Split MacArgs in two.

`MacArgs` is an enum with three variants: `Empty`, `Delimited`, and `Eq`. It's
used in two ways:
- For representing attribute macro arguments (e.g. in `AttrItem`), where all
  three variants are used.
- For representing function-like macros (e.g. in `MacCall` and `MacroDef`),
  where only the `Delimited` variant is used.

In other words, `MacArgs` is used in two quite different places due to them
having partial overlap. I find this makes the code hard to read. It also leads
to various unreachable code paths, and allows invalid values (such as
accidentally using `MacArgs::Empty` in a `MacCall`).

This commit splits `MacArgs` in two:
- `DelimArgs` is a new struct just for the "delimited arguments" case. It is
  now used in `MacCall` and `MacroDef`.
- `AttrArgs` is a renaming of the old `MacArgs` enum for the attribute macro
  case. Its `Delimited` variant now contains a `DelimArgs`.

Various other related things are renamed as well.

These changes make the code clearer, avoids several unreachable paths, and
disallows the invalid values.

compiler/rustc_parse/src/lib.rs

@@ -15,8 +15,7 @@ extern crate tracing;
 use rustc_ast as ast;
 use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
-use rustc_ast::Attribute;
-use rustc_ast::{AttrItem, MetaItem};
+use rustc_ast::{AttrItem, Attribute, MetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
@@ -257,10 +256,12 @@ pub fn parse_cfg_attr(
     parse_sess: &ParseSess,
 ) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
     match attr.get_normal_item().args {
-        ast::MacArgs::Delimited(dspan, delim, ref tts) if !tts.is_empty() => {
+        ast::AttrArgs::Delimited(ast::DelimArgs { dspan, delim, ref tokens })
+            if !tokens.is_empty() =>
+        {
             let msg = "wrong `cfg_attr` delimiters";
             crate::validate_attr::check_meta_bad_delim(parse_sess, dspan, delim, msg);
-            match parse_in(parse_sess, tts.clone(), "`cfg_attr` input", |p| p.parse_cfg_attr()) {
+            match parse_in(parse_sess, tokens.clone(), "`cfg_attr` input", |p| p.parse_cfg_attr()) {
                 Ok(r) => return Some(r),
                 Err(mut e) => {
                     e.help(&format!("the valid syntax is `{}`", CFG_ATTR_GRAMMAR_HELP))