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rfcs/text/1721-crt-static.md

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Summary

Enable the compiler to select whether a target dynamically or statically links to a platform's standard C runtime ("CRT") through the introduction of three orthogonal and otherwise general purpose features, one of which will likely never become stable and can be considered an implementation detail of std. These features do not require the compiler or language to have intrinsic knowledge of the existence of C runtimes.

The end result is that rustc will be able to reuse its existing standard library binaries for the MSVC and musl targets to build code that links either statically or dynamically to libc.

The design herein additionally paves the way for improved support for dllimport/dllexport, and cpu-specific features, particularly when combined with a std-aware cargo.

Motivation

Today all targets of rustc hard-code how they link to the native C runtime. For example the x86_64-unknown-linux-gnu target links to glibc dynamically, x86_64-unknown-linux-musl links statically to musl, and x86_64-pc-windows-msvc links dynamically to MSVCRT. There are many use cases, however, where these decisions are not suitable. For example binaries on Alpine Linux want to link dynamically to musl and creating portable binaries on Windows is most easily done by linking statically to MSVCRT.

Today rustc has no mechanism for accomplishing this besides defining an entirely new target specification and distributing a build of the standard library for it. Because target specifications must be described by a target triple, and target triples have preexisting conventions into which such a scheme does not fit, we have resisted doing so.

Detailed design

This RFC introduces three separate features to the compiler and Cargo. When combined they will enable the compiler to change whether the C standard library is linked dynamically or statically. In isolation each feature is a natural extension of existing features, and each should be useful on its own.

A key insight is that, for practical purposes, the object code for the standard library does not need to change based on how the C runtime is being linked; though it is true that on Windows, it is generally important to properly manage the use of dllimport/dllexport attributes based on the linkage type, and C code does need to be compiled with specific options based on the linkage type. So it is technically possible to produce Rust executables and dynamic libraries that either link to libc statically or dynamically from a single std binary by correctly manipulating the arguments to the linker.

A second insight is that there are multiple existing, unserved use cases for configuring features of the hardware architecture, underlying platform, or runtime 1, which require the entire 'world', possibly including std, to be compiled a certain way. C runtime linkage is another example of this requirement.

From these observations we can design a cross-platform solution spanning both Cargo and the compiler by which Rust programs may link to either a dynamic or static C library, using only a single std binary. As future work this RFC discusses how the proposed scheme scheme can be extended to rebuild std specifically for a particular C-linkage scenario, which may have minor advantages on Windows due to issues around dllimport and dllexport; and how this scheme naturally extends to recompiling std in the presence of modified CPU features.

This RFC does not propose unifying how the C runtime is linked across platforms (e.g. always dynamically or always statically) but instead leaves that decision to each target, and to future work.

In summary the new mechanics are:

  • Specifying C runtime linkage via -C target-feature=+crt-static or -C target-feature=-crt-static. This extends -C target-feature to mean not just "CPU feature" ala LLVM, but "feature of the Rust target". Several existing properties of this flag, the ability to add, with +, or remove, with -, the feature, as well as the automatic lowering to cfg values, are crucial to later aspects of the design. This target feature will be added to targets via a small extension to the compiler's target specification.
  • Lowering cfg values to Cargo build script environment variables. This will enable build scripts to understand all enabled features of a target (like crt-static above) to, for example, compile C code correctly on MSVC.
  • Lazy link attributes. This feature is only required by std's own copy of the libc crate, and only because std is distributed in binary form and it may yet be a long time before Cargo itself can rebuild std.

Specifying dynamic/static C runtime linkage

A new target-feature flag will now be supported by the compiler for relevant targets: crt-static. This can be enabled and disabled in the compiler via:

rustc -C target-feature=+crt-static ...
rustc -C target-feature=-crt-static ...

Currently all target-feature flags are passed through straight to LLVM, but this proposes extending the meaning of target-feature to Rust-target-specific features as well. Target specifications will be able to indicate what custom target-features can be defined, and most existing targets will define a new crt-static feature which is turned off by default (except for musl).

The default of crt-static will be different depending on the target. For example x86_64-unknown-linux-musl will have it on by default, whereas arm-unknown-linux-musleabi will have it turned off by default.

