Rollup merge of #93463 - dtolnay:_args, r=cjgillot

Rename _args -> args in format_args expansion

As observed in https://github.com/rust-lang/rust/pull/91359#discussion_r786058960, prior to that PR this variable was sometimes never used, such as in the case of:

```rust
println!("");

// used to expand to:
::std::io::_print(
    ::core::fmt::Arguments::new_v1(
        &["\n"],
        &match () {
            _args => [],
        },
    ),
);
```

so the leading underscore in `_args` was used to suppress an unused variable lint. However after #91359 the variable is always used when present, as the unused case would instead expand to:

```rust
::std::io::_print(::core::fmt::Arguments::new_v1(&["\n"], &[]));
```

compiler/rustc_builtin_macros/src/format.rs

@@ -791,7 +791,7 @@ impl<'a, 'b> Context<'a, 'b> {
         // Thus in the not nicely ordered case we emit the following instead:
         //
         //     match (&$arg0, &$arg1, …) {
-        //         _args => [ArgumentV1::new(_args.$i, …), ArgumentV1::new(_args.$j, …), …]
+        //         args => [ArgumentV1::new(args.$i, …), ArgumentV1::new(args.$j, …), …]
         //     }
         //
         // for the sequence of indices $i, $j, … governed by fmt_arg_index_and_ty.
@@ -804,7 +804,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 self.ecx.expr_addr_of(expansion_span, P(e.take()))
             } else {
                 let def_site = self.ecx.with_def_site_ctxt(span);
-                let args_tuple = self.ecx.expr_ident(def_site, Ident::new(sym::_args, def_site));
+                let args_tuple = self.ecx.expr_ident(def_site, Ident::new(sym::args, def_site));
                 let member = Ident::new(sym::integer(arg_index), def_site);
                 self.ecx.expr(def_site, ast::ExprKind::Field(args_tuple, member))
             };
@@ -828,7 +828,7 @@ impl<'a, 'b> Context<'a, 'b> {
                     .map(|e| self.ecx.expr_addr_of(e.span.with_ctxt(self.macsp.ctxt()), e))
                     .collect();
 
-                let pat = self.ecx.pat_ident(self.macsp, Ident::new(sym::_args, self.macsp));
+                let pat = self.ecx.pat_ident(self.macsp, Ident::new(sym::args, self.macsp));
                 let arm = self.ecx.arm(self.macsp, pat, args_array);
                 let head = self.ecx.expr(self.macsp, ast::ExprKind::Tup(heads));
                 self.ecx.expr_match(self.macsp, head, vec![arm])

compiler/rustc_span/src/symbol.rs

@@ -272,7 +272,6 @@ symbols! {
         __H,
         __S,
         __try_var,
-        _args,
         _d,
         _e,
         _task_context,
@@ -324,6 +323,7 @@ symbols! {
         append_const_msg,
         arbitrary_enum_discriminant,
         arbitrary_self_types,
+        args,
         arith_offset,
         arm,
         arm_target_feature,