Constness -> enum Const { Yes(Span), No }

Same idea for `Unsafety` & use new span for better diagnostics.

src/librustdoc/doctree.rs

@@ -203,7 +203,7 @@ pub struct Impl<'hir> {
     pub unsafety: hir::Unsafety,
     pub polarity: hir::ImplPolarity,
     pub defaultness: hir::Defaultness,
-    pub constness: ast::Constness,
+    pub constness: hir::Constness,
     pub generics: &'hir hir::Generics<'hir>,
     pub trait_: &'hir Option<hir::TraitRef<'hir>>,
     pub for_: &'hir hir::Ty<'hir>,