Lowering cfg values to Cargo build script environment variables

Cargo will begin to forward cfg values from the compiler into build scripts. Currently the compiler supports --print cfg as a flag to print out internal cfg directives, which Cargo uses to implement platform-specific dependencies.

When Cargo runs a build script it already sets a number of environment variables, and it will now set a family of CARGO_CFG_* environment variables as well. For each key printed out from rustc --print cfg, Cargo will set an environment variable for the build script to learn about.

For example, locally rustc --print cfg prints:

target_os="linux"
target_family="unix"
target_arch="x86_64"
target_endian="little"
target_pointer_width="64"
target_env="gnu"
unix
debug_assertions

And with this Cargo would set the following environment variables for build script invocations for this target.

export CARGO_CFG_TARGET_OS=linux
export CARGO_CFG_TARGET_FAMILY=unix
export CARGO_CFG_TARGET_ARCH=x86_64
export CARGO_CFG_TARGET_ENDIAN=little
export CARGO_CFG_TARGET_POINTER_WIDTH=64
export CARGO_CFG_TARGET_ENV=gnu
export CARGO_CFG_UNIX
export CARGO_CFG_DEBUG_ASSERTIONS

As mentioned in the previous section, the linkage of the C standard library will be specified as a target feature, which is lowered to a cfg value, thus giving build scripts the ability to modify compilation options based on C standard library linkage. One important complication here is that cfg values in Rust may be defined multiple times, and this is the case with target features. When a cfg value is defined multiple times, Cargo will create a single environment variable with a comma-separated list of values.

So for a target with the following features enabled

target_feature="sse"
target_feature="crt-static"

Cargo would convert it to the following environment variable:

export CARGO_CFG_TARGET_FEATURE=sse,crt-static

Through this method build scripts will be able to learn how the C standard library is being linked. This is crucially important for the MSVC target where code needs to be compiled differently depending on how the C library is linked.

This feature ends up having the added benefit of informing build scripts about selected CPU features as well. For example once the target_feature #[cfg] is stabilized build scripts will know whether SSE/AVX/etc are enabled features for the C code they might be compiling.

After this change, the gcc-rs crate will be modified to check for the CARGO_CFG_TARGET_FEATURE directive, and parse it into a list of enabled features. If the crt-static feature is not enabled it will compile C code on the MSVC target with /MD, indicating dynamic linkage. Otherwise if the value is static it will compile code with /MT, indicating static linkage. Because today the MSVC targets use dynamic linkage and gcc-rs compiles C code with /MD, gcc-rs will remain forward and backwards compatible with existing and future Rust MSVC toolchains until such time as the decision is made to change the MSVC toolchain to +crt-static by default.

Lazy link attributes

The final feature that will be added to the compiler is the ability to "lazily" interpret the linkage requirements of a native library depending on values of cfg at compile time of downstream crates, not of the crate with the #[link] directives. This feature is never intended to be stabilized, and is instead targeted at being an unstable implementation detail of the libc crate linked to std (but not the stable libc crate deployed to crates.io).

Specifically, the #[link] attribute will be extended with a new argument that it accepts, cfg(..), such as:

#[link(name = "foo", cfg(bar))]

This cfg indicates to the compiler that the #[link] annotation only applies if the bar directive is matched. This interpretation is done not during compilation of the crate in which the #[link] directive appears, but during compilation of the crate in which linking is finally performed. The compiler will then use this knowledge in two ways:

  • When dllimport or dllexport needs to be applied, it will evaluate the final compilation unit's #[cfg] directives and see if upstream #[link] directives apply or not.

  • When deciding what native libraries should be linked, the compiler will evaluate whether they should be linked or not depending on the final compilation's #[cfg] directives and the upstream #[link] directives.

Customizing linkage to the C runtime

With the above features, the following changes will be made to select the linkage of the C runtime at compile time for downstream crates.

