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Document a bit more how the SDK version actually works

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This commit is contained in:
Mads Marquart 2024-09-29 14:45:08 +02:00
parent 612796c420
commit 0bebedd799
1 changed files with 41 additions and 7 deletions
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48
compiler/rustc_codegen_ssa/src/back/link.rs
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@ -2959,11 +2959,12 @@ pub(crate) fn are_upstream_rust_objects_already_included(sess: &Session) -> bool
}
}
/// We need to communicate four things to the linker on Apple/Darwin targets:
/// We need to communicate five things to the linker on Apple/Darwin targets:
/// - The architecture.
/// - The operating system (and that it's an Apple platform).
/// - The deployment target.
/// - The environment / ABI.
/// - The deployment target.
/// - The SDK version.
fn add_apple_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor) {
if !sess.target.is_like_osx {
return;
@ -3039,7 +3040,38 @@ fn add_apple_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavo
let (major, minor, patch) = current_apple_deployment_target(&sess.target);
let min_version = format!("{major}.{minor}.{patch}");
// Lie about the SDK version, we don't know it here
// The SDK version is used at runtime when compiling with a newer SDK / version of Xcode:
// - By dyld to give extra warnings and errors, see e.g.:
// <https://github.com/apple-oss-distributions/dyld/blob/dyld-1165.3/common/MachOFile.cpp#L3029>
// <https://github.com/apple-oss-distributions/dyld/blob/dyld-1165.3/common/MachOFile.cpp#L3738-L3857>
// - By system frameworks to change certain behaviour. For example, the default value of
// `-[NSView wantsBestResolutionOpenGLSurface]` is `YES` when the SDK version is >= 10.15.
// <https://developer.apple.com/documentation/appkit/nsview/1414938-wantsbestresolutionopenglsurface?language=objc>
//
// We do not currently know the actual SDK version though, so we have a few options:
// 1. Use the minimum version supported by rustc.
// 2. Use the same as the deployment target.
// 3. Use an arbitary recent version.
// 4. Omit the version.
//
// The first option is too low / too conservative, and means that users will not get the
// same behaviour from a binary compiled with rustc as with one compiled by clang.
//
// The second option is similarly conservative, and also wrong since if the user specified a
// higher deployment target than the SDK they're compiling/linking with, the runtime might
// make invalid assumptions about the capabilities of the binary.
//
// The third option requires that `rustc` is periodically kept up to date with Apple's SDK
// version, and is also wrong for similar reasons as above.
//
// The fourth option is bad because while `ld`, `otool`, `vtool` and such understand it to
// mean "absent" or `n/a`, dyld doesn't actually understand it, and will end up interpreting
// it as 0.0, which is again too low/conservative.
//
// Currently, we lie about the SDK version, and choose the second option.
//
// FIXME(madsmtm): Parse the SDK version from the SDK root instead.
// <https://github.com/rust-lang/rust/issues/129432>
let sdk_version = &*min_version;
// From the man page for ld64 (`man ld`):
@ -3053,11 +3085,13 @@ fn add_apple_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavo
cmd.link_args(&["-platform_version", platform_name, &*min_version, sdk_version]);
} else {
// cc == Cc::Yes
//
// We'd _like_ to use `-target` everywhere, since that can uniquely
// communicate all the required details, but that doesn't work on GCC,
// and since we don't know whether the `cc` compiler is Clang, GCC, or
// something else, we fall back to other options that also work on GCC
// when compiling for macOS.
// communicate all the required details except for the SDK version
// (which is read by Clang itself from the SDKROOT), but that doesn't
// work on GCC, and since we don't know whether the `cc` compiler is
// Clang, GCC, or something else, we fall back to other options that
// also work on GCC when compiling for macOS.
//
// Targets other than macOS are ill-supported by GCC (it doesn't even
// support e.g. `-miphoneos-version-min`), so in those cases we can