The user's hooks may try to enable yas-minor-mode before yasnippet.el
has finished loading. In [1], we catch and ignore the errors arising
from this, but it's better if we can avoid hitting the error in the
first place.
This means we have 3 separate measures to avoid the problem, but since
this bug can leave Emacs unusable due to a failing post-command-hook,
a bit of redundant safety is not a bad thing.
* yasnippet.el (yas-minor-mode): Only turn on if yasnippet is fully
loaded.
(yas--font-lock-keywords): Disable user hooks.
[1]: "Avoid potential problems with user's hooks"
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