Rollup merge of #69211 - petrochenkov:prevtok, r=Centril

parser: Simplify treatment of macro variables in `Parser::bump`

Follow-up to https://github.com/rust-lang/rust/pull/69006.

Token normalization for `$ident` and `$lifetime` is merged directly into `bump`.
Special "unknown macro variable" diagnostic for unexpected `$`s is removed as preventing legal code from compiling (as a result `bump` also doesn't call itself recursively anymore and can't make `prev_token` inconsistent).

r? @Centril

src/test/ui/issues/issue-6596-1.rs

@@ -1,7 +1,7 @@
 macro_rules! e {
     ($inp:ident) => (
         $nonexistent
-        //~^ ERROR unknown macro variable `nonexistent`
+        //~^ ERROR expected expression, found `$`
     );
 }
 

src/test/ui/issues/issue-6596-1.stderr

@@ -1,8 +1,8 @@
-error: unknown macro variable `nonexistent`
+error: expected expression, found `$`
   --> $DIR/issue-6596-1.rs:3:9
    |
 LL |         $nonexistent
-   |         ^^^^^^^^^^^^ unknown macro variable
+   |         ^^^^^^^^^^^^ expected expression
 ...
 LL |     e!(foo);
    |     -------- in this macro invocation

src/test/ui/issues/issue-6596-2.rs

@@ -3,7 +3,7 @@
 macro_rules! g {
     ($inp:ident) => (
         { $inp $nonexistent }
-        //~^ ERROR unknown macro variable `nonexistent`
+        //~^ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `$`
     );
 }
 

src/test/ui/issues/issue-6596-2.stderr

@@ -1,8 +1,8 @@
-error: unknown macro variable `nonexistent`
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `$`
   --> $DIR/issue-6596-2.rs:5:16
    |
 LL |         { $inp $nonexistent }
-   |                ^^^^^^^^^^^^ unknown macro variable
+   |                ^^^^^^^^^^^^ expected one of 8 possible tokens
 ...
 LL |     g!(foo);
    |     -------- in this macro invocation

src/test/ui/proc-macro/auxiliary/generate-dollar-ident.rs

@@ -0,0 +1,17 @@
+// force-host
+// no-prefer-dynamic
+
+#![feature(proc_macro_hygiene)]
+#![feature(proc_macro_quote)]
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::*;
+
+#[proc_macro]
+pub fn dollar_ident(input: TokenStream) -> TokenStream {
+    let black_hole = input.into_iter().next().unwrap();
+    quote! {
+        $black_hole!($$var);
+    }
+}

src/test/ui/proc-macro/generate-dollar-ident.rs

@@ -0,0 +1,18 @@
+// Proc macros can generate token sequence `$ IDENT`
+// without it being recognized as an unknown macro variable.
+
+// check-pass
+// aux-build:generate-dollar-ident.rs
+
+extern crate generate_dollar_ident;
+use generate_dollar_ident::*;
+
+macro_rules! black_hole {
+    ($($tt:tt)*) => {};
+}
+
+black_hole!($var);
+
+dollar_ident!(black_hole);
+
+fn main() {}