Fix invalid tag closing when leaving expansion "original code"

src/librustdoc/html/highlight.rs

@@ -428,6 +428,27 @@ impl<'a, F: Write> TokenHandler<'a, '_, F> {
             }
         }
     }
+
+    /// Used when we're done with the current expansion "original code" (ie code before expansion).
+    /// We close all tags inside `Class::Original` and only keep the ones that were not closed yet.
+    fn close_original_tag(&mut self) {
+        let mut classes_to_reopen = Vec::new();
+        while let Some(mut class_info) = self.class_stack.open_classes.pop() {
+            if class_info.class == Class::Original {
+                while let Some(class_info) = classes_to_reopen.pop() {
+                    self.class_stack.open_classes.push(class_info);
+                }
+                class_info.close_tag(self.out);
+                return;
+            }
+            class_info.close_tag(self.out);
+            if !class_info.pending_exit {
+                class_info.closing_tag = None;
+                classes_to_reopen.push(class_info);
+            }
+        }
+        panic!("Didn't find `Class::Original` to close");
+    }
 }
 
 impl<F: Write> Drop for TokenHandler<'_, '_, F> {
@@ -476,7 +497,9 @@ fn end_expansion<'a, W: Write>(
     expanded_codes: &'a [ExpandedCode],
     span: Span,
 ) -> Option<&'a ExpandedCode> {
-    token_handler.class_stack.exit_elem();
+    // We close `Class::Original` and everything open inside it.
+    token_handler.close_original_tag();
+    // Then we check if we have another macro expansion on the same line.
     let expansion = get_next_expansion(expanded_codes, token_handler.line, span);
     if expansion.is_none() {
         token_handler.close_expansion();

tests/rustdoc/macro/auxiliary/one-line-expand.rs

@@ -0,0 +1,15 @@
+//@ force-host
+//@ no-prefer-dynamic
+//@ compile-flags: --crate-type proc-macro
+
+#![crate_type = "proc-macro"]
+#![crate_name = "just_some_proc"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn repro(_args: TokenStream, _input: TokenStream) -> TokenStream {
+    "struct Repro;".parse().unwrap()
+}

tests/rustdoc/macro/one-line-expand.rs

@@ -0,0 +1,19 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/148184>.
+// It ensures that the macro expansion correctly handles its "class stack".
+
+//@ compile-flags: -Zunstable-options --generate-macro-expansion
+//@ aux-build:one-line-expand.rs
+
+#![crate_name = "foo"]
+
+extern crate just_some_proc;
+
+//@ has 'src/foo/one-line-expand.rs.html'
+//@ has - '//*[@class="comment"]' '//'
+//@ has - '//*[@class="original"]' '#[just_some_proc::repro]'
+//@ has - '//*[@class="original"]/*[@class="attr"]' '#[just_some_proc::repro]'
+//@ has - '//code/*[@class="kw"]' 'struct '
+
+//
+#[just_some_proc::repro]
+struct Repro;