Deprecate the unstable concat_idents!

`concat_idents` has been around unstably for a long time, but there is
now a better (but still unstable) way to join identifiers using
`${concat(...)}` syntax with `macro_metavar_expr_concat`. This resolves
a lot of the problems with `concat_idents` and is on a better track
toward stabilization, so there is no need to keep both versions around.
`concat_idents!` still has a lot of use in the ecosystem so deprecate it
before removing, as discussed in [1].

Link: #124225
[1]: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/Removing.20.60concat_idents.60

Author: tgross35

library/core/src/macros/mod.rs

@@ -1137,6 +1137,10 @@ pub(crate) mod builtin {
         issue = "29599",
         reason = "`concat_idents` is not stable enough for use and is subject to change"
     )]
+    #[deprecated(
+        since = "1.87.0",
+        note = "use `${concat(...)}` with the `macro_metavar_expr_concat` feature instead"
+    )]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! concat_idents {

library/core/src/prelude/v1.rs

@@ -59,6 +59,7 @@ pub use crate::hash::macros::Hash;
 
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 #[doc(no_inline)]
 pub use crate::{
     assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,

library/std/src/lib.rs

@@ -704,6 +704,7 @@ pub use core::primitive;
 // Re-export built-in macros defined through core.
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 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,

library/std/src/prelude/v1.rs

@@ -46,6 +46,7 @@ pub use crate::result::Result::{self, Err, Ok};
 // Re-exported built-in macros
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
+#[cfg_attr(bootstrap, allow(deprecated_in_future))]
 #[doc(no_inline)]
 pub use core::prelude::v1::{
     assert, cfg, column, compile_error, concat, concat_idents, env, file, format_args,

src/doc/unstable-book/src/library-features/concat-idents.md

@@ -2,7 +2,10 @@
 
 The tracking issue for this feature is: [#29599]
 
+This feature is deprecated, to be replaced by [`macro_metavar_expr_concat`].
+
 [#29599]: https://github.com/rust-lang/rust/issues/29599
+[`macro_metavar_expr_concat`]: https://github.com/rust-lang/rust/issues/124225
 
 ------------------------
 

tests/ui/feature-gates/feature-gate-concat_idents.rs

@@ -1,3 +1,5 @@
+#![expect(deprecated)] // concat_idents is deprecated
+
 const XY_1: i32 = 10;
 
 fn main() {

tests/ui/feature-gates/feature-gate-concat_idents.stderr

@@ -1,5 +1,5 @@
 error[E0658]: use of unstable library feature `concat_idents`: `concat_idents` is not stable enough for use and is subject to change
-  --> $DIR/feature-gate-concat_idents.rs:5:13
+  --> $DIR/feature-gate-concat_idents.rs:7:13
    |
 LL |     let a = concat_idents!(X, Y_1);
    |             ^^^^^^^^^^^^^
@@ -9,7 +9,7 @@ LL |     let a = concat_idents!(X, Y_1);
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: use of unstable library feature `concat_idents`: `concat_idents` is not stable enough for use and is subject to change
-  --> $DIR/feature-gate-concat_idents.rs:6:13
+  --> $DIR/feature-gate-concat_idents.rs:8:13
    |
 LL |     let b = concat_idents!(X, Y_2);
    |             ^^^^^^^^^^^^^

tests/ui/feature-gates/feature-gate-concat_idents2.rs

@@ -1,3 +1,5 @@
+#![expect(deprecated)] // concat_idents is deprecated
+
 fn main() {
     concat_idents!(a, b); //~ ERROR `concat_idents` is not stable enough
                           //~| ERROR cannot find value `ab` in this scope

tests/ui/feature-gates/feature-gate-concat_idents2.stderr

@@ -1,5 +1,5 @@
 error[E0658]: use of unstable library feature `concat_idents`: `concat_idents` is not stable enough for use and is subject to change
-  --> $DIR/feature-gate-concat_idents2.rs:2:5
+  --> $DIR/feature-gate-concat_idents2.rs:4:5
    |
 LL |     concat_idents!(a, b);
    |     ^^^^^^^^^^^^^
@@ -9,7 +9,7 @@ LL |     concat_idents!(a, b);
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0425]: cannot find value `ab` in this scope
-  --> $DIR/feature-gate-concat_idents2.rs:2:5
+  --> $DIR/feature-gate-concat_idents2.rs:4:5
    |
 LL |     concat_idents!(a, b);
    |     ^^^^^^^^^^^^^^^^^^^^ not found in this scope

tests/ui/feature-gates/feature-gate-concat_idents3.rs

@@ -1,3 +1,5 @@
+#![expect(deprecated)] // concat_idents is deprecated
+
 const XY_1: i32 = 10;
 
 fn main() {