Rust bindings to LLVM's C API.

# Usage

```toml
[dependencies]
llvm-sys = "201.0.0-rc1"
```

There must be a compatible version of LLVM available. By default `llvm-sys`
will look for `llvm-config` on `PATH` to find a system-wide copy of LLVM and
use that if it is a compatible version. Custom options for finding LLVM
on your system can be specified via environment variables. See
[LLVM compatibility](#llvm-compatibility) for more information.

## Documentation

[![documentation on docs.rs](https://img.shields.io/docsrs/llvm-sys)](https://docs.rs/llvm-sys/)

See the `examples` directory in this repository for API examples. There also
exist some other projects using these bindings which may be
informative or useful:

 * Safe, "Rusty" APIs for using LLVM:
   * [Inkwell](https://github.com/TheDan64/inkwell)
   * [llvm-ir](https://github.com/cdisselkoen/llvm-ir)
 * [Tari's merthc](https://gitlab.com/taricorp/merthc)
 * [Wilfred's BF compiler](https://crates.io/crates/bfc)
 * Jay Phelps wrote about [building a minimal compiler targeting
   WebAssembly](https://medium.com/@jayphelps/93e8c193fdb4)
    - [Lyle extended this for arm64 using clang](https://github.com/lyledean1/llvm-hello-world-rust)
 * [Lyle wrote a small PoC compiler Calculon using LLVM with LALRPOP](https://github.com/lyledean1/calculon)

Most of the interfaces are not documented in these bindings. Refer to the
[LLVM documentation](http://llvm.org/docs/) for more information, particularly
the [generated API documentation](http://llvm.org/doxygen/).

If you have your own project using these bindings that you think is worth
mentioning here, by all means let me know.

## Cargo features

This crate provides a few feature flags that can be important to particular
uses. These are mostly useful only at build-time; If you encounter problems
building something that depends on llvm-sys, enabling one or more option may be
useful; for example:

    cargo build --features=llvm-sys/prefer-dynamic

Most crates that depend on llvm-sys should not automatically enable any of these
features, because the appropriateness of any given option usually depends on the
configuration of the system on which the library is being compiled.

 * Dynamic/static linking preferences `prefer-dynamic`, `force-dynamic`,
   `prefer-static`, `force-static`. Only one may be specified, indicating how
   LLVM should be linked. If none are enabled, `prefer-static` is assumed.
   (Prior to crate version `191`, the default linking behavior is
   `force-static`.)

   With dynamic linking, the crate will link against a LLVM shared library
   (a .dll on Windows, .so on Linux and so forth) which must be available at
   runtime; static linking instead uses a static library archive (.lib on
   Windows or .a on most Unix-like systems) that includes all of the library
   code in the final binary (increasing file size but eliminating any dependency
   on additional files to run the program).

   If a `prefer-` option is enabled, the crate will attempt to use the specified
   kind of linking (static or dynamic) but fall back to the other if the first
   kind is unavailable. Setting a `force-` option will attempt only to do the
   specified kind of linking and fail otherwise.

 * `strict-versioning`: causes the build to fail if a version of LLVM exactly
   matching the current crate version is not found. If enabled, linking crate
   version 150.1.0 against LLVM 16.0 (for example) would fail whereas it is
   normally permitted.

   This may be useful if there is known to be an incompatibility between some
   LLVM versions, causing the build to fail rather than possibly causing errors
   at runtime.

 * `disable-alltargets-init`: disable building the functions that initialize
   LLVM components for all supported targets. These functions need to be
   compiled from C, so if they are unneeded then turning them off can allow
   the build to proceed if a working C compiler is unavailable.

 * `no-llvm-linking`: prevents llvm-sys from instructing the compiler to link
   against any LLVM libraries. This can be useful if another crate links LLVM in
   some different way (preventing conflicts) but means the other crate must
   ensure llvm-sys' library requirements are satisfied.

## Build requirements

llvm-sys requires a copy of `llvm-config` corresponding to the desired version of
LLVM to build: `llvm-config` allows it to probe what libraries need to be linked
and what compiler options are required.

Binary distributions of LLVM (including the official release packages) generally
**do not** include a copy of `llvm-config`, making them unsuited to use for building
programs with `llvm-sys`. Known exceptions (that *do* include a copy of `llvm-config`)
include:

 * Official Debian/Ubuntu packages from [apt.llvm.org](https://apt.llvm.org/)
 * Fedora/Red Hat's `llvm-devel` package
 * Arch Linux's [`llvm`](https://archlinux.org/packages/extra/x86_64/llvm/) package

If a suitable binary package is not available for your platform, compiling from
source is usually the best option. See [Compiling LLVM in this document](#compiling-llvm)
for details.

---

It may be difficult or even impossible to provide a compatible LLVM version
system-wide for a given project (consider a program using two libraries that
internally use different versions of LLVM!) so environment variables can be set
to help the build scripts find your copy of the libraries. This is also helpful
if you are unable to provide a system-wide version of LLVM but can still
compile it yourself.

`LLVM_SYS_<version>_PREFIX` specifies the install prefix for a compiled and
installed copy of the libraries, where `<version>` is the major version of
`llvm-sys` (for example, `LLVM_SYS_201_PREFIX`). The llvm-sys build scripts
will look for a `llvm-config` binary in the directory `<PREFIX>/bin/` in
addition to searching for a copy on `$PATH`, and verify that the LLVM version
reported by `llvm-config` is compatible with the current crate version.

