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/validate_attr.rs

@@ -3,7 +3,8 @@
 use crate::parse_in;
 
 use rustc_ast::tokenstream::DelimSpan;
-use rustc_ast::{self as ast, Attribute, MacArgs, MacArgsEq, MacDelimiter, MetaItem, MetaItemKind};
+use rustc_ast::MetaItemKind;
+use rustc_ast::{self as ast, AttrArgs, AttrArgsEq, Attribute, DelimArgs, MacDelimiter, MetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, FatalError, PResult};
 use rustc_feature::{AttributeTemplate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
@@ -24,7 +25,7 @@ pub fn check_meta(sess: &ParseSess, attr: &Attribute) {
         Some(BuiltinAttribute { name, template, .. }) if *name != sym::rustc_dummy => {
             check_builtin_attribute(sess, attr, *name, *template)
         }
-        _ if let MacArgs::Eq(..) = attr.get_normal_item().args => {
+        _ if let AttrArgs::Eq(..) = attr.get_normal_item().args => {
             // All key-value attributes are restricted to meta-item syntax.
             parse_meta(sess, attr)
                 .map_err(|mut err| {
@@ -42,13 +43,13 @@ pub fn parse_meta<'a>(sess: &'a ParseSess, attr: &Attribute) -> PResult<'a, Meta
         span: attr.span,
         path: item.path.clone(),
         kind: match &item.args {
-            MacArgs::Empty => MetaItemKind::Word,
-            MacArgs::Delimited(dspan, delim, t) => {
+            AttrArgs::Empty => MetaItemKind::Word,
+            AttrArgs::Delimited(DelimArgs { dspan, delim, tokens }) => {
                 check_meta_bad_delim(sess, *dspan, *delim, "wrong meta list delimiters");
-                let nmis = parse_in(sess, t.clone(), "meta list", |p| p.parse_meta_seq_top())?;
+                let nmis = parse_in(sess, tokens.clone(), "meta list", |p| p.parse_meta_seq_top())?;
                 MetaItemKind::List(nmis)
             }
-            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => {
+            AttrArgs::Eq(_, AttrArgsEq::Ast(expr)) => {
                 if let ast::ExprKind::Lit(token_lit) = expr.kind
                     && let Ok(lit) = ast::Lit::from_token_lit(token_lit, expr.span)
                 {
@@ -78,7 +79,7 @@ pub fn parse_meta<'a>(sess: &'a ParseSess, attr: &Attribute) -> PResult<'a, Meta
                     return Err(err);
                 }
             }
-            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => MetaItemKind::NameValue(lit.clone()),
+            AttrArgs::Eq(_, AttrArgsEq::Hir(lit)) => MetaItemKind::NameValue(lit.clone()),
         },
     })
 }