bug#50015: Rust packages are not reproducible

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

Rust packages, which are essentially empty, are not bit-reproducible:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix challenge rust-rocket-codegen --substitute-urls='https://ci.guix.gnu.org https://bordeaux.guix.gnu.org' 
/gnu/store/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7 contents differ:
  no local build for '/gnu/store/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7'
  https://ci.guix.gnu.org/nar/lzip/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7: 0j6zf2afvc49jnp7i6z7yvbxm0bmw8yc65hz3lncgvw5lc6z1bc1
  https://bordeaux.guix.gnu.org/nar/lzip/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7: 015kb637b56mqcsg3f6x1qggm2bybiszji2069gb913wxbj6rs7w
  differing file:
    /share/cargo/registry/rocket_codegen-0.4.7.crate

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive
ludo@ribbon ~/src/guix$ ./pre-inst-env guix challenge rust-rocket-codegen
/gnu/store/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7 contents differ:
  no local build for '/gnu/store/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7'
  https://ci.guix.gnu.org/nar/lzip/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7: 0j6zf2afvc49jnp7i6z7yvbxm0bmw8yc65hz3lncgvw5lc6z1bc1
  https://bordeaux.guix.gnu.org/nar/lzip/09hlac18bwl1kcnhdig7z1v2j8ly1czw-rust-rocket-codegen-0.4.7: 015kb637b56mqcsg3f6x1qggm2bybiszji2069gb913wxbj6rs7w
  differing file:
    /share/cargo/registry/rocket_codegen-0.4.7.crate

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive
$ git log |head -1
commit 973842acbc2d0dc1ab41738a534d4abda6d9efa7
--8<---------------cut here---------------end--------------->8---

The diffoscope output suggests it’s about timestamps on one file in the
.crate archive:

--8<---------------cut here---------------start------------->8---
│ │ │ │   --- /tmp/guix-directory.ii5wmv/share/cargo/registry/rocket_codegen-0.4.7.crate
│ │ │ ├── +++ /tmp/guix-directory.uTTKSw/share/cargo/registry/rocket_codegen-0.4.7.crate
│ │ │ │ ├── rocket_codegen-0.4.7.crate-content
│ │ │ │ │ ├── file list
│ │ │ │ │ │ @@ -1,67 +1,67 @@
│ │ │ │ │ │ --rw-r--r--   0        0        0     1293 2021-07-27 15:22:18.000000 rocket_codegen-0.4.7/Cargo.toml
[…]
│ │ │ │ │ │ +-rw-r--r--   0        0        0     1293 2021-07-27 22:01:49.000000 rocket_codegen-0.4.7/Cargo.toml
--8<---------------cut here---------------end--------------->8---

Does that ring a bell?

Thanks,
Ludo’.

PS: I noticed this via
    <http://data.guix.gnu.org/revision/973842acbc2d0dc1ab41738a534d4abda6d9efa7/package-reproducibility>
    with help from Chris.  Fixing this could noticeably improve our
    stats.  :-)