cargo login with 2nd alternate registry overwrites 1st alternate registry's token in ~/.cargo/credentials

[carols10cents]

Problem

When working on crates.io, I often run an instance locally and interact with it through cargo. We also have a staging instance where I test things out. So this is 2 alternate registries in addition to the main crates.io.

My ~/.cargo/config looks like this:

[registries]
staging = { index = "https://github.com/rust-lang/staging.crates.io-index" }
local = { index = "file:///Users/carolnichols/rust/crates.io/tmp/index-bare" }

If I run cargo login --registry staging [token] then run cargo login --registry local [other-token], ~/.cargo/credentials only contains the credentials for the local registry.

Steps

1. Using rustc 1.39.0 (4560ea788 2019-11-04)/cargo 1.39.0 (1c6ec66 2019-09-30), create a ~/.cargo/config containing 2 alternate registries as shown above.

2. Move your existing ~/.cargo/credentials file elsewhere if you don't want to mess it up reproducing this.

3. Run cargo login abcde to set your token for the main crates.io registry.

4. Observe that ~/.cargo/credentials contains these contents, as expected:

[registry]
token = "abcde"

4. Run cargo login --registry staging fghij.
5. Observe that ~/.cargo/credentials contains these contents, as expected:

[registries.staging]
token = "fghij"

[registry]
token = "abcde"

6. Run cargo login --registry local klmno.
7. Observe that ~/.cargo/credentials contains these contents, with local and the main registry present as expected, but with staging gone:

[registries.local]
token = "klmno"

[registry]
token = "abcde"

The contents I expected to have would contain tokens for all 3 registries, like:

[registries.local]
token = "klmno"

[registries.staging]
token = "fghij"

[registry]
token = "abcde"

Possible Solution(s)

Unclear, I was going to go poking around after filing this.

Notes

Output of cargo version: cargo 1.39.0 (1c6ec66d5 2019-09-30), current stable on macOS.

I was also able to reproduce with rustc 1.41.0-nightly (27d6f55f4 2019-12-11)/cargo 1.41.0-nightly (626f0f40e 2019-12-03).

[giraffate]

When registries is set for key,

cargo/src/cargo/util/config/mod.rs

Lines 1327 to 1330 in 19a0de2

	(
	"registries".into(),
	CV::Table(map, file.path().to_path_buf()),
	)

it seems to overwrite value here.

cargo/src/cargo/util/config/mod.rs

Line 1355 in 19a0de2

toml.as_table_mut().unwrap().insert(key, value.into_toml());

When key registries has the value, it seems better to fix to merge the original value with the new value, but what about?