add -transition=checkimports switch

[WalterBright]

This is added to provide deprecation warnings to ease the transition to the new import lookup method introduced by #5445

[MartinNowak]

I also added this to the changelog, see 3cadde0.
Just out of curiosity, why this change of mind?

[MartinNowak]

Also can we please move on w/ the other import/lookup/module fixes?
#5472
#5485

[9rnsr]

I have a question about the -checkimports switch.

dmd test.d

• Use new lookup algorithm without any information messages.
• It's intentional because Walter hates complation slow down to show messaged for old behavor.

dmd -transition=import test.d

• Use old lookup algorithm without any information messages.
• Not bad.

dmd -transition=import -transition=checkimports test.d

• Use old lookup algorithm with informative deprecation messages.
• not bad.

dmd -transition=checkimports test.d

• Use OLD lookup algorithm with informative deprecation messages.
• Why we cannot use the new algorithm with information? It's inconvenient for code upgrade path.

[9rnsr]

I found another confusing behavior.

dmd -transition=import -transition=checkimports test.d

• -transition=import is overwritten by -transition=checkimports.
• Current -transition=checkimports silently chooses old lookup rule, so it's looked lik being working as exected.

dmd -transition=checkimports -transition=import test.d

• -transition=checkimports is overwritten by -transition=import.
• The information messages is killed by -transition=import, so it's very confusing.

[9rnsr]

OK, I noticed that we cannot get intentional result on this line.

When -transition=checkimports is specified, global.params.bug10378 is set to false, so this Dsymbol.search will return the new lookup algorithm result always.

[9rnsr]

I'm working on:

https://trello.com/c/qmmkxYUs/167-issue-15811-transition-import-and-transition-checkimport-have-oddly-behaviors

[ibuclaw]

Why not make this part of the deprecation command-line switch?

[9rnsr]

Because Walter didn't agree with introducing (little?) compilation speed slowdown to show informative deprecation messages...