Cargo fetches submodules of git dependencies each time you run cargo update

[jyn514]

Problem

Cargo updates submodules much more frequently than it needs to; I think it's cloning the submodule fresh, when it could just reuse the submodule from the last update.

Steps

1. cargo new cargo-submodule-weirdness && cd cargo-submodule-weirdness
2. Paste the following into Cargo.toml:

[dependencies]
boring = { git = "https://github.com/cloudflare/boring", branch = "master" }
boring-sys = { git = "https://github.com/cloudflare/boring", branch = "master" }
hyper-boring = { git = "https://github.com/cloudflare/boring", branch = "master" }
tokio-boring = { git = "https://github.com/cloudflare/boring", branch = "master" }

3. cargo update -p boring --precise 44426292e061ec7030b4e049528c876bba26deba
4. cargo update -p boring --precise c85bf0940e13e92e085816718c6ae97fda8ca1aa

Note that cargo clones https://github.com/google/boringssl.git anew each time, even though it doesn't need to.

Possible Solution(s)

Reuse the cloned submodule between commits.

Notes

Log output for a --precise hash that cargo hasn't seen before:

[2022-01-10T23:22:55Z DEBUG cargo::core::workspace] find_root - trying /home/jnelson/rust-community/Cargo.toml
[2022-01-10T23:22:55Z DEBUG cargo::core::workspace] find_root - trying /home/jnelson/Cargo.toml
[2022-01-10T23:22:55Z DEBUG cargo::core::workspace] find_root - trying /home/Cargo.toml
[2022-01-10T23:22:55Z DEBUG cargo::core::workspace] find_root - trying /Cargo.toml
[2022-01-10T23:22:55Z DEBUG cargo::core::workspace] find_members - only me as a member
[2022-01-10T23:22:55Z DEBUG cargo::core::registry] load/missing  https://github.com/cloudflare/boring?branch=master#c4e1f474
[2022-01-10T23:22:55Z DEBUG cargo::core::registry] loading source https://github.com/cloudflare/boring?branch=master#c4e1f474
[2022-01-10T23:22:55Z DEBUG cargo::sources::config] loading: https://github.com/cloudflare/boring?branch=master#c4e1f474
[2022-01-10T23:22:55Z INFO  cargo::sources::git::utils] update submodules for: "/home/jnelson/.local/lib/cargo/git/checkouts/boring-2cc70a61acb7a8a8/c4e1f47/"
    Updating git submodule `https://github.com/google/boringssl.git`
[2022-01-10T23:22:55Z DEBUG cargo::sources::git::utils] can't use github fast path with `rev = "067cfd92f4d7da0edfa073b096d090b98a83b860"`
[2022-01-10T23:22:55Z DEBUG cargo::sources::git::utils] skipping gc as there's only 0 pack files
[2022-01-10T23:22:55Z DEBUG cargo::sources::git::utils] doing a fetch for https://github.com/google/boringssl.git
[2022-01-10T23:22:55Z DEBUG cargo::sources::git::utils] initiating fetch of ["+refs/heads/*:refs/remotes/origin/*", "+HEAD:refs/remotes/origin/HEAD"] from https://github.com/google/boringssl.git

and for a cached hash:

[2022-01-10T23:23:50Z DEBUG cargo::core::workspace] find_root - trying /home/jnelson/rust-community/Cargo.toml
[2022-01-10T23:23:50Z DEBUG cargo::core::workspace] find_root - trying /home/jnelson/Cargo.toml
[2022-01-10T23:23:50Z DEBUG cargo::core::workspace] find_root - trying /home/Cargo.toml
[2022-01-10T23:23:50Z DEBUG cargo::core::workspace] find_root - trying /Cargo.toml
[2022-01-10T23:23:50Z DEBUG cargo::core::workspace] find_members - only me as a member
[2022-01-10T23:23:50Z DEBUG cargo::core::registry] load/missing  https://github.com/cloudflare/boring?branch=master#c85bf094
[2022-01-10T23:23:50Z DEBUG cargo::core::registry] loading source https://github.com/cloudflare/boring?branch=master#c85bf094
[2022-01-10T23:23:50Z DEBUG cargo::sources::config] loading: https://github.com/cloudflare/boring?branch=master#c85bf094
[2022-01-10T23:23:50Z INFO  cargo::sources::git::utils] update submodules for: "/home/jnelson/.local/lib/cargo/git/checkouts/boring-2cc70a61acb7a8a8/c85bf09/"
[2022-01-10T23:23:50Z INFO  cargo::sources::git::utils] update submodules for: "/home/jnelson/.local/lib/cargo/git/checkouts/boring-2cc70a61acb7a8a8/c85bf09/boring-sys/deps/boringssl/"
[2022-01-10T23:23:50Z DEBUG cargo::ops::resolve] avoid_patch_ids={}

Version

cargo 1.59.0-nightly (fcef61230 2021-12-17)
release: 1.59.0-nightly
commit-hash: fcef61230c3b6213b6b0d233a36ba4ebd1649ec3
commit-date: 2021-12-17
host: x86_64-unknown-linux-gnu
libgit2: 1.3.0 (sys:0.13.23 vendored)
libcurl: 7.80.0-DEV (sys:0.4.51+curl-7.80.0 vendored ssl:OpenSSL/1.1.1l)
os: Pop!_OS 20.04 (focal) [64-bit]

[ehuss]

Yea, unfortunately cargo currently doesn't handle submodules well. I'm going to close as a duplicate of #7987, which contains a little more background information.