Lint level control

[epage]

Sometimes it's useful to be able to change the alert level of specific lints for a particular item or an entire module. Clippy and rustc lints have already stabilized syntax like #[deny(lint_name)] and #[allow(lint_name)], and our syntax choices would ideally be reasonably compatible with those.

The kinds of controls we'd like to include are:

• Forbid: change not allowed regardless of version bump
• Major change: only allowed in major version bumps
• Minor change: only allowed in minor version bumps
• Informational: allowed in all version bumps, but nevertheless reported; for example, if human judgment is necessary to determine whether the change is concerning or not
• Ignored: not reported at all

The lint level control adjustments should allow both raising and lowering the level of a given lint. For example:

• semver-minor lints should be possible to be upgraded to semver-major
• semver-major lints should be possible to be downgraded to semver-minor
• it should be possible to annotate an entire module as downgrading all semver lints to ignored / informational, e.g. in macro-only or explicitly-unstable modules.