Add new rustdoc-ui test for custom_code_classes_in_docs feature

Author: GuillaumeGomez

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

@@ -8,3 +8,11 @@
 //~| NOTE see issue #79483 <https://github.com/rust-lang/rust/issues/79483>
 //~| HELP add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
 pub struct Bar;
+
+/// ```ASN.1
+/// int main(void) { return 0; }
+/// ```
+//~^^^ WARNING custom classes in code blocks will change behaviour
+//~| NOTE see issue #79483 <https://github.com/rust-lang/rust/issues/79483>
+//~| HELP add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
+pub struct Bar2;

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

@@ -10,5 +10,16 @@ LL | | /// ```
    = help: add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
    = note: found these custom classes: class=language-c
 
-warning: 1 warning emitted
+warning: custom classes in code blocks will change behaviour
+  --> $DIR/feature-gate-custom_code_classes_in_docs.rs:12:1
+   |
+LL | / /// ```ASN.1
+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
+
+warning: 2 warnings emitted