unknown feature `llvm_asm` when compile rust-src

Question

I tried to compile rust-src using cargo xbuild but get this error:

error[E0635]: unknown feature `llvm_asm`
-> .cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.28/src/lib.rs:3:12
3 | #![feature(llvm_asm)]

How can I fix this error? It's seem like xbuild tries to compile the new rust-src with an old rustc. I want it to also use the old rust-src.

I can't update to a newer rustc version as it results in lots of "R_x86_32 relocation" errors, so I would prefer to use the 2020-03-24 version.

Minimal example

command

cargo new --bin test

rustup component add rust-src

cargo install cargo-xbuild

cd test

ls test
Cargo.toml  rust-toolchain  src  x86_64-unknown-none.json

rust-toolchain

nightly-2020-03-24

x86_64-unknown-none.json

{
  "llvm-target": "x86_64-unknown-none",
  "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
  "arch": "x86_64",
  "target-endian": "little",
  "target-pointer-width": "64",
  "target-c-int-width": "32",
  "os": "none",
  "executables": true,
  "linker-flavor": "ld.lld",
  "linker": "rust-lld",
  "panic-strategy": "abort",
  "disable-redzone": true,
  "features": "-mmx,-sse,+soft-float"
}

src/main.rs

#![no_std]                // don't link the Rust standard library
#![no_main]               // disable all Rust-level entry points
#![allow(non_snake_case)] // disable non snake case name warning

use core::panic::PanicInfo;

#[no_mangle]
pub extern "C" fn _start() -> ! {
    loop {}
}

#[panic_handler]
pub fn MyPacnicHandler(_panicInfo: &PanicInfo) -> ! {
    loop {}
}

compile

cargo xbuild --target x86_64-unknown-none

rustc --version

rustc 1.44.0-nightly (1edd389cc 2020-03-23)

You cannot compile new nightly code with an old nightly compiler — there's no stability guarantees for nightly builds. You need to use new code and a new compiler or old code and an old compiler. You've already eliminated one of those possibilities, leaving you with only the other. — Shepmaster
how can i use old code then? should i download old code from github and copy it to specific folder? — William Taylor
How to specify the exact version of a dependency?; Set specific version of the dependency of a project's dependency in Cargo.toml or Cargo.lock — Shepmaster
in my case, it a component called 'rust-src', not some crate or dependency. — William Taylor
It's hard to answer your question because it doesn't include a minimal reproducible example. We can't tell what crates (and their versions), types, traits, fields, etc. are present in the code. It would make it easier for us to help you if you try to reproduce your error on the Rust Playground if possible, otherwise in a brand new Cargo project, then edit your question to include the additional info. There are Rust-specific MRE tips you can use to reduce your original code for posting here. Thanks! — Shepmaster

sjero sjero · Accepted Answer · 2020-05-13T20:36:13

This is a bug in cargo-xbuild. Basically, cargo xbuild unconditionally fetches the latest compiler_builtins.

A patch has been merged, but is not yet in the latest crates.io release. See this PR: https://github.com/rust-osdev/cargo-xbuild/pull/75/commits/eede1a1d4c08064763f1943c0920de2270260b33

unknown feature `llvm_asm` when compile rust-src

Minimal example

2 Answers