[bug#52296] [PATCH 1/4] gnu: Add rust-memchr-2.3.

[jgart]

On Sun, 05 Dec 2021 14:05:36 +0100 Nicolas Goaziou <mail@nicolasgoaziou.fr> 
wrote:
> Hello,
> 
> jgart via Guix-patches via <guix-patches@gnu.org> writes:
> 
> > * gnu/packages/crates-io.scm (rust-memchr-2.3): New variable.
> 
> We usually don't add a new Rust package for minor version bumps after
> 1.0. The way to go would be to relax version requirements in
> Cargo.toml so we can use rust-memchr-2. See for example
> rust-average-0.13 package.
> 
> The same goes for rust-slog-async-2.5 and rust-tempfile-3.1.

Hi Nicolas,

Thank you for taking the time to review this patch set.

rust-slog and rust-slog-async have the exact same version in the toml file:

```
[dependencies.slog]
version = "~2.5"

[dependencies.slog-async]
version = "~2.5"
```

The versions currently packaged in guix are slog 2.5.2 and slog-async 2.6.0.

I'm stumped on how to differentiate those two using `substitute*`

What do you suggest I do here?