Add counter balance to build probe drawback

rust-lang · epage · Sep 13, 2025 · Sep 15, 2025 · Sep 15, 2025 · Sep 15, 2025
commit 545ef5257edcfb0d41c62bb8498c4fac6b4087d2
diff --git a/text/3857-cfg-version.md b/text/3857-cfg-version.md
@@ -283,7 +283,16 @@ While that isn't always true today (e.g. some Cargo features go from "unknown" w
 having distinct implementations for different Rust versions can make the testing matrix more complex.
 Tools like [`cargo hack`](https://crates.io/crates/cargo-hack) can help which can run commands on not just one toolchain version but also the every version starting with the MSRV with a command like `cargo hack --rust-version --version-step 1 check`.
 
-This does not help with probes for specific implementations of nightly features which still requires having a `build.rs`.
+This does not help with identifying nightlies that a feature is available on or compatible with which will still require a `build.rs`.
+In terms of doing this via build probes,
+Cargo team has previously rejected support for build probes
+([source](https://github.com/rust-lang/cargo/issues/11244#issuecomment-2326780810)).
+Whether build probes or nightly version checks,
+auto-enabling nightly features
+(rather than having users opt-in)
+runs counter to the spirit of nightly (works like stable except where you opt-in)
+and cause problems if the checks are incorrect which has broken many crates nightly builds in the past
+and has even caused friction in changing unstable features within the compiler.
 
 Libraries could having ticking time bombs that accidentally break or have undesired behavior for some future Rust version that can't be found until we hit that version.