Fix 59191 - ICE when macro replaces crate root with non-module item

src/librustc_expand/expand.rs

@@ -363,7 +363,15 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                 krate.attrs = vec![];
                 krate.module = ast::Mod { inner: orig_mod_span, items: vec![], inline: true };
             }
-            _ => unreachable!(),
+            Some(ast::Item { span, kind, .. }) => {
+                self.cx.span_fatal(
+                    span,
+                    &format!(
+                        "expected crate top-level item to be a module after macro expansion, found a {}",
+                        kind.descriptive_variant()
+                    ),
+                );
+            }
         };
         self.cx.trace_macros_diag();
         krate

src/test/ui/proc-macro/auxiliary/issue-59191.rs

@@ -0,0 +1,16 @@
+// edition:2018
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn no_main(_attrs: TokenStream, _input: TokenStream) -> TokenStream {
+    let new_krate = r#"
+        fn main() {}
+    "#;
+    new_krate.parse().unwrap()
+}

src/test/ui/proc-macro/issue-59191-replace-root-with-fn.rs

@@ -0,0 +1,7 @@
+// edition:2018
+// aux-crate:issue_59191=issue-59191.rs
+// Test that using a macro to replace the entire crate tree with a non-'mod' item errors out nicely.
+// `issue_59191::no_main` replaces whatever's passed in with `fn main() {}`.
+#![feature(custom_inner_attributes)]
+#![issue_59191::no_main]
+//~^ ERROR expected crate top-level item to be a module after macro expansion, found a function

src/test/ui/proc-macro/issue-59191-replace-root-with-fn.stderr

@@ -0,0 +1,8 @@
+error: expected crate top-level item to be a module after macro expansion, found a function
+  --> $DIR/issue-59191-replace-root-with-fn.rs:6:1
+   |
+LL | #![issue_59191::no_main]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+