Cannot compile instant for custom target

[Takashiidobe]

Repo to reproduce here: https://github.com/Takashiidobe/rvim/tree/test-building-x86_64-unknown-none

I was trying to compile one of my projects on a generic x86_64 target, which is defined by this target file:

// 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",
  "linker-flavor": "gcc",
  "target-endian": "little",
  "target-pointer-width": "64",
  "target-c-int-width": "32",
  "arch": "x86_64",
  "os": "none",
  "disable-redzone": true,
  "features": "-mmx,-sse,+soft-float",
  "executables": true
}

To do that I ran this command inside of my project to build it:
$ cargo build --target ./x86_64-unknown-none.json

Predictably this outputs an error:

   Compiling cfg-if v1.0.0
   Compiling smallvec v1.7.0
   Compiling scopeguard v1.1.0
   Compiling bitflags v1.3.2
error[E0463]: can't find crate for `core`
  |
  = note: the `x86_64-unknown-none-17810034709879873439` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-unknown-none-17810034709879873439`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `scopeguard` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed

So I need to build the stdlib. I did this by swapping to nightly:
$ rustup default nightly
and then running this:
$ cargo build -Z build-std --target ./x86_64-unknown-none.json

which outputs this:

Compiling compiler_builtins v0.1.49
   Compiling core v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core)
   Compiling libc v0.2.106
   Compiling cc v1.0.69
   Compiling memchr v2.4.1
   Compiling std v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std)
   Compiling parking_lot_core v0.8.5
   Compiling unwind v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/unwind)
   Compiling rustc-std-workspace-core v1.99.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/rustc-std-workspace-core)
   Compiling alloc v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc)
   Compiling cfg-if v0.1.10
   Compiling adler v0.2.3
   Compiling rustc-demangle v0.1.21
   Compiling rustc-std-workspace-alloc v1.99.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/rustc-std-workspace-alloc)
   Compiling panic_unwind v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/panic_unwind)
   Compiling panic_abort v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/panic_abort)
   Compiling gimli v0.25.0
   Compiling object v0.26.2
   Compiling hashbrown v0.11.0
   Compiling std_detect v0.1.5 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/stdarch/crates/std_detect)
   Compiling miniz_oxide v0.4.0
   Compiling addr2line v0.16.0
   Compiling proc_macro v0.0.0 (/Users/takashi/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/proc_macro)
   Compiling cfg-if v1.0.0
   Compiling scopeguard v1.1.0
   Compiling smallvec v1.7.0
   Compiling bitflags v1.3.2
   Compiling unicode-segmentation v1.8.0
   Compiling instant v0.1.12
   Compiling lock_api v0.4.5
error[E0658]: use of unstable library feature 'restricted_std'
  |
  = help: add `#![feature(restricted_std)]` to the crate attributes to enable

For more information about this error, try `rustc --explain E0658`.
error: could not compile `instant` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed

I'm guessing this can be fixed by adding #![feature(restricted_std)] at the crate root for either this project or a dependent, but I'm not sure how this works in practice.

If nightly support and custom targets aren't a priority, feel free to close.
If this is coming from somewhere else, I can move the issue over there.

Thanks :)