Function span_lint 

pub fn span_lint<T: LintContext>(
    cx: &T,
    lint: &'static Lint,
    sp: impl Into<MultiSpan>,
    msg: impl Into<DiagMessage>,
)

Emit a basic lint message with a msg and a span.

This is the most primitive of our lint emission methods and can be a good way to get a new lint started.

Usually it’s nicer to provide more context for lint messages. Be sure the output is understandable when you use this method.

NOTE: Lint emissions are always bound to a node in the HIR, which is used to determine the lint level. For the span_lint function, the node that was passed into the LintPass::check_* function is used.

If you’re emitting the lint at the span of a different node than the one provided by the LintPass::check_* function, consider using span_lint_hir instead. This is needed for #[allow] and #[expect] attributes to work on the node highlighted in the displayed warning.

If you’re unsure which function you should use, you can test if the #[expect] attribute works where you would expect it to. If it doesn’t, you likely need to use span_lint_hir instead.

Example

error: usage of mem::forget on Drop type
  --> tests/ui/mem_forget.rs:17:5
   |
17 |     std::mem::forget(seven);
   |     ^^^^^^^^^^^^^^^^^^^^^^^