nested nonmergeable lists from config cli got merged with file configs

[weihanglo]

Problem

From my observation, this only happens to nested non-mergeable list like registries.*.credential-provider. It doesn't happen to doc.browser (see #16208).

Reproducible example

#[cargo_test]
fn nonmergeable_lists() {
    let cli_arg = "registries.example.credential-provider=['cli', 'cli-arg']";
    let gctx = GlobalContextBuilder::new()
        .config_arg(cli_arg)
        .cwd("foo")
        .build();
    let provider = gctx
        .get::<Option<RegistryConfig>>("registries.example")
        .unwrap()
        .unwrap()
        .credential_provider
        .unwrap();
    assert_eq!(provider.path.raw_value(), "cli");
    assert_eq!(provider.args, ["cli-arg"]);

    // Now create a file-based config
    write_config_at(
        &paths::root().join("foo/.cargo/config.toml"),
        "\
[registries.example]
credential-provider = ['c', 'd']
",
    );

    let gctx = GlobalContextBuilder::new()
        .config_arg(cli_arg)
        .cwd("foo")
        .build();
    let provider = gctx
        .get::<Option<RegistryConfig>>("registries.example")
        .unwrap()
        .unwrap()
        .credential_provider
        .unwrap();
    // This seems wrong and shouldn't merge
    assert_eq!(provider.path.raw_value(), "c");
    assert_eq!(provider.args, ["d", "cli", "cli-arg"]);
}

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

latest master: 0f14d9d

I haven't checked if it was buggy since #15066 or it is a recent regression.

[weihanglo]

This seems only happen to non-mergeable list, as this test passes for normal list

#[cargo_test]
fn merges_array() {
    // Array entries are appended.
    write_config_toml(
        "
        [build.nested]
        rustflags = ['--file']
        ",
    );
    let gctx = GlobalContextBuilder::new()
        .config_arg("build.nested.rustflags = ['--cli']")
        .build();
    assert_eq!(
        gctx.get::<Vec<String>>("build.nested.rustflags").unwrap(),
        ["--file", "--cli"]
    );

    // With normal env.
    let gctx = GlobalContextBuilder::new()
        .env("CARGO_BUILD_NESTED_RUSTFLAGS", "--env1 --env2")
        .config_arg("build.nested.rustflags = ['--cli']")
        .build();
    assert_eq!(
        gctx.get::<Vec<String>>("build.nested.rustflags").unwrap(),
        ["--file", "--env1", "--env2", "--cli"]
    );

    // With advanced-env.
    let gctx = GlobalContextBuilder::new()
        .unstable_flag("advanced-env")
        .env("CARGO_BUILD_NESTED_RUSTFLAGS", "--env")
        .config_arg("build.nested.rustflags = ['--cli']")
        .build();
    assert_eq!(
        gctx.get::<Vec<String>>("build.nested.rustflags").unwrap(),
        ["--file", "--env", "--cli"]
    );

    // Merges multiple instances.
    let gctx = GlobalContextBuilder::new()
        .config_arg("build.nested.rustflags=['--one']")
        .config_arg("build.nested.rustflags=['--two']")
        .build();
    assert_eq!(
        gctx.get::<Vec<String>>("build.nested.rustflags").unwrap(),
        ["--file", "--one", "--two"]
    );
}