Change expansion error to be non-fatal

Changes the error handler for inner attributes that replace the root
with a non-module. Previously it would emit a fatal error. It now emits
an empty expasion and a non-fatal error like the existing handler for a
failed expansion.

src/librustc_expand/expand.rs

@@ -364,7 +364,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                 krate.module = ast::Mod { inner: orig_mod_span, items: vec![], inline: true };
             }
             Some(ast::Item { span, kind, .. }) => {
-                self.cx.span_fatal(
+                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 {}",

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

@@ -3,5 +3,6 @@
 // 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

@@ -1,8 +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:6:1
+  --> $DIR/issue-59191-replace-root-with-fn.rs:7:1
    |
 LL | #![issue_59191::no_main]
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+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`.