testament: error instead of silently ignore invalid targets; remove pointless alias target vs targets; document matrix; DRY

[timotheecour]

• document matrix in testament.rst
• remove code duplication (parseTargets content was duplicated)
• silently ignoring invalid spec doesn't make sense, this PR errors instead
• remove pointless aliasing targets vs target and instead only allow targets which is much more common (and fix tests); helps with searching especially for users not aware that the less common target even existed
• lowercase targets
• followup [cleanup, testament] targets: use cpp instead of c++ everywhere (was by far the most common) #15961 by enforcing it to avoid things like add testcase for #15937 #15933 (comment)

this will do the right thing:

XDG_CONFIG_HOME= nim r -f testament/testament.nim --targets:'js c' r tests/stdlib/thashes.nim

these will fail:

XDG_CONFIG_HOME= nim r -f testament/testament.nim --targets:'js c nonexistant' r tests/stdlib/thashes.nim

or after adding an invalid target in tests/stdlib/thashes.nim:

XDG_CONFIG_HOME= nim r -f testament/testament.nim r tests/stdlib/thashes.nim

[Araq]

These aliases are not "useless", they let me get away with writing specs without constantly looking at the documentation. A world where "target" and "targets" are different is the really confusing world.

I'm merging this anyway, just trying to outline the ideas behind the testament aliases.

[Araq]

Since testament is now an official tool, I added back the aliases "c++" and "target".

[timotheecour]

Since testament is now an official tool, I added back the aliases "c++" and "target".

ok for backward compatiblity argument (but probably a warning would be best in future work);

These aliases are not "useless", they let me get away with writing specs without constantly looking at the documentation. A world where "target" and "targets" are different is the really confusing world.

nim (stdlib etc) rightfully avoids this kind of aliasing usually, and testament seems to be an odd exception; aliases complicate other tasks such as grepping, and lead to people copying their favorite alias and making the problem worse.

I prefer a tool that is strict but also suggests spell corrections, and it's in fact easy to write a generic spellSuggest as a followup after #16067, via a completely generic case statement macro:

spellSuggest:
  case arg
  of "targets": ...
  of "output": ...
  else: raise newException(ValueError, spellCorrect)

which adds spell corrections automatically simply by invoking a generic macro spellSuggest:, and without having to add any logic to existing code beyond invoking spellSuggest:

output:"""
target:"c"
"""

=> invalid spec target: did you mean targets?

This would be generally applicable (nim cmdline etc)

[Araq]

invalid spec target: did you mean targets?
This would be generally applicable (nim cmdline etc)

I don't see this to work well for testament as I usually run batches of tests. However, TabNine is getting so good at this that we don't have to do anything here, not anymore.