### LLVM compatibility

Because the LLVM C [API stability guarantees][c-api-stability] are relatively
weak, this crate enforces that the LLVM release in use match the one it was made
for. The crate version is constructed by treating the LLVM version as a real
number and multiplying by 10, ignoring any fractional part. Thus `llvm-sys`
version 191 is compatible with LLVM 19.1.x, and `llvm-sys` 201 would be compatible
with LLVM 20.1.x.

[c-api-stability]: http://llvm.org/docs/DeveloperPolicy.html#c-api-changes

The build scripts will not enforce this compatibility matrix strictly,
permitting compilation against any version of LLVM that is at least as new as
the crate target version. This is safe in most cases because the LLVM C API is
meant to maintain binary compatibility across releases with the exception of
when functions are deprecated and later removed. An incompatible LLVM version
will generally fail to compile with a link-time error, rather than cause runtime
errors. Where versions are known to break binary compatibility, the build script
will prevent compilation.

Depending on your use of the C API, your program may require that only a
version of LLVM exactly matching your crate version be allowed. This can be set
with the cargo feature flag `strict-versioning` or by setting the environment
variable `LLVM_SYS_<version>_STRICT_VERSIONING` (where `<version>` is the target
crate version) to any value.

llvm-sys blocklists some versions of LLVM that are known to be
binary-incompatible. If you're feeling lucky, setting
`LLVM_SYS_<version>_IGNORE_BLOCKLIST` to "YES" will permit the use of
blocklisted library versions (which may cause vexing bugs).

---

This crate declares that it links to `llvm-<MAJOR VERSION>`, not just `llvm`.
This makes it possible to declare a crate that depends on multiple
versions of `llvm-sys` (corresponding to different versions of LLVM) as long as
only one of them is actually used:

```toml
llvm-sys-191 = { package = "llvm-sys", version = "191", optional = true }
llvm-sys-201 = { package = "llvm-sys", version = "201", optional = true }
```

This requires that the target LLVM version (`llvm-20` for instance) be declared
as the linking target rather than just `llvm` because Cargo requires that all
linked libraries be unique regardless of what is actually enabled. Note that
although Cargo will not prevent you from enabling multiple versions of LLVM at
once as a result, doing so will likely cause errors at link time.

## Compiling LLVM

If you need to compile LLVM or manage multiple versions,
[llvmenv](https://crates.io/crates/llvmenv) may simplify the process. Consider
using it if you don't have special requirements or previous experience with
LLVM!

While the [getting started guide](http://llvm.org/docs/GettingStarted.html) is
the official guide to compiling LLVM, this section will attempt to provide
minimum guidance in creating usable libraries. If you encounter problems, refer
to the official documentation.

### Download sources

Download and unpack a copy of the source for the required version.

```sh
wget https://github.com/llvm/llvm-project/archive/refs/tags/llvmorg-20.1.0-rc1.tar.gz
tar xJf llvmorg-20.1.0-rc1.tar.xz
```

Note that you do not need to compile Clang or the test suite.

### Configure

Configure the build using [CMake][cmake] (you will need a copy of CMake
installed).

[cmake]: https://cmake.org/

```sh
mkdir -p llvm-3.9.0.src/build
cd llvm-3.9.0.src/build
cmake .. -DCMAKE_INSTALL_PREFIX=$HOME/llvm-3.9.0
```

Some of the useful options that can be specified at configuration-time
(via `-D` passed to CMake):

 * `CMAKE_INSTALL_PREFIX` sets the location to install everything in the install
   step (later). In the above example this is under your home directory.
 * `CMAKE_BUILD_TYPE` specifies the build mode, one of Debug, Release,
   MinSizeRel or RelWithDebInfo. Unless you plan to debug LLVM itself,
   Release or MinSizeRel is probably a good choice.
 * `LLVM_ENABLE_ASSERTIONS` enables internal sanity checks, highly recommended
   when writing and debugging your own program that uses LLVM because it can
   detect many usage errors that could otherwise result in hard-to-debug
   crashes.

Passing `-G <generator>` to CMake will make it use a different build system, but
by default it will select one appropriate to your system. If you have
[ninja][ninja] available, it is recommended due to its speed (`-G Ninja`).

[ninja]: https://ninja-build.org/

### Compile and install

```sh
cmake --build . --target install
```

This will automatically invoke the build system and copy binaries into the
prefix specified at configuration-time when done.
Some build tools (like Visual Studio on Windows) support all configurations
concurrently so you also need to specify the build type (which defaults to Debug
on Windows), adding an option like `--config MinSizeRel` to this invocation of
cmake.

After building and installing via CMake, you can compile llvm-sys against it.

```sh
cd your/crate/path
LLVM_SYS_39_PREFIX=$HOME/llvm-3.9.0 cargo build
```

## Windows

You must use a version of Rust that uses the same compiler as you build LLVM
with, either MSVC or MinGW. Fortunately, a mismatch like this will cause errors
at compile-time when llvm-config provides options which are supported by only
one of them, so if you're using the other it will cause the build to fail.

## Cross-compilation

Will theoretically work, but hasn't been tested. Let us know if you try.
