proc_macro: Accept $crate as an identifier if it comes from the compiler

src/libsyntax_ext/proc_macro_server.rs

@@ -150,6 +150,8 @@ impl FromInternal<(TokenStream, &'_ ParseSess, &'_ mut Vec<Self>)>
             Question => op!('?'),
             SingleQuote => op!('\''),
 
+            Ident(ident, false) if ident.name == keywords::DollarCrate.name() =>
+                tt!(Ident::dollar_crate()),
             Ident(ident, is_raw) => tt!(Ident::new(ident.name, is_raw)),
             Lifetime(ident) => {
                 let ident = ident.without_first_quote();
@@ -359,6 +361,10 @@ impl Ident {
         }
         Ident { sym, is_raw, span }
     }
+    fn dollar_crate(span: Span) -> Ident {
+        // `$crate` is accepted as an ident only if it comes from the compiler.
+        Ident { sym: keywords::DollarCrate.name(), is_raw: false, span }
+    }
 }
 
 // FIXME(eddyb) `Literal` should not expose internal `Debug` impls.

src/test/ui/proc-macro/auxiliary/dollar-crate.rs

@@ -0,0 +1,12 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn normalize(input: TokenStream) -> TokenStream {
+    input.into_iter().collect()
+}

src/test/ui/proc-macro/dollar-crate.rs

@@ -0,0 +1,16 @@
+// compile-pass
+// aux-build:dollar-crate.rs
+
+extern crate dollar_crate;
+
+type S = u8;
+
+macro_rules! check { () => {
+    dollar_crate::normalize! {
+        type A = $crate::S;
+    }
+}}
+
+check!();
+
+fn main() {}