Fix a bug in which the visibility of a use declaration defining a name in one namespace (e.g. the value namespace) is overridden by a later use declaration defining the same name in the other namespace (e.g. the type namespace).

tests & rustdoc still broken

rustdoc still broken

to do "in-situ" visits.

Change build_reduced_graph.rs so the fields def and module of NsDef are never both Some unless the NsDef represents a duplicate definition (see issue 26421).

Define a newtype `NameBinding` for `Rc<RefCell<Option<NsDef>>>` and refactor `NameBindings` to be a `NameBinding` for each namespace.

Replace uses of `NameBindings` with `NameBinding` where only one binding is being used (in `NamespaceResult`, `Target,` etc).

Refactor away `resolve_definition_of_name_in_module` and `NameDefinition`.

And use VariantData instead of P<VariantData> in Item_ and Variant_

Currently it is possible to do the following:

 - define a module named `Foo` and then a unit or tuple struct also named `Foo`
 - define any struct named `Foo` and then a module named `Foo`

This commit introduces a warning for both of these cases.

paths, and construct paths for all definitions. Also, stop rewriting
DefIds for closures, and instead just load the closure data from
the original def-id, which may be in another crate.

they are being used as an opaque "position identifier"

PathSegment, PatIdent, ExprWhile, ExprLoop, ExprBreak and ExprAgain - they need Idents for resolve

This could be a [breaking-change] if your lint or syntax extension (is that even possible?) uses HIR attributes or literals.

Fixes #22968
Probably fixes #27602