use get_diagnostic_item in even more places

[ada4a]

continuation of #15519, see there for the commit structure

AFAICT this finally resolves #7784

changelog: none

[rustbot]

r? @blyxyas

rustbot has assigned @blyxyas.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[ada4a]

While working on this, I've noticed two common patterns:

1. if-else-chains with repeated is_diagnostic_item checks:

   if cx.tcx.is_diagnostic_item(sym::ptr_null, method_defid) {
       OmitFollowedCastReason::Null(qpath)
   } else if cx.tcx.is_diagnostic_item(sym::ptr_null_mut, method_defid) {
       OmitFollowedCastReason::NullMut(qpath)
   } else {
       OmitFollowedCastReason::None

   which can be replaced with a match:

    match cx.tcx.get_diagnostic_name(method_defid) {
        Some(sym::ptr_null) => OmitFollowedCastReason::Null(qpath),
        Some(sym::ptr_null_mut) => OmitFollowedCastReason::NullMut(qpath),
        _ => OmitFollowedCastReason::None,
    }

2. ||-d is_diagnostic_item checks:

   cx.tcx.is_diagnostic_item(sym::cmp_ord_min, id) || cx.tcx.is_diagnostic_item(sym::cmp_ord_max, id)

   which I guess can be replaced with this (recommended in Use get_diagnostic_name #7784):

   matches!(cx.tcx.get_diagnostic_name(id), Some(sym::cmp_ord_min | sym::cmp_ord_max))

   or, given that these usually appear in let-chains, with something like this:

   if let Some(name) = cx.tcx.get_diagnostic_name(id)
   && matches!(name, sym::cmp_ord_min | sym::cmp_ord_max)

   which I personally like better and is what I used throughout the PRs, though you can of course disagree

And I wonder whether I should try making this into internal lints, even if without the suggestions at first?

[samueltardieu]

And I wonder whether I should try making this into internal lints, even if without the suggestions at first?

Why not? I also systematically suggest using the more efficient form when I do reviews, but I didn't go through the existing code. If this is not too much work, you might attempt it.