At the moment, this moves only a single diagnostic, but the idea is
reafactor the rest to use the same pattern. We are going to have a
single file per diagnostic. This file will define diagnostics code,
rendering range and fixes, if any. It'll also have all of the tests.
This is similar to how we deal with assists.
After we refactor all diagnostics to follow this pattern, we'll probably
move them to a new `ide_diagnostics` crate.
Not that we intentionally want to test all diagnostics on this layer,
despite the fact that they are generally emitted in the guts on the
compiler. Diagnostics care to much about the end presentation
details/fixes to be worth-while "unit" testing. So, we'll unit-test only
the primary output of compilation process (types and name res tables),
and will use integrated UI tests for diagnostics.
The idea here is to eventually get rid of `dyn Diagnostic` and
`DiagnosticSink` infrastructure altogether, and just have a `enum
hir::Diagnostic` instead.
The problem with `dyn Diagnostic` is that it is defined in the lowest
level of the stack (hir_expand), but is used by the highest level (ide).
As a first step, we free hir_expand and hir_def from `dyn Diagnostic`
and kick the can up to `hir_ty`, as an intermediate state. The plan is
then to move DiagnosticSink similarly to the hir crate, and, as final
third step, remove its usage from the ide.
One currently unsolved problem is testing. You can notice that the test
which checks precise diagnostic ranges, unresolved_import_in_use_tree,
was moved to the ide layer. Logically, only IDE should have the infra to
render a specific range.
At the same time, the range is determined with the data produced in
hir_def and hir crates, so this layering is rather unfortunate. Working
on hir_def shouldn't require compiling `ide` for testing.
