lsp-flycheck is rust specific about lsp-mode HOT 4 CLOSED

Documentation:
Syntax checker to use for the current buffer.

If unset or nil, automatically select a suitable syntax checker
from ‘flycheck-checkers’ on every syntax check.

If set to a syntax checker only use this syntax checker and never
select one from ‘flycheck-checkers’ automatically.  The syntax
checker is used regardless of whether it is contained in
‘flycheck-checkers’ or ‘flycheck-disabled-checkers’.  If the
syntax checker is unusable in the current buffer an error is
signaled.