustdoc_fence_token(sub.trim()));
                     is_doctest = is_codeblock && is_rust;
                     continue;
                 }

crates/rust-analyzer/src/markdown.rs

@@ -27,9 +27,8 @@ pub(crate) fn format_docs(src: &str) -> String {
             in_code_block ^= true;
 
             if in_code_block {
-                is_rust = header
-                    .split(',')
-                    .all(|sub| RUSTDOC_CODE_BLOCK_ATTRIBUTES_RUST_SPECIFIC.contains(&sub.trim()));
+                is_rust =
+                    header.split(',').all(|sub| is_rust_specific_code_block_attribute(sub.trim()));
 
                 if is_rust {
                     line = "```rust";
@@ -42,6 +41,13 @@ pub(crate) fn format_docs(src: &str) -> String {
     processed_lines.join("\n")
 }
 
+fn is_rust_specific_code_block_attribute(attr: &str) -> bool {
+    if RUSTDOC_CODE_BLOCK_ATTRIBUTES_RUST_SPECIFIC.contains(&attr) {
+        return true;
+    }
+    attr.starts_with('E') && attr.len() == 5 && attr[1..].parse::<u32>().is_ok()
+}
+
 fn code_line_ignored_by_rustdoc(line: &str) -> bool {
     let trimmed = line.trim();
     trimmed == "#" || trimmed.starts_with("# ") || trimmed.starts_with("#\t")
@@ -81,6 +87,12 @@ mod tests {
         assert_eq!(format_docs(comment), "```rust\nlet z = 55;\n```");
     }
 
+    #[test]
+    fn test_format_docs_handles_error_codes() {
+        let comment = "```compile_fail,E0641\nlet b = 0 as *const _;\n```";
+        assert_eq!(format_docs(comment), "```rust\nlet b = 0 as *const _;\n```");
+    }
+
     #[test]
     fn test_format_docs_skips_comments_in_rust_block() {
         let comment =