RFC: cfg_os_version_min

text/3750-cfg-os-version-min.md

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+- Feature Name: `cfg_os_version_min`
+- Start Date: 2024-12-27
+- RFC PR: [rust-lang/rfcs#3750](https://github.com/rust-lang/rfcs/pull/3750)
+- Rust Issue: [rust-lang/rust#136866](https://github.com/rust-lang/rust/issues/136866)
+
+# Summary
+[summary]: #summary
+
+A new `cfg` predicate `os_version_min` that allows users to declare the minimum primary (target-defined) API level required/supported by a block.
+E.g. `cfg!(os_version_min("windows", "6.1.7600"))` would match Windows version >= 6.1.7600.
+
+# Motivation
+[motivation]: #motivation
+
+Operating systems and their libraries are continually advancing, adding and sometimes removing APIs or otherwise changing behaviour.
+Versioning of APIs is common so that developers can target the set of APIs they support.
+Crates, including the standard library, must account for various API version requirements for the crate to be able to run.
+Rust currently has no mechanism for crates to compile different code (or to gracefully fail to compile) depending on the minimum targeted API version.
+This leads to the following issues:
+
+* Relying on dynamic detection of API support has a runtime cost.
+The standard library often performs [dynamic API detection](https://github.com/rust-lang/rust/blob/f283d3f02cf3ed261a519afe05cde9e23d1d9278/library/std/src/sys/windows/compat.rs) falling back to older (and less ideal) APIs or forgoing entire features when a certain API is not available.
+For example, the [current `Mutex` impl](https://github.com/rust-lang/rust/blob/234099d1d12bef9d6e81a296222fbc272dc51d89/library/std/src/sys/windows/mutex.rs#L1-L20) has a Windows 7 fallback. Users who only ever intend to run their code on newer versions of Windows will still pay a runtime cost for this dynamic API detection.
+Providing a mechanism for specifying which minimum API version the user cares about, allows for statically specifying which APIs a binary can use.
+* Certain features cannot be dynamically detected and thus limit possible implementations.
+The libc crate must use [a raw syscall on Android for `accept4`](https://github.com/rust-lang/libc/pull/1968), because this was only exposed in libc in version 21 of the Android API.
+Additionally libstd must dynamically load `signal` for all versions of Android despite it being required only for versions 19 and below.
+In the future there might be similar changes where there is no way to implement a solution for older versions.
+* Trying to compile code with an implicit dependency on a API version greater than what is supported by the target platform leads to linker errors.
+For example, the `x86_64-pc-windows-msvc` target's rustc implementation requires `SetThreadErrorMode` which was introduced in Windows 7.
+This means trying to build the compiler on older versions of Windows will fail with [a less than helpful linker error](https://github.com/rust-lang/rust/issues/35471).
+* Bumping the minimum supported version of a platform in Rust is a large endeavour.
+By adding this feature, we enable [rustc to more gradually raise the supported version](https://github.com/rust-lang/rust/pull/104385#issuecomment-1453520239) or to have more "levels" of version support.
+This would allow for having the "default" supported target be higher than the "minimum" supported target.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Rust targets are often thought of as monoliths.
+The thought is that if you compile a binary for that target, that binary should be able to run on any system that fits that target's description.
+However, this is not actually true.
+For example, when compiling for `x86_64-pc-windows-msvc` and linking with the standard library, my binary has implicitly taken a dependency on a set of APIs that Windows exposes for certain functionality.
+If I try to run my binary on older systems that do not have those APIs, then my binary will fail to run.
+When compiling for a certain target, you are therefore declaring a dependency on a minimum target API version that you rely on for your binary to run.
+
+Each standard library target uses a sensible minimum API version. for `x86_64-pc-windows-msvc` the minimum API version is "10.0.10240" which corresponds to Windows 10's initial release.
+For `x86_64-win7-pc-windows-msvc` the minimum API version is "6.1.7600" which corresponds to Windows 7.
+However, inferring the API version from the target name isn't ideal especially as it can change over time.
+
+Instead you use the `os_version_min` predicates to specify the minimum API levels of various parts of the operating system.  For example:
+
+* `os_version_min("windows", <string>)` would test the [minimum build version](https://gaijin.at/en/infos/windows-version-numbers) of Windows.
+* `os_version_min("libc", <string>)` would test the version of libc.
+* `os_version_min("kernel", <string>)` would test the version of the kernel.
+
+Let’s use `os_version_min("windows", …)` for a simple example.
+
+```rust
+pub fn random_u64() -> u64 {
+    let mut rand = 0_u64.to_ne_bytes();
+    if cfg!(os_version_min("windows", "10.0.10240")) {
+        // For an API version greater or equal to Windows 10, we use `ProcessPrng`
+        unsafe { ProcessPrng(rand.as_mut_ptr(), rand.len()) };
+    } else {
+        // Otherwise we fallback to `RtlGenRandom`
+        unsafe { RtlGenRandom(rand.as_mut_ptr().cast(), rand.len() as u32) };
+    }
+    u64::from_ne_bytes(rand)
+}
+```
+
+A more involved example would be to attempt to dynamically load the symbol.
+On macOS we use weak linking to do this:
+
+```rust
+// Always available under these conditions.
