Add regression test for #148184

Author: GuillaumeGomez

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;