Introduce an RFC for config aliases

Author: tgross35

text/0000-cfg-alias.md

@@ -0,0 +1,165 @@
+- Feature Name: `cfg_alias`
+- Start Date: 2025-04-23
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue:
+  [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+
+[summary]: #summary
+
+This RFC introduces a way to name configuration predicates for easy reuse
+throughout a crate.
+
+```rust
+#![cfg_alias(
+    x86_linux,
+    all(any(target_arch = "x86", target_arch = "x86_64"), target_os = "linux")
+)]
+
+#[cfg(x86_linux)]
+fn foo() { /* ... */ }
+
+#[cfg(not(x86_linux))]
+fn foo() { /* ... */ }
+```
+
+# Motivation
+
+[motivation]: #motivation
+
+It is very common that the same `#[cfg(...)]` options need to be repeated in
+multiple places. Often this is because a `cfg(...)` needs to be matched with a
+`cfg(not(...))`, or because code cannot easily be reorganized to group all code
+for a specific `cfg` into a module. The solution is usually to copy a `#[cfg]`
+group around, which is error-prone and noisy.
+
+Adding aliases to config predicates reduces the amount of code that needs to be
+duplicated, and giving it a name provides an easy way to show what a group of
+configuration is intended to represent.
+
+Something to this effect can be done using build scripts. This requires reading
+various Cargo environment variables and potentially doing string manipulation
+(for splitting target features), so it is often inconvenient enough to not be
+worth doing. Allowing aliases to be defined within the crate and with the same
+syntax as the `cfg` itself makes this much easier.
+
+# Guide-level explanation
+
+[guide-level-explanation]: #guide-level-explanation
+
+There is a new crate-level attribute that takes a name and a `cfg` predicate:
+
+```rust
+#![cfg_alias(some_alias, predicate)]
+```
+
+`predicate` can be anything that usually works within `#[cfg(...)]`, including
+`all`, `any`, and `not`.
+
+Once an alias is defined, `name` can be used as if it had been passed via
+`--cfg`:
+
+```rust
+#[cfg(some_alias)]
+struct Foo { /* ... */ }
+
+#[cfg(not(some_alias))]
+struct Foo { /* ... */ }
+
+#[cfg(all(some_alias, target_os = "linux"))]
+fn bar() { /* ... */ }
+```
+
+# Reference-level explanation
+
+[reference-level-explanation]: #reference-level-explanation
+
+The new crate-level attribute is introduced:
+
+```text
+CfgAliasAttribute:
+    cfg_alias(IDENTIFIER, ConfigurationPredicate)
+```
+
+The identifier is added to the `cfg` namespace. It must not conflict with any
+builtin configuration names, or with those passed via `--cfg`.
+
+Once defined, the alias can be used as a regular predicate.
+
+The alias is only usable after it has been defined. For example, the following
+will emit an unknown configuration lint:
+
+```rust
+#![cfg_attr(some_alias, some_attribute)]
+// warning: unexpected_cfgs
+//
+// The lint could mention that `some_alias` was found in the
+// crate but is not available here.
+
+#![cfg_alias(some_alias, true)]
+```
+
+RFC Question:
+
+Two ways to implement this are with (1) near-literal substitution, or (2)
+checking whether the alias should be set or not at the time it is defined. Is
+there any user-visible behavior that would make us need to specify one or the
+other?
+
+If we go with the first option, we should limit to a single expansion to avoid
+recursing (as is done for `#define` in C).
+
+# Drawbacks
+
+[drawbacks]: #drawbacks
+
+- This does not support more general attribute aliases, such as
+  `#![alias(foo = derive(Clone, Copy, Debug, Default)`. This seems better suited
+  for something like `declarative_attribute_macros` in [RFC3697].
+
+[RFC3697]: https://github.com/rust-lang/rfcs/pull/3697
+
+# Rationale and alternatives
+
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- The syntax `cfg_alias(name, predicate)` was chosen for similarity with
+  `cfg_attr(predicate, attributes)`. Alternatives include:
+  - `cfg_alias(name = predicate)`
+- It may be possible to have `#[cfg_alias(...)]` work as an outer macro and only
+  apply to a specific scope. This likely is not worth the complexity.
+
+# Prior art
+
+[prior-art]: #prior-art
+
+In C it is possible to modify the define map in source:
+
+```c
+# if (defined(__x86_64__) || defined(__i386__)) && defined(__SSE2__)
+#define X86_SSE2
+#endif
+
+#ifdef X86_SSE2
+// ...
+#endif
+```
+
+# Unresolved questions
+
+[unresolved-questions]: #unresolved-questions
+
+Questions to resolve before this RFC could merge:
+
+- Which syntax should be used?
+- Substitution vs. evaluation at define time (the question under the
+  reference-level explanation)
+
+# Future possibilities
+
+[future-possibilities]: #future-possibilities
+
+- A `--cfg-alias` CLI option would provide a way for Cargo to interact with this
+  feature, such as defining config aliases in the workspace `Cargo.toml` for
+  reuse in multiple crates.