[Bug 885525]

[M-wada]

After fix of Bug 798663(landed on Tb 17.0.2), "Gmail IMAA or not" is
detected by X-GM-EXT-1 capability response, and is saved in
kGmailImapCapability bit of Capability flags, and is used as indicator
of "Gmail IMAP", ad is propaