Some parts of LSP need to use cached diagnostics as sent from the LSP
server unmodified. Rather than fixing invalid line numbers when
diagnostics are first set, fix them when they are displayed to the user
(e.g. in show() or one of the get_next/get_prev family of functions).
* feat(diagnostic): add vim.diagnostic.match()
Provide vim.diagnostic.match() to generate a diagnostic from a string and
a Lua pattern.
* feat(diagnostic): add tolist() and fromlist()
When severity_sort is true, higher severities should be displayed before
lower severities (e.g. ERROR is displayed over WARN).
Also improved the test case for this.
This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).
More discussion at [1].
[1]: https://github.com/neovim/neovim/pull/15585
