Check for macros in built-in attributes that don't support them.

compiler/rustc_expand/src/module.rs

@@ -3,6 +3,7 @@ use rustc_ast::ptr::P;
 use rustc_ast::{token, Attribute, Inline, Item};
 use rustc_errors::{struct_span_err, DiagnosticBuilder};
 use rustc_parse::new_parser_from_file;
+use rustc_parse::validate_attr;
 use rustc_session::parse::ParseSess;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Ident};
@@ -168,7 +169,25 @@ fn mod_file_path_from_attr(
     dir_path: &Path,
 ) -> Option<PathBuf> {
     // Extract path string from first `#[path = "path_string"]` attribute.
-    let path_string = sess.first_attr_value_str_by_name(attrs, sym::path)?.as_str();
+    let first_path = attrs.iter().find(|at| at.has_name(sym::path))?;
+    let path_string = match first_path.value_str() {
+        Some(s) => s.as_str(),
+        None => {
+            // This check is here mainly to catch attempting to use a macro,
+            // such as #[path = concat!(...)]. This isn't currently supported
+            // because otherwise the InvocationCollector would need to defer
+            // loading a module until the #[path] attribute was expanded, and
+            // it doesn't support that (and would likely add a bit of
+            // complexity). Usually bad forms are checked in AstValidator (via
+            // `check_builtin_attribute`), but by the time that runs the macro
+            // is expanded, and it doesn't give an error.
+            validate_attr::emit_fatal_malformed_builtin_attribute(
+                &sess.parse_sess,
+                first_path,
+                sym::path,
+            );
+        }
+    };
 
     // On windows, the base path might have the form
     // `\\?\foo\bar` in which case it does not tolerate