lint/ty: move fns to avoid abstraction violation
This PR moves `transparent_newtype_field` and `is_zst` to `LateContext` where they are used, rather than being on the `VariantDef` and `TyS` types, hopefully addressing @eddyb's concern [from this comment](https://github.com/rust-lang/rust/pull/74340#discussion_r456534910).
If a symbol name can only be imported from one place for a type, and
as long as it was not glob-imported anywhere in the current crate, we
can trim its printed path and print only the name.
This has wide implications on error messages with types, for example,
shortening `std::vec::Vec` to just `Vec`, as long as there is no other
`Vec` importable anywhere.
This adds a new '-Z trim-diagnostic-paths=false' option to control this
feature.
On the good path, with no diagnosis printed, we should try to avoid
issuing this query, so we need to prevent trimmed_def_paths query on
several cases.
This change also relies on a previous commit that differentiates
between `Debug` and `Display` on various rustc types, where the latter
is trimmed and presented to the user and the former is not.
This commit moves `transparent_newtype_field` and `is_zst` to
`LateContext` where they are used, rather than being on the `VariantDef`
and `TyS` types.
Signed-off-by: David Wood <david@davidtw.co>
