Tricky error with `rustc-std-workspace-core` crate.

[crlf0710]

I'm not very sure whether this is a bug or not. But let me write down the reproduction steps. I'm using the newest stable.
Cargo.toml:

[package]
name = "dummy"
version = "0.0.1"
edition = "2018"

[dependencies]
core = { version = "1.0", package = "rustc-std-workspace-core"}

src/lib.rs

#![no_std]

compiler output:

   Compiling dummy v0.1.0 (L:\ThirdParty\rust\dummy)
error[E0433]: failed to resolve: could not find `prelude` in `core`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0433`.
error: Could not compile `dummy`.

To learn more, run the command again with --verbose.

The rustc-std-workspace-core crate here is the one on crates.io. @alexcrichton

[crlf0710]

If i add this to lib.rs:

#[derive(Clone)]
struct S;

There'll be extra error:

error[E0433]: failed to resolve: could not find `clone` in `$crate`
 --> src\lib.rs:3:10
  |
3 | #[derive(Clone)]
  |          ^^^^^ could not find `clone` in `$crate`

[Mark-Simulacrum]

Why are you using core = { version = "1.0", package = "rustc-std-workspace-core"} in Cargo.toml? That should only be for rustc-std deps, it shouldn't generally be used by anyone else.

I believe what you're finding is that you've overridden core crate with what is essentially an empty crate which is not going to contain Clone appropriately...

cc @petrochenkov

[crlf0710]

i'm trying to add these dependencies to the crate winapi. And i'm checking whether these hacks works outside rustbuild. Then i saw this error.
cc #63725

[Mark-Simulacrum]

See e.g. how libc does it for a working example. https://github.com/rust-lang/libc/blob/master/src/lib.rs#L41

You'll only want to replace core with rustc-std-workspace-core if a rustc-dep-of-std feature is set or so

[crlf0710]

https://github.com/unicode-rs/unicode-width/blob/master/Cargo.toml#L22
This also fails when running cargo build --features core.
@alexcrichton Could you also release a new dot version for rustc-std-workspace-core? I'm not entirely sure what the content should be, maybe something like this:

extern crate core;
pub use core::*;

[alexcrichton]

Hm can you explain why it's necessary? The rustc-std-workspace-core crate from crates.io should never actually be compiled, the feature should never be turned on by consumers.

[crlf0710]

I just think it might be a little strange if the most important crates in the ecosystem has an optional dependency that's causing compilation errors when enabled~

Maybe it's not a must. but if the rustc-std-workspace-* crates on crates.io can work as a drop-in replacement for the sysroot corresponding crates, it might be slightly more natural?

[mati865]

@crlf0710 you are not supposed to use it directly or have it enabled by default.
libc is a good example as it pulls this crate only when rustc-std-workspace-core feature is enabled meaning it never gets enabled outside of libstd.

[Mark-Simulacrum]

I'm going to close this as won't fix.