Overhaul MacArgs::Eq.

The value in `MacArgs::Eq` is currently represented as a `Token`.
Because of `TokenKind::Interpolated`, `Token` can be either a token or
an arbitrary AST fragment. In practice, a `MacArgs::Eq` starts out as a
literal or macro call AST fragment, and then is later lowered to a
literal token. But this is very non-obvious. `Token` is a much more
general type than what is needed.

This commit restricts things, by introducing a new type `MacArgsEqKind`
that is either an AST expression (pre-lowering) or an AST literal
(post-lowering). The downside is that the code is a bit more verbose in
a few places. The benefit is that makes it much clearer what the
possibilities are (though also shorter in some other places). Also, it
removes one use of `TokenKind::Interpolated`, taking us a step closer to
removing that variant, which will let us make `Token` impl `Copy` and
remove many "handle Interpolated" code paths in the parser.

Things to note:
- Error messages have improved. Messages like this:
  ```
  unexpected token: `"bug" + "found"`
  ```
  now say "unexpected expression", which makes more sense. Although
  arbitrary expressions can exist within tokens thanks to
  `TokenKind::Interpolated`, that's not obvious to anyone who doesn't
  know compiler internals.
- In `parse_mac_args_common`, we no longer need to collect tokens for
  the value expression.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -13,7 +13,7 @@ use rustc_ast::util::comments::{gather_comments, Comment, CommentStyle};
 use rustc_ast::util::parser;
 use rustc_ast::{self as ast, BlockCheckMode, PatKind, RangeEnd, RangeSyntax};
 use rustc_ast::{attr, Term};
-use rustc_ast::{GenericArg, MacArgs};
+use rustc_ast::{GenericArg, MacArgs, MacArgsEq};
 use rustc_ast::{GenericBound, SelfKind, TraitBoundModifier};
 use rustc_ast::{InlineAsmOperand, InlineAsmRegOrRegClass};
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
@@ -472,11 +472,18 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             MacArgs::Empty => {
                 self.print_path(&item.path, false, 0);
             }
-            MacArgs::Eq(_, token) => {
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => {
                 self.print_path(&item.path, false, 0);
                 self.space();
                 self.word_space("=");
-                let token_str = self.token_to_string_ext(token, true);
+                let token_str = self.expr_to_string(expr);
+                self.word(token_str);
+            }
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                self.print_path(&item.path, false, 0);
+                self.space();
+                self.word_space("=");
+                let token_str = self.literal_to_string(lit);
                 self.word(token_str);
             }
         }
@@ -818,6 +825,10 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         Self::to_string(|s| s.print_expr(e))
     }
 
+    fn literal_to_string(&self, lit: &ast::Lit) -> String {
+        Self::to_string(|s| s.print_literal(lit))
+    }
+
     fn tt_to_string(&self, tt: &TokenTree) -> String {
         Self::to_string(|s| s.print_tt(tt, false))
     }