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.

6 files changed, 13 insertions, 13 deletions

diff --git a/src/test/ui/attributes/issue-90873.rs b/src/test/ui/attributes/issue-90873.rs
index 0f62d415308..1411f61744d 100644
--- a/src/test/ui/attributes/issue-90873.rs
+++ b/src/test/ui/attributes/issue-90873.rs
@@ -1,9 +1,9 @@
 #![u=||{static d=||1;}]
-//~^ unexpected token
+//~^ unexpected expression
 //~| cannot find attribute `u` in this scope
 //~| missing type for `static` item
 
 #![a={impl std::ops::Neg for i8 {}}]
-//~^ ERROR unexpected token
+//~^ ERROR unexpected expression
 //~| ERROR cannot find attribute `a` in this scope
 //~| ERROR `main` function not found in crate `issue_90873`
diff --git a/src/test/ui/attributes/issue-90873.stderr b/src/test/ui/attributes/issue-90873.stderr
index fbdc05ef6f0..0852bb7ca8b 100644
--- a/src/test/ui/attributes/issue-90873.stderr
+++ b/src/test/ui/attributes/issue-90873.stderr
@@ -1,4 +1,4 @@
-error: unexpected token: `||
+error: unexpected expression: `||
            {
                static d: _ = || 1;
            }`
@@ -7,7 +7,7 @@ error: unexpected token: `||
 LL | #![u=||{static d=||1;}]
    |      ^^^^^^^^^^^^^^^^^
 
-error: unexpected token: `{
+error: unexpected expression: `{
            impl std::ops::Neg for i8 {}
        }`
   --> $DIR/issue-90873.rs:6:6
diff --git a/src/test/ui/attributes/key-value-expansion-on-mac.rs b/src/test/ui/attributes/key-value-expansion-on-mac.rs
index 95bc1c04961..c1d68d8cda9 100644
--- a/src/test/ui/attributes/key-value-expansion-on-mac.rs
+++ b/src/test/ui/attributes/key-value-expansion-on-mac.rs
@@ -7,8 +7,8 @@ macro_rules! bar {
 
 // FIXME?: `bar` here expands before `stringify` has a chance to expand.
 // `#[rustc_dummy = ...]` is validated and dropped during expansion of `bar`,
-// the "unexpected token" errors comes from the validation.
-#[rustc_dummy = stringify!(b)] //~ ERROR unexpected token: `stringify!(b)`
+// the "unexpected expression" errors comes from the validation.
+#[rustc_dummy = stringify!(b)] //~ ERROR unexpected expression: `stringify!(b)`
 bar!();
 
 fn main() {}
diff --git a/src/test/ui/attributes/key-value-expansion-on-mac.stderr b/src/test/ui/attributes/key-value-expansion-on-mac.stderr
index fa9ea543765..64ab892d997 100644
--- a/src/test/ui/attributes/key-value-expansion-on-mac.stderr
+++ b/src/test/ui/attributes/key-value-expansion-on-mac.stderr
@@ -1,4 +1,4 @@
-error: unexpected token: `stringify!(b)`
+error: unexpected expression: `stringify!(b)`
   --> $DIR/key-value-expansion-on-mac.rs:11:17
    |
 LL | #[rustc_dummy = stringify!(b)]
diff --git a/src/test/ui/attributes/key-value-expansion.rs b/src/test/ui/attributes/key-value-expansion.rs
index 08121413ee9..83d601e5e3a 100644
--- a/src/test/ui/attributes/key-value-expansion.rs
+++ b/src/test/ui/attributes/key-value-expansion.rs
@@ -18,13 +18,13 @@ macro_rules! bug {
 
 // Any expressions containing macro call `X` that's more complex than `X` itself.
 // Parentheses will work.
-bug!((column!())); //~ ERROR unexpected token: `(7u32)`
+bug!((column!())); //~ ERROR unexpected expression: `(7u32)`
 
 // Original test case.
 
 macro_rules! bug {
     () => {
-        bug!("bug" + stringify!(found)); //~ ERROR unexpected token: `"bug" + "found"`
+        bug!("bug" + stringify!(found)); //~ ERROR unexpected expression: `"bug" + "found"`
     };
     ($test:expr) => {
         #[doc = $test]
@@ -46,7 +46,7 @@ macro_rules! doc_comment {
 macro_rules! some_macro {
     ($t1: ty) => {
         doc_comment! {format!("{coor}", coor = stringify!($t1)).as_str()}
-        //~^ ERROR unexpected token: `{
+        //~^ ERROR unexpected expression: `{
     };
 }
 
diff --git a/src/test/ui/attributes/key-value-expansion.stderr b/src/test/ui/attributes/key-value-expansion.stderr
index 1b7cb76b553..1b776322aaa 100644
--- a/src/test/ui/attributes/key-value-expansion.stderr
+++ b/src/test/ui/attributes/key-value-expansion.stderr
@@ -1,10 +1,10 @@
-error: unexpected token: `(7u32)`
+error: unexpected expression: `(7u32)`
   --> $DIR/key-value-expansion.rs:21:6
    |
 LL | bug!((column!()));
    |      ^^^^^^^^^^^
 
-error: unexpected token: `"bug" + "found"`
+error: unexpected expression: `"bug" + "found"`
   --> $DIR/key-value-expansion.rs:27:14
    |
 LL |         bug!("bug" + stringify!(found));
@@ -15,7 +15,7 @@ LL | bug!();
    |
    = note: this error originates in the macro `bug` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: unexpected token: `{
+error: unexpected expression: `{
                let res =
                    ::alloc::fmt::format(::core::fmt::Arguments::new_v1(&[""],
                            &[::core::fmt::ArgumentV1::new_display(&"u8")]));