Function diag_lint_level 

pub fn diag_lint_level<'a, D: Diagnostic<'a, ()> + 'a>(
    sess: &'a Session,
    lint: &'static Lint,
    level: LevelAndSource,
    span: Option<MultiSpan>,
    decorate: D,
)

The innermost function for emitting lints implementing the Diagnostic trait.

If you are looking to implement a lint, look for higher level functions, for example:

• TyCtxt::emit_node_span_lint
• TyCtxt::node_span_lint
• TyCtxt::node_lint
• LintContext::opt_span_lint

This function will replace lint_level once all its callers have been replaced with diag_lint_level.