Remove Cursor::append.

It's a weird function: it lets you modify the token stream in the middle
of iteration. There is only one call site, and it is only used for the
rare `ProceduralMasquerade` legacy case.

compiler/rustc_ast/src/tokenstream.rs

@@ -25,7 +25,7 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
-use std::{fmt, iter, mem};
+use std::{fmt, iter};
 
 /// When the main Rust parser encounters a syntax-extension invocation, it
 /// parses the arguments to the invocation as a token tree. This is a very
@@ -683,16 +683,6 @@ impl Cursor {
         self.index
     }
 
-    pub fn append(&mut self, new_stream: TokenStream) {
-        if new_stream.is_empty() {
-            return;
-        }
-        let index = self.index;
-        let stream = mem::take(&mut self.stream);
-        *self = TokenStream::from_streams(smallvec![stream, new_stream]).into_trees();
-        self.index = index;
-    }
-
     pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
         self.stream.0[self.index..].get(n).map(|(tree, _)| tree)
     }

compiler/rustc_expand/src/proc_macro_server.rs

@@ -522,11 +522,11 @@ impl server::TokenStream for Rustc<'_, '_> {
                     // FIXME: It needs to be removed, but there are some
                     // compatibility issues (see #73345).
                     if group.flatten {
-                        cursor.append(group.stream);
+                        tts.append(&mut self.into_trees(group.stream));
-                        continue;
+                    } else {
-                    }
                         tts.push(TokenTree::Group(group));
                     }
+                }
                 Some(tt) => tts.push(tt),
                 None => return tts,
             }