First, the libc crate will be modified to contain blocks along the lines of:

cfg_if! {
    if #[cfg(target_env = "musl")] {
        #[link(name = "c", cfg(target_feature = "crt-static"), kind = "static")]
        #[link(name = "c", cfg(not(target_feature = "crt-static")))]
        extern {}
    } else if #[cfg(target_env = "msvc")] {
        #[link(name = "msvcrt", cfg(not(target_feature = "crt-static")))]
        #[link(name = "libcmt", cfg(target_feature = "crt-static"))]
        extern {}
    } else {
        // ...
    }
}

This informs the compiler that, for the musl target, if the CRT is statically linked then the library named c is included statically in libc.rlib. If the CRT is linked dynamically, however, then the library named c will be linked dynamically. Similarly for MSVC, a static CRT implies linking to libcmt and a dynamic CRT implies linking to msvcrt (as we do today).

Finally, an example of compiling for MSVC and linking statically to the C runtime would look like:

set RUSTFLAGS=-C target-feature=+crt-static
cargo build --target x86_64-pc-windows-msvc

and similarly, compiling for musl but linking dynamically to the C runtime would look like:

RUSTFLAGS='-C target-feature=-crt-static' cargo build --target x86_64-unknown-linux-musl

Future work

The features proposed here are intended to be the absolute bare bones of support needed to configure how the C runtime is linked. A primary drawback, however, is that it's somewhat cumbersome to select the non-default linkage of the CRT. Similarly, however, it's cumbersome to select target CPU features which are not the default, and these two situations are very similar. Eventually it's intended that there's an ergonomic method for informing the compiler and Cargo of all "compilation codegen options" over the usage of RUSTFLAGS today.

Furthermore, it would have arguably been a "more correct" choice for Rust to by default statically link to the CRT on MSVC rather than dynamically. While this would be a breaking change today due to how C components are compiled, if this RFC is implemented it should not be a breaking change to switch the defaults in the future, after a reasonable transition period.

The support in this RFC implies that the exact artifacts that we're shipping will be usable for both dynamically and statically linking the CRT. Unfortunately, however, on MSVC code is compiled differently if it's linking to a dynamic library or not. The standard library uses very little of the MSVCRT, so this won't be a problem in practice for now, but runs the risk of binding our hands in the future. It's intended, though, that Cargo will eventually support custom-compiling the standard library. The crt-static feature would simply be another input to this logic, so Cargo would custom-compile the standard library if it differed from the upstream artifacts, solving this problem.

References

Drawbacks

  • Working with RUSTFLAGS can be cumbersome, but as explained above it's planned that eventually there's a much more ergonomic configuration method for other codegen options like target-cpu which would also encompass the linkage of the CRT.

  • Adding a feature which is intended to never be stable (#[link(.., cfg(..))]) is somewhat unfortunate but allows sidestepping some of the more thorny questions with how this works. The stable semantics will be that for some targets the --cfg crt_link=... directive affects the linkage of the CRT, which seems like a worthy goal regardless.

  • The lazy semantics of #[link(cfg(..))] are not so obvious from the name (no other cfg attribute is treated this way). But this seems a minor issue since the feature serves one implementation-specif purpose and isn't intended for stabilization.

Alternatives

  • One alternative is to add entirely new targets, for example x86_64-pc-windows-msvc-static. Unfortunately though we don't have a great naming convention for this, and it also isn't extensible to other codegen options like target-cpu. Additionally, adding a new target is a pretty heavyweight solution as we'd have to start distributing new artifacts and such.

  • Another possibility would be to start storing metadata in the "target name" along the lines of x86_64-pc-windows-msvc+static. This is a pretty big design space, though, which may not play well with Cargo and build scripts, so for now it's preferred to avoid this rabbit hole of design if possible.

  • Finally, the compiler could simply have an environment variable which indicates the CRT linkage. This would then be read by the compiler and by build scripts, and the compiler would have its own back channel for changing the linkage of the C library along the lines of #[link(.., cfg(..))] above.

  • Another approach has been proposed recently that has rustc define an environment variable to specify the C runtime kind.

  • Instead of extending the semantics of -C target-feature beyond "CPU features", we could instead add a new flag for the purpose, e.g. -C custom-feature.

Unresolved questions

  • What happens during the cfg to environment variable conversion for values that contain commas? It's an unusual corner case, and build scripts should not depend on such values, but it needs to be handled sanely.

  • Is it really true that lazy linking is only needed by std's libc? What about in a world where we distribute more precompiled binaries than just std?