+#[cfg(any(
+    os_version_min("macos", "11.0"),
+    os_version_min("ios", "14.0"),
+    os_version_min("tvos", "14.0"),
+    os_version_min("watchos", "7.0"),
+    os_version_min("visionos", "1.0")
+))]
+let preadv = {
+    extern "C" {
+        fn preadv(libc::c_int, *const libc::iovec, libc::c_int, off64_t) -> isize;
+    }
+    Some(preadv)
+};
+
+// Otherwise `preadv` needs to be weakly linked.
+// We do that using a `weak!` macro, defined elsewhere.
+#[cfg(not(any(
+    os_version_min("macos", "11.0"),
+    os_version_min("ios", "14.0"),
+    os_version_min("tvos", "14.0"),
+    os_version_min("watchos", "7.0"),
+    os_version_min("visionos", "1.0")
+)))]
+weak!(fn preadv(libc::c_int, *const libc::iovec, libc::c_int, off64_t) -> isize);
+
+if let Some(preadv) = preadv {
+    preadv(...) // Use preadv, it's available
+} else {
+    // ... fallback impl
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `os_version_min` predicate allows users to conditionally compile code based on the API version supported by the target platform using `cfg`.
+It requires a key and a version string.
+The key can be either a `target_os` string or else one of a set of target-defined strings.
+Version strings are always target defined (see [Versioning Schema][versioning-schema]) and will be compared against the target's supported version.
+For example, `#[cfg(os_version_min("macos", "11.0"))]` has the key `macos` and the minimum version `11.0`, which will match any macOS version greater than or equal to macOS 11 Big Sur.
+If a target doesn't support a key, then the `cfg` will always return `false`.
+
+Each target platform will set the minimum API versions it supports for each key.
+
+## The standard library
+[the-standard-library]: #the-standard-library
+
+Currently the standard library is pre-compiled meaning that only a single version of each API can be supported, which must be the minimum version.
+Third party crates can choose to use a higher API level so long as it's compatible with the baseline API version.
+However, there is currently no support for setting a `os_version_min` version above the target's baseline (see [Future Possibilities][future-possibilities]).
+
+## Versioning Schema
+[versioning-schema]: #versioning-schema
+
+Version strings can take on nearly any form and while there are some standard formats, such as semantic versioning or release dates, projects/platforms can change schemas or provide aliases for some or all of their releases.
+Because of this diversity in version strings, each target will be responsible for validating the version, and defining comparisons on it.
+
+## Linting
+[linting]: #linting
+
+By default `os_version_min` will be linted by `check_cfg` in a similar way to `target_os`.
+That is, all valid values for `target_os` will be accepted as valid keys for `os_version_min` on all platforms.
+The list of additional keys supported by the target will be consulted, which will then be allowed on a per-target basis.
+
+## Future Compatibility
+[future-compatibility]: #future-compatibility
+
+The functions for parsing and comparing version strings may need to be updated whenever a new API is added, when the version format changes, or when new aliases need to be added.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Each supported platform will need to implement version string parsing logic (or re-use some provided defaults), maintain the logic in response to future changes, and update any version alias tables.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The overall mechanism proposed here builds on other well established primitives in Rust such as `cfg`.
+A mechanism which tries to bridge cross-platform differences under one `min_target_api_version` predicate [was suggested](https://github.com/rust-lang/rfcs/blob/b0f94000a3ddbd159013e100e48cd887ba2a0b54/text/0000-min-target-api-version.md) but was rejected due to different platforms having divergent needs.
+
+For many platforms, the `target_os` name and the `os_version_min` name will be identical.
+Even platforms that have multiple possible `versions` relevant to the OS will still have one primary version.
+E.g. for `linux` the primary version would refer to the kernel with `libc` being a secondary OS library version.
+Therefore it would make sense for the primary target OS version to be a property of `target_os`.
+E.g.: `cfg(target_os("macos", min_version = "..."))`.
+This means we'd need a more general syntax for `libc` and potentially other versioned libraries where the target OS is ambiguous.
+
+# Prior art
+[prior-art]: #prior-art
+
+In C it's common to be able to target different versions based on a preprocessor macro.
+For example, on Windows `WINVER` can be used:
+
+```c
+// If the minimum version is at least Windows 10
+#if (WINVER >= _WIN32_WINNT_WIN10)
+// ...
+#endif
+```
+
+This RFC is a continuation of [RFC #3379](https://github.com/rust-lang/rfcs/pull/3379) more narrowly scoped to just `os_version_min`.
+That RFC was in turn an updated version of [this RFC draft](https://github.com/rust-lang/rfcs/pull/3036), with the changes reflecting conversations from the draft review process and [further Zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/CFG.20OS.20Redux.20.28migrated.29/near/294738760).
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Custom targets usually specify their configurations in JSON files.
+It is unclear how the target maintainers would add version comparison information to these files.
+
+What exactly should the syntax be?
+Should we draw a distinction between cases where the `os_version_min` directly implies a specific `target_os` and cases where it doesn't (see alternatives)?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+* The compiler could allow setting a higher minimum OS version than the target's default.
+* With the `build-std` feature, each target could optionally support lowering the API version below the default.