Auto merge of rust-lang#148434 - oli-obk:inherent-const-impl, r=fee1-dead

Inherent const impl

Some constifications are annoying because we need to repeat `T: Trait` bounds from an impl block on the individual constified `const fn`s as `T: [const] Trait`. We've brainstormed solutions before, and one would be to have separate `const impl` blocks or sth. However the final syntax will look, I decided to just impl this syntax and either have sth nice on nightly to work with or at least move the discussion along.

Also interacts with the discussion around `impl const Trait for Type` vs `const impl Trait for Type`, as we may want to use the latter to keep inherent and trait impls in sync (unless we come up with even another scheme).

* [ ] rustdoc + tests
* [ ] macro stability /regression tests

r? `@fee1-dead`

cc `@traviscross` `@rust-lang/project-const-traits`

Author: bors

clippy_lints/src/derivable_impls.rs

@@ -232,6 +232,7 @@ impl<'tcx> LateLintPass<'tcx> for DerivableImpls {
             of_trait: Some(of_trait),
             items: [child],
             self_ty,
+            constness,
             ..
         }) = item.kind
             && !cx.tcx.is_automatically_derived(item.owner_id.to_def_id())
@@ -247,7 +248,7 @@ impl<'tcx> LateLintPass<'tcx> for DerivableImpls {
             && !attrs.iter().any(|attr| attr.doc_str().is_some())
             && cx.tcx.hir_attrs(impl_item_hir).is_empty()
         {
-            let is_const = of_trait.constness == hir::Constness::Const;
+            let is_const = constness == hir::Constness::Const;
             if adt_def.is_struct() {
                 check_struct(
                     cx,

clippy_utils/src/ast_utils/mod.rs

@@ -495,20 +495,22 @@ pub fn eq_item_kind(l: &ItemKind, r: &ItemKind) -> bool {
                 of_trait: lot,
                 self_ty: lst,
                 items: li,
+                constness: lc,
             }),
             Impl(ast::Impl {
                 generics: rg,
                 of_trait: rot,
                 self_ty: rst,
                 items: ri,
+                constness: rc,
             }),
         ) => {
             eq_generics(lg, rg)
                 && both(lot.as_deref(), rot.as_deref(), |l, r| {
                     matches!(l.safety, Safety::Default) == matches!(r.safety, Safety::Default)
                         && matches!(l.polarity, ImplPolarity::Positive) == matches!(r.polarity, ImplPolarity::Positive)
                         && eq_defaultness(l.defaultness, r.defaultness)
-                        && matches!(l.constness, ast::Const::No) == matches!(r.constness, ast::Const::No)
+                        && matches!(lc, ast::Const::No) == matches!(rc, ast::Const::No)
                         && eq_path(&l.trait_ref.path, &r.trait_ref.path)
                 })
                 && eq_ty(lst, rst)