QuasiQuote submode handling produces many warnings

[purcell]

I've just spent hours tracking down prolific errors like the following, which occur only when editing certain Haskell files:

Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 1888
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 2100
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 2100
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 2343
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 2343
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 1119
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 1119
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 1888
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 1888
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 2100
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 2100
Invalid face reference: 1
Invalid face reference: nil
Invalid face reference: 2343
Invalid face reference: nil [2 times]
Invalid face reference: 1
Invalid face reference: nil [4 times]
Invalid face reference: 2343
Invalid face reference: 1

I discovered that the cause is that haskell-syntactic-face-function does not conform with the behaviour expected of a font-lock-syntactic-face-function, because those functions must always return a face name.

Particularly in the case of quasiquoting, where the quasiquote is recognised as a member of haskell-font-lock-quasi-quote-modes, the function hands off propertization to the language mode (e.g. sql-mode) before deliberately returning nil. Indeed, to return anything else overrides the sub-mode's work.

So it seems like we're abusing syntactic faces, and should find another approach. However, I'm not sure of the best solution.

(Observed in both Emacs 25.3 and 26/27 development snapshots: I haven't tested with older versions.)

[dminuoso]

Is there a workaround to suppress these warnings? I've been driven crazy by this, it made it hard to debug an unrelated issue.

[purcell]

Is there a workaround to suppress these warnings? I've been driven crazy by this, it made it hard to debug an unrelated issue.

Yes, this should do the trick:

(setq haskell-font-lock-quasi-quote-modes nil)

[purcell]

I wonder if we should actually just strip out this functionality. Users can always use mmm-mode or similar to handle quasiquoted regions, and that is rather more battle-tested in general.