Allow running rustdoc on proc-macro crates without specifying '--crate-type proc-macro'

Add a test to make sure that this works

Author: Aaron1011

src/librustc_interface/passes.rs

@@ -473,22 +473,38 @@ fn configure_and_expand_inner<'a>(
         ast_validation::check_crate(sess, &krate)
     });
 
-    krate = time(sess, "maybe creating a macro crate", || {
-        let crate_types = sess.crate_types.borrow();
-        let num_crate_types = crate_types.len();
-        let is_proc_macro_crate = crate_types.contains(&config::CrateType::ProcMacro);
-        let is_test_crate = sess.opts.test;
-        syntax_ext::proc_macro_harness::inject(
-            &sess.parse_sess,
-            &mut resolver,
-            krate,
-            is_proc_macro_crate,
-            has_proc_macro_decls,
-            is_test_crate,
-            num_crate_types,
-            sess.diagnostic(),
-        )
-    });
+
+    let crate_types = sess.crate_types.borrow();
+    let is_proc_macro_crate = crate_types.contains(&config::CrateType::ProcMacro);
+
+    // For backwards compatibility, we don't try to run proc macro injection
+    // if rustdoc is run on a proc macro crate without '--crate-type proc-macro' being
+    // specified. This should only affect users who manually invoke 'rustdoc', as
+    // 'cargo doc' will automatically pass the proper '--crate-type' flags.
+    // However, we do emit a warning, to let such users know that they should
+    // start passing '--crate-type proc-macro'
+    if has_proc_macro_decls && sess.opts.actually_rustdoc && !is_proc_macro_crate {
+        let mut msg = sess.diagnostic().struct_warn(&"Trying to document proc macro crate \
+            without passing '--crate-type proc-macro to rustdoc");
+
+        msg.warn("The generated documentation may be incorrect");
+        msg.emit()
+    } else {
+        krate = time(sess, "maybe creating a macro crate", || {
+            let num_crate_types = crate_types.len();
+            let is_test_crate = sess.opts.test;
+            syntax_ext::proc_macro_harness::inject(
+                &sess.parse_sess,
+                &mut resolver,
+                krate,
+                is_proc_macro_crate,
+                has_proc_macro_decls,
+                is_test_crate,
+                num_crate_types,
+                sess.diagnostic(),
+            )
+        });
+    }
 
     // Done with macro expansion!