Rollup merge of #68758 - daboross:fix-59191, r=petrochenkov

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

Hi,

This should fix #59191! My friend and I are working on learning the rustc codebase through contributions, so please feel free to mention anything amiss or that could be done better.

The code adds an explicit case for when a macro applied to the crate root (via an inner attribute) replaces it with something nonsensical, like a function. The crate root must be a module, and the error message reflects this.

---

I should note that there are a few other weird edge cases here, like if they do output a module, it succeeds but uses that module's name as a prefix for all names in the crate. I'm assuming that's an issue for stabilizing #54726, though.

src/librustc_expand/expand.rs

@@ -363,7 +363,17 @@ 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, .. }) => {
+                krate.attrs = vec![];
+                krate.module = ast::Mod { inner: orig_mod_span, items: vec![], inline: true };
+                self.cx.span_err(
+                    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 +1,8 @@
+// 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)]
+//~^ ERROR `main` function not found in crate `issue_59191_replace_root_with_fn` [E0601]
+#![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 +1,17 @@
+error: expected crate top-level item to be a module after macro expansion, found a function
+  --> $DIR/issue-59191-replace-root-with-fn.rs:7:1
+   |
+LL | #![issue_59191::no_main]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0601]: `main` function not found in crate `issue_59191_replace_root_with_fn`
+  --> $DIR/issue-59191-replace-root-with-fn.rs:5:1
+   |
+LL | / #![feature(custom_inner_attributes)]
+LL | |
+LL | | #![issue_59191::no_main]
+   | |________________________^ consider adding a `main` function to `$DIR/issue-59191-replace-root-with-fn.rs`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0601`.