fix: inner attribute followed by outer attribute causing ICE

compiler/rustc_ast/src/tokenstream.rs

@@ -221,12 +221,6 @@ impl AttrAnnotatedTokenStream {
                     for attr in &data.attrs {
                         match attr.style {
                             crate::AttrStyle::Outer => {
-                                assert!(
-                                    inner_attrs.len() == 0,
-                                    "Found outer attribute {:?} after inner attrs {:?}",
-                                    attr,
-                                    inner_attrs
-                                );
                                 outer_attrs.push(attr);
                             }
                             crate::AttrStyle::Inner => {

src/test/ui/parser/auxiliary/issue-89971-outer-attr-following-inner-attr-ice.rs

@@ -0,0 +1,13 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_derive(ICE)]
+pub fn derive(_: TokenStream) -> TokenStream {
+    r#"#[allow(missing_docs)] struct X { }"#.parse().unwrap()
+}

src/test/ui/parser/issue-89971-outer-attr-following-inner-attr-ice.rs

@@ -0,0 +1,15 @@
+// aux-build:issue-89971-outer-attr-following-inner-attr-ice.rs
+
+#[macro_use]
+extern crate issue_89971_outer_attr_following_inner_attr_ice;
+
+fn main() {
+    Mew();
+    X {};
+}
+
+#![deny(missing_docs)]
+//~^ ERROR an inner attribute is not permitted in this context
+#[derive(ICE)]
+#[deny(missing_docs)]
+struct Mew();

src/test/ui/parser/issue-89971-outer-attr-following-inner-attr-ice.stderr

@@ -0,0 +1,18 @@
+error: an inner attribute is not permitted in this context
+  --> $DIR/issue-89971-outer-attr-following-inner-attr-ice.rs:11:1
+   |
+LL | #![deny(missing_docs)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | struct Mew();
+   | ------------- the inner attribute doesn't annotate this struct
+   |
+   = note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files
+help: to annotate the struct, change the attribute from inner to outer style
+   |
+LL - #![deny(missing_docs)]
+LL + #[deny(missing_docs)]
+   | 
+
+error: aborting due to previous error
+