与编译器支持有关的笔记
这本书使用了一个内置的编译器目标，thumbv7m-none-eabi，Rust团队为其分发了一个 rust-std 组件，rust-std 是跟 core 和 std 一样的预先编译好的crates的集合。
如果你想尝试为一个不同的目标架构复制这本书的内容，你需要考虑下Rust为(编译)目标所提供的支持在什么级别。
LLVM 支持
自Rust 1.28以来，官方的Rust编译器，rustc，使用LLVM生成(机器)代码。Rust对某个架构提供的最小级别的支持是在rustc中让它的LLVM后端可用。通过运行下面的命令，你可以看到所有的 rustc 通过LLVM支持的架构:
$ # 运行这个命令，你需要安装`cargo-binutils`
$ cargo objdump -- -version
LLVM (http://llvm.org/):
LLVM version 7.0.0svn
Optimized build.
Default target: x86_64-unknown-linux-gnu
Host CPU: skylake
Registered Targets:
aarch64 - AArch64 (little endian)
aarch64_be - AArch64 (big endian)
arm - ARM
arm64 - ARM64 (little endian)
armeb - ARM (big endian)
hexagon - Hexagon
mips - Mips
mips64 - Mips64 [experimental]
mips64el - Mips64el [experimental]
mipsel - Mipsel
msp430 - MSP430 [experimental]
nvptx - NVIDIA PTX 32-bit
nvptx64 - NVIDIA PTX 64-bit
ppc32 - PowerPC 32
ppc64 - PowerPC 64
ppc64le - PowerPC 64 LE
sparc - Sparc
sparcel - Sparc LE
sparcv9 - Sparc V9
systemz - SystemZ
thumb - Thumb
thumbeb - Thumb (big endian)
wasm32 - WebAssembly 32-bit
wasm64 - WebAssembly 64-bit
x86 - 32-bit X86: Pentium-Pro and above
x86-64 - 64-bit X86: EM64T and AMD64
如果LLVM支持了你感兴趣的架构，但是构建的rustc没有使能其后端(自Rust 1.28以来AVR就是这样的情况)，那么你将需要修改Rust源码以启用它。PR rust-lang/rust#52787 的前两个commits能让你知道需要做的改变。
换句话说，如果LLVM不支持这个架构，但是LLVM的一个分支支持，在编译rustc之前你将需要使用这个分支替代原来的LLVM 。Rust编译系统允许这么做且理论上，它应该只需要修改llvm的子模组让它指向那个分支就可以了。
如果只有一些供应商提供的GCC支持你的目标架构，你可以选择使用mrustc，一个非官方的Rust编译器，去把你的Rust程序翻译成C代码然后使用GCC编译它。
内置的目标
一个编译目标不仅是它的架构。每个目标都有一个与它关联的规范，其中描述了它的架构，它的操作系统和默认的链接器。
Rust编译器知道几个目标。这些被内置进编译器里且可以通过下列的命令被列出来:
$ rustc --print target-list | column
aarch64-fuchsia mipsisa32r6el-unknown-linux-gnu
aarch64-linux-android mipsisa64r6-unknown-linux-gnuabi64
aarch64-pc-windows-msvc mipsisa64r6el-unknown-linux-gnuabi64
aarch64-unknown-cloudabi msp430-none-elf
aarch64-unknown-freebsd nvptx64-nvidia-cuda
aarch64-unknown-hermit powerpc-unknown-linux-gnu
aarch64-unknown-linux-gnu powerpc-unknown-linux-gnuspe
aarch64-unknown-linux-musl powerpc-unknown-linux-musl
aarch64-unknown-netbsd powerpc-unknown-netbsd
aarch64-unknown-none powerpc-wrs-vxworks
aarch64-unknown-none-softfloat powerpc-wrs-vxworks-spe
aarch64-unknown-openbsd powerpc64-unknown-freebsd
aarch64-unknown-redox powerpc64-unknown-linux-gnu
aarch64-uwp-windows-msvc powerpc64-unknown-linux-musl
aarch64-wrs-vxworks powerpc64-wrs-vxworks
arm-linux-androideabi powerpc64le-unknown-linux-gnu
arm-unknown-linux-gnueabi powerpc64le-unknown-linux-musl
arm-unknown-linux-gnueabihf riscv32i-unknown-none-elf
arm-unknown-linux-musleabi riscv32imac-unknown-none-elf
arm-unknown-linux-musleabihf riscv32imc-unknown-none-elf
armebv7r-none-eabi riscv64gc-unknown-linux-gnu
armebv7r-none-eabihf riscv64gc-unknown-none-elf
armv4t-unknown-linux-gnueabi riscv64imac-unknown-none-elf
armv5te-unknown-linux-gnueabi s390x-unknown-linux-gnu
armv5te-unknown-linux-musleabi sparc-unknown-linux-gnu
armv6-unknown-freebsd sparc64-unknown-linux-gnu
armv6-unknown-netbsd-eabihf sparc64-unknown-netbsd
armv7-linux-androideabi sparc64-unknown-openbsd
armv7-unknown-cloudabi-eabihf sparcv9-sun-solaris
armv7-unknown-freebsd thumbv6m-none-eabi
armv7-unknown-linux-gnueabi thumbv7a-pc-windows-msvc
armv7-unknown-linux-gnueabihf thumbv7em-none-eabi
armv7-unknown-linux-musleabi thumbv7em-none-eabihf
armv7-unknown-linux-musleabihf thumbv7m-none-eabi
armv7-unknown-netbsd-eabihf thumbv7neon-linux-androideabi
armv7-wrs-vxworks-eabihf thumbv7neon-unknown-linux-gnueabihf
armv7a-none-eabi thumbv7neon-unknown-linux-musleabihf
armv7a-none-eabihf thumbv8m.base-none-eabi
armv7r-none-eabi thumbv8m.main-none-eabi
armv7r-none-eabihf thumbv8m.main-none-eabihf
asmjs-unknown-emscripten wasm32-unknown-emscripten
hexagon-unknown-linux-musl wasm32-unknown-unknown
i586-pc-windows-msvc wasm32-wasi
i586-unknown-linux-gnu x86_64-apple-darwin
i586-unknown-linux-musl x86_64-fortanix-unknown-sgx
i686-apple-darwin x86_64-fuchsia
i686-linux-android x86_64-linux-android
i686-pc-windows-gnu x86_64-linux-kernel
i686-pc-windows-msvc x86_64-pc-solaris
i686-unknown-cloudabi x86_64-pc-windows-gnu
i686-unknown-freebsd x86_64-pc-windows-msvc
i686-unknown-haiku x86_64-rumprun-netbsd
i686-unknown-linux-gnu x86_64-sun-solaris
i686-unknown-linux-musl x86_64-unknown-cloudabi
i686-unknown-netbsd x86_64-unknown-dragonfly
i686-unknown-openbsd x86_64-unknown-freebsd
i686-unknown-uefi x86_64-unknown-haiku
i686-uwp-windows-gnu x86_64-unknown-hermit
i686-uwp-windows-msvc x86_64-unknown-hermit-kernel
i686-wrs-vxworks x86_64-unknown-illumos
mips-unknown-linux-gnu x86_64-unknown-l4re-uclibc
mips-unknown-linux-musl x86_64-unknown-linux-gnu
mips-unknown-linux-uclibc x86_64-unknown-linux-gnux32
mips64-unknown-linux-gnuabi64 x86_64-unknown-linux-musl
mips64-unknown-linux-muslabi64 x86_64-unknown-netbsd
mips64el-unknown-linux-gnuabi64 x86_64-unknown-openbsd
mips64el-unknown-linux-muslabi64 x86_64-unknown-redox
mipsel-unknown-linux-gnu x86_64-unknown-uefi
mipsel-unknown-linux-musl x86_64-uwp-windows-gnu
mipsel-unknown-linux-uclibc x86_64-uwp-windows-msvc
mipsisa32r6-unknown-linux-gnu x86_64-wrs-vxworks
使用下列的命令你可以打印出某个目标的规范:
$ rustc +nightly -Z unstable-options --print target-spec-json --target thumbv7m-none-eabi
{
"abi-blacklist": [
"stdcall",
"fastcall",
"vectorcall",
"thiscall",
"win64",
"sysv64"
],
"arch": "arm",
"data-layout": "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64",
"emit-debug-gdb-scripts": false,
"env": "",
"executables": true,
"is-builtin": true,
"linker": "arm-none-eabi-gcc",
"linker-flavor": "gcc",
"llvm-target": "thumbv7m-none-eabi",
"max-atomic-width": 32,
"os": "none",
"panic-strategy": "abort",
"relocation-model": "static",
"target-c-int-width": "32",
"target-endian": "little",
"target-pointer-width": "32",
"vendor": ""
}
如果这些内置的目标都不适合你的目标系统，你将必须通过使用JSON格式编写你自己的目标规范来生成一个自制的目标，在下个章节中会讲到。
rust-std 组件
Rust团队通过rustup为某些内置的目标分发rust-std组件。这个组件是跟 core 和 std 一样的预先编译好的crates的集合，它要求交叉编译。
通过运行下列的命令，你可以找到具有一个rust-std组件的目标列表:
$ rustup target list | column
aarch64-apple-ios mipsel-unknown-linux-musl
aarch64-fuchsia nvptx64-nvidia-cuda
aarch64-linux-android powerpc-unknown-linux-gnu
aarch64-pc-windows-msvc powerpc64-unknown-linux-gnu
aarch64-unknown-linux-gnu powerpc64le-unknown-linux-gnu
aarch64-unknown-linux-musl riscv32i-unknown-none-elf
aarch64-unknown-none riscv32imac-unknown-none-elf
aarch64-unknown-none-softfloat riscv32imc-unknown-none-elf
arm-linux-androideabi riscv64gc-unknown-linux-gnu
arm-unknown-linux-gnueabi riscv64gc-unknown-none-elf
arm-unknown-linux-gnueabihf riscv64imac-unknown-none-elf
arm-unknown-linux-musleabi s390x-unknown-linux-gnu
arm-unknown-linux-musleabihf sparc64-unknown-linux-gnu
armebv7r-none-eabi sparcv9-sun-solaris
armebv7r-none-eabihf thumbv6m-none-eabi
armv5te-unknown-linux-gnueabi thumbv7em-none-eabi
armv5te-unknown-linux-musleabi thumbv7em-none-eabihf
armv7-linux-androideabi thumbv7m-none-eabi
armv7-unknown-linux-gnueabi thumbv7neon-linux-androideabi
armv7-unknown-linux-gnueabihf thumbv7neon-unknown-linux-gnueabihf
armv7-unknown-linux-musleabi thumbv8m.base-none-eabi
armv7-unknown-linux-musleabihf thumbv8m.main-none-eabi
armv7a-none-eabi thumbv8m.main-none-eabihf
armv7r-none-eabi wasm32-unknown-emscripten
armv7r-none-eabihf wasm32-unknown-unknown
asmjs-unknown-emscripten wasm32-wasi
i586-pc-windows-msvc x86_64-apple-darwin
i586-unknown-linux-gnu x86_64-apple-ios
i586-unknown-linux-musl x86_64-fortanix-unknown-sgx
i686-linux-android x86_64-fuchsia
i686-pc-windows-gnu x86_64-linux-android
i686-pc-windows-msvc x86_64-pc-windows-gnu
i686-unknown-freebsd x86_64-pc-windows-msvc
i686-unknown-linux-gnu x86_64-rumprun-netbsd
i686-unknown-linux-musl x86_64-sun-solaris
mips-unknown-linux-gnu x86_64-unknown-cloudabi
mips-unknown-linux-musl x86_64-unknown-freebsd
mips64-unknown-linux-gnuabi64 x86_64-unknown-linux-gnu (default)
mips64-unknown-linux-muslabi64 x86_64-unknown-linux-gnux32
mips64el-unknown-linux-gnuabi64 x86_64-unknown-linux-musl
mips64el-unknown-linux-muslabi64 x86_64-unknown-netbsd
mipsel-unknown-linux-gnu x86_64-unknown-redox
如果你的目标没有rust-std组件，或者你正在使用一个自制的目标，那么你必须要使用一个开发版的工具链去构建标准库。看下一页，关于构建自制目标。