Remove the deprecated concat_idents! macro

In [137653], the lang and libs-API teams did a joint FCP to deprecate
and eventually remove the long-unstable `concat_idents!` macro. The
deprecation is landing in 1.88, so do the removal here (target version
1.90).

This macro has been superseded by the more recent `${concat(...)}`
metavariable expression language feature, which avoids some of the
limitations of `concat_idents!`. The metavar expression is unstably
available under the [`macro_metavar_expr_concat`] feature.

History is mildly interesting here: `concat_idents!` goes back to 2011
when it was introduced with 513276e ("Add #concat_idents[] and
about the same:

    let asdf_fdsa = "<.<";
    assert(#concat_idents[asd,f_f,dsa] == "<.<");

    assert(#ident_to_str[use_mention_distinction]
           == "use_mention_distinction");

(That test existed from introduction until its removal here.)

Closes: https://www.github.com/rust-lang/rust/issues/29599

[137653]: https://www.github.com/rust-lang/rust/pull/137653
[`macro_metavar_expr_concat`]: https://www.github.com/rust-lang/rust/issues/124225

Author: tgross35

core/src/macros/mod.rs

@@ -1109,45 +1109,6 @@ pub(crate) mod builtin {
         ($name:expr $(,)?) => {{ /* compiler built-in */ }};
     }
 
-    /// Concatenates identifiers into one identifier.
-    ///
-    /// This macro takes any number of comma-separated identifiers, and
-    /// concatenates them all into one, yielding an expression which is a new
-    /// identifier. Note that hygiene makes it such that this macro cannot
-    /// capture local variables. Also, as a general rule, macros are only
-    /// allowed in item, statement or expression position. That means while
-    /// you may use this macro for referring to existing variables, functions or
-    /// modules etc, you cannot define a new one with it.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(concat_idents)]
-    ///
-    /// # fn main() {
-    /// fn foobar() -> u32 { 23 }
-    ///
-    /// let f = concat_idents!(foo, bar);
-    /// println!("{}", f());
-    ///
-    /// // fn concat_idents!(new, fun, name) { } // not usable in this way!
-    /// # }
-    /// ```
-    #[unstable(
-        feature = "concat_idents",
-        issue = "29599",
-        reason = "`concat_idents` is not stable enough for use and is subject to change"
-    )]
-    #[deprecated(
-        since = "1.88.0",
-        note = "use `${concat(...)}` with the `macro_metavar_expr_concat` feature instead"
-    )]
-    #[rustc_builtin_macro]
-    #[macro_export]
-    macro_rules! concat_idents {
-        ($($e:ident),+ $(,)?) => {{ /* compiler built-in */ }};
-    }
-
     /// Concatenates literals into a byte slice.
     ///
     /// This macro takes any number of comma-separated literals, and concatenates them all into

core/src/prelude/v1.rs

@@ -58,10 +58,9 @@ pub use crate::fmt::macros::Debug;
 pub use crate::hash::macros::Hash;
 
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow(deprecated)]
 #[doc(no_inline)]
 pub use crate::{
-    assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,
+    assert, cfg, column, compile_error, concat, env, file, format_args,
     format_args_nl, include, include_bytes, include_str, line, log_syntax, module_path, option_env,
     stringify, trace_macros,
 };

std/src/lib.rs

@@ -281,7 +281,6 @@
 #![feature(cfg_target_thread_local)]
 #![feature(cfi_encoding)]
 #![feature(char_max_len)]
-#![feature(concat_idents)]
 #![feature(core_float_math)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
@@ -717,10 +716,9 @@ pub use core::primitive;
 pub use core::todo;
 // Re-export built-in macros defined through core.
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow(deprecated)]
 pub use core::{
-    assert, assert_matches, cfg, column, compile_error, concat, concat_idents, const_format_args,
-    env, file, format_args, format_args_nl, include, include_bytes, include_str, line, log_syntax,
+    assert, assert_matches, cfg, column, compile_error, concat, const_format_args, env, file,
+    format_args, format_args_nl, include, include_bytes, include_str, line, log_syntax,
     module_path, option_env, stringify, trace_macros,
 };
 // Re-export macros defined in core.

std/src/prelude/v1.rs

@@ -45,10 +45,9 @@ pub use crate::result::Result::{self, Err, Ok};
 
 // Re-exported built-in macros
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow(deprecated)]
 #[doc(no_inline)]
 pub use core::prelude::v1::{
-    assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,
+    assert, cfg, column, compile_error, concat, env, file, format_args,
     format_args_nl, include, include_bytes, include_str, line, log_syntax, module_path, option_env,
     stringify, trace_macros, Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd,
 };