resolve: Make proc macro stubs less stubby

Create real working and registered (even if dummy) `SyntaxExtension`s for them.
This improves error recovery and allows to avoid all special cases for proc macro stubs (except for the error on use, of course).

The introduced dummy `SyntaxExtension`s can be used for any other inappropriately resolved macros as well.

1 files changed, 21 insertions, 1 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 04f124685cb..82386b78f1d 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -10,7 +10,7 @@ use crate::parse::token;
 use crate::ptr::P;
 use crate::symbol::{kw, sym, Ident, Symbol};
 use crate::{ThinVec, MACRO_ARGUMENTS};
-use crate::tokenstream::{self, TokenStream};
+use crate::tokenstream::{self, TokenStream, TokenTree};
 
 use errors::{DiagnosticBuilder, DiagnosticId};
 use smallvec::{smallvec, SmallVec};
@@ -640,6 +640,26 @@ impl SyntaxExtension {
         }
     }
 
+    pub fn dummy_bang(edition: Edition) -> SyntaxExtension {
+        fn expander<'cx>(_: &'cx mut ExtCtxt<'_>, span: Span, _: &[TokenTree])
+                         -> Box<dyn MacResult + 'cx> {
+            DummyResult::any(span)
+        }
+        SyntaxExtension::default(SyntaxExtensionKind::LegacyBang(Box::new(expander)), edition)
+    }
+
+    pub fn dummy_derive(edition: Edition) -> SyntaxExtension {
+        fn expander(_: &mut ExtCtxt<'_>, _: Span, _: &ast::MetaItem, _: Annotatable)
+                    -> Vec<Annotatable> {
+            Vec::new()
+        }
+        SyntaxExtension::default(SyntaxExtensionKind::Derive(Box::new(expander)), edition)
+    }
+
+    pub fn non_macro_attr(mark_used: bool, edition: Edition) -> SyntaxExtension {
+        SyntaxExtension::default(SyntaxExtensionKind::NonMacroAttr { mark_used }, edition)
+    }
+
     pub fn expn_info(&self, call_site: Span, descr: Symbol) -> ExpnInfo {
         ExpnInfo {
             call_site,