Remove obsolete private_in_public lint in nightly.

[thenorili]

RFC 2145 is on nightly now, deprecating the public_in_private lint. https://rust-lang.github.io/rfcs/2145-type-privacy.html

public_in_private has been superceded by three new lints. The first two are warn-by-default and the third is allow-by-default. See the excerpt below for some details.

This change skips this lint in nightly in favor of the new warn-by-default lints.

This change revealed a bug in the 'nightly_features' config option for color-spantrace -- it doesn't exist! The lint it was guarding was never turned on. This adds a minimal build script to check for nightly to color-spantrace. Its functionality is tested in the eyre crate, which seems sufficient to me.

The proof this works is that it builds without the warning in nightly : )

After this change there are only two more build warnings for eyre and color-spantrace, the future-incompat dependency warning and the more serious filename collision.

Lint private_interfaces is reported when a type with visibility x is used in primary interface of an item with effective visibility y and x < y. This lint is warn-by-default.

Lint private_bounds is reported when a type or trait with visibility
x is used in secondary interface of an item with effective
visibility y and x < y. This lint is warn-by-default.

Lint unnameable_types is reported when effective visibility of a
type is larger than module in which it can be named, either
directly, or through reexports, or through trivial type aliases
(type X = Y;, no generics on both sides). This lint is
allow-by-default.

Compatibility lint private_in_public is never reported and removed.

[thenorili]

Missed rust-fmt! I have blocked myself from making the mistake again : )

vscode settings.json

    "[rust]": {
        "editor.defaultFormatter": "rust-lang.rust-analyzer",
        "editor.formatOnSave": true,
    },

[yaahc]

looks good to me ^_^