RFC: Add target configuration

text/0000-cfg-target.md

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+- Feature Name: `cfg-target`
+- Start Date: 2020-09-27
+- RFC PR: [rust-lang/rfcs#3239](https://github.com/rust-lang/rfcs/pull/3239)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+This proposes a new `cfg`: `target`, which matches the entire target triple
+string (e.g. `arm-unknown-linux-gnueabihf`). This also adds a `CARGO_CFG_TARGET`
+environment variable for parity with other `CARGO_CFG_*` variables.
+
+In addition, this proposes a shorthand `cfg(target(...))` to match multiple
+components of a target string at once.
+
+# Motivation
+[motivation]: #motivation
+
+To `#[cfg]` against a specific target, a `build.rs` script is required to emit a
+custom `cfg` based on the `TARGET` environment variable. Adding a build script
+increases compile time and makes a crate incompatible with certain build
+systems.
+
+Otherwise, all available components would need to be specified separately:
+`target_arch`, `target_vendor`, `target_os`, `target_env` and `target_abi`.
+This can be very cumbersome. Note that the target ABI cannot currently be
+`#[cfg]`-ed against, so a `build.rs` is still necessary to match all target
+components.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This would act like existing `target_*` configurations (except `target_feature`)
+but match against all components.
+
+```rust
+#[cfg(target = "x86_64-apple-ios-macabi")]
+mod mac_catalyst;
+```
+
+This includes `#[cfg_attr(target = "...", attr)]`.
+
+It would also support to specify each `target_*` inside a new `target`
+attribute as follows:
+
+```rust
+// So we can for example rewrite:
+#[cfg(all(target_os = "linux", target_arch = "arm"))]
+// as:
+#[cfg(target(os = "linux", arch = "arm"))]
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+`target` is a key-value option set once with the target's Rust triple.
+
+Example values:
+
+- `"aarch64-apple-darwin"`
+- `"arm-unknown-linux-gnueabihf"`
+- `"x86_64-apple-ios-macabi"`
+- `"x86_64-pc-windows-gnu"`
+- `"x86_64-pc-windows-msvc"`
+- `"x86_64-unknown-linux-gnu"`
+
+## Semantics of target with attributes
+
+The shorthand form of `#[cfg(target(os = "linux))]` is expanded and entirely
+equivalent to `#[cfg(target_os = "linux")]` (and so on for `arch` and the other
+potential attributes).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Configuring against specific targets can be overly strict and could make
+  certain `#[cfg]`s miss similar configurations with small changes.
+
+  For example: `aarch64-unknown-none` does not match
+  `aarch64-unknown-none-softfloat`, yet one would likely want to include ABI
+  variants. The same concern applies to the target vendor.
+
+  A potential solution would be to allow glob matching (e.g.
+  `aarch64-unknown-none*`), but that is not within the scope of this proposal
+  because it is not currently used in other `#[cfg]`s.
+
+- The `CARGO_CFG_TARGET` environment variable is redundant with the existing
+  `TARGET`. However, including it would be consistent with other `CARGO_CFG_*`
+  variables.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+We can keep the existing work-around of checking the `TARGET` environment
+variable in a `build.rs` script. However, that increases compile time and makes
+a crate incompatible with certain build systems.
+
+# Prior art
+[prior-art]: #prior-art
+
+- [Target component configurations](https://doc.rust-lang.org/reference/conditional-compilation.html#set-configuration-options):
+  `target_arch`, `target_vendor`, `target_os`, and `target_env`.
+
+- `TARGET` and `CARGO_CFG_TARGET_*`
+  [environment variables for `build.rs`](https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts).
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- How do we ensure a project does not miss configurations similar to the ones
+  being `#[cfg]`-ed against with this feature? Perhaps this should be added as a
+  Clippy lint that's off by default.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+This would enable `#[cfg]`-ing against a specific target ABI (e.g. `macabi`,
+`eabihf`). However, that is not the motivation for this proposal and should be
+handled separately.