unstable book; document `macro_metavar_expr_concat` #139416

mejrs · 2025-04-05T17:16:23Z

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rustbot · 2025-04-05T17:16:28Z

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mejrs · 2025-04-05T17:17:16Z

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

@@ -6,6 +6,8 @@ The tracking issue for this feature is: [#29599]

 ------------------------

+> This feature is expected to be removed in favor of [`macro_metavar_expr_concat`](./macro-metavar-expr-concat.md).


see #t-libs > Removing `concat_idents`

mejrs · 2025-04-05T17:19:19Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+
+### Overview
+
+`macro_rules!` macros cannot create new identifiers and use them in ident positions.


I'm not sure about the "in ident position" phrasing. Is there a better (or maybe stronger/weaker) way of describing where macros canot be used?

It's fine to say that we can't create identifiers at all, since this isn't possible on stable. This section could be something like:

In stable Rust, there is no way to create new identifiers by joining identifiers to literals or other identifiers without using procedural macros such as paste. #![feature(macro_metavar_expr_concat)] introduces a way to do this, using the concat metavariable expression:

// example

The above expands to:

// expansion

I've used this wording at the top but kept this section as is, I prefer a "show a locked door before you show a key" approach.

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

ehuss · 2025-04-07T20:59:58Z

r? @tgross35
maybe?
or
@joshtriplett

tgross35 · 2025-04-08T00:06:09Z

Happy to review this one. One general note: concat_idents is in the process of being deprecated/removed within a few months, so I don’t think it makes sense to introduce macro_metavar_expr_concat with a comparison to concat_idents. If anything just a short note that it supersedes that feature.

tgross35 · 2025-04-08T01:45:22Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+
+### Overview
+
+`macro_rules!` macros cannot create new identifiers and use them in ident positions.


It's fine to say that we can't create identifiers at all, since this isn't possible on stable. This section could be something like:

In stable Rust, there is no way to create new identifiers by joining identifiers to literals or other identifiers without using procedural macros such as paste. #![feature(macro_metavar_expr_concat)] introduces a way to do this, using the concat metavariable expression:

// example

The above expands to:

// expansion

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

tgross35 · 2025-04-08T01:48:24Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+### Syntax
+
+This feature builds upon the metavariable expression syntax `${ .. }` as specified in [RFC 3086] ([`macro_metavar_expr`]).
+ `concat` is available like `${ concat(items) }`, where `items` is a comma separated sequence of idents and/or string literals.


s/string literals/literals, I think we want to support numeric literals as well (or maybe already do?)

numerics are not supported: https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=db1dbedf669a4061ff6ebceccf9cd7ad

#![feature(macro_metavar_expr_concat)] macro_rules! cursed_add { ($a:literal, $b:literal) => { ${ concat($a, $b) } } } fn main() { let x = cursed_add!(42, 73); dbg!(x); }

error: metavariables of `${concat(..)}` must be of type `ident`, `literal` or `tt` --> src/main.rs:4:18 | 4 | ${ concat($a, $b) } | ^ | = note: currently only string literals are supported

Fair enough. It would probably still be fine to leave it as open ended "literals" so docs don't need to update if this changes, but 🤷

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

tgross35 · 2025-04-08T02:19:52Z

@rustbot author for the above

rustbot · 2025-04-08T02:19:56Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

mejrs · 2025-04-08T15:17:59Z

@rustbot ready

tgross35 · 2025-04-08T17:09:36Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+A common use case is the need to create names for structs or functions. The following cannot be done on stable Rust[^1]:
+
+```rust,compile_fail
+macro_rules! create_some_structs {
+    ($name:ident) => {
+        // Invalid syntax
+        pub struct First$name;
+        // Also invalid syntax
+        pub struct Second($name);
+        // Macros are not allowed in this position
+        // (This restriction is what makes `concat_idents!` useless)
+        pub struct concat_idents!(Third, $name);
+    }
+}
+# create_some_structs!(Thing);
+```


This bit can be eliminated, macro_metavar_expr_concat shouldn't be discussed in terms of concat_idents's limitations.

tgross35 · 2025-04-08T17:10:08Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+# create_some_structs!(Thing);
+```
+
+`#![feature(macro_metavar_expr_concat)]` provides the `concat` metavariable to concatenate idents:


`#![feature(macro_metavar_expr_concat)]` provides the `concat` metavariable expression for creating new identifiers:

tgross35 · 2025-04-08T17:26:11Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+### Syntax
+
+This feature builds upon the metavariable expression syntax `${ .. }` as specified in [RFC 3086] ([`macro_metavar_expr`]).
+ `concat` is available like `${ concat(items) }`, where `items` is a comma separated sequence of idents and/or string literals.


Fair enough. It would probably still be fine to leave it as open ended "literals" so docs don't need to update if this changes, but 🤷

tgross35 · 2025-04-08T17:27:46Z

src/doc/unstable-book/src/language-features/macro-metavar-expr-concat.md

+In stable Rust, there is no way to create new identifiers by joining identifiers to literals or other identifiers without using procedural macros such as [`paste`].
+ `#![feature(macro_metavar_expr_concat)]` introduces a way to do this, using the concat metavariable expression.
+
+> This feature is not to be confused with [`macro_metavar_expr`] or [`concat_idents`].


> This feature uses the syntax from [`macro_metavar_expr`] but is otherwise > independent. It replaces the old unstable feature [`concat_idents`].

mejrs · 2025-04-09T20:26:34Z

@rustbot ready

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unstable book; document macro_metavar_expr_concat

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unstable book; document `macro_metavar_expr_concat` #139416

unstable book; document `macro_metavar_expr_concat` #139416

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mejrs commented Apr 9, 2025

		@@ -6,6 +6,8 @@ The tracking issue for this feature is: [#29599]

		------------------------

		> This feature is expected to be removed in favor of [`macro_metavar_expr_concat`](./macro-metavar-expr-concat.md).


		### Overview

		`macro_rules!` macros cannot create new identifiers and use them in ident positions.

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mejrs commented Apr 9, 2025

unstable book; document `macro_metavar_expr_concat` #139416

unstable book; document `macro_metavar_expr_concat` #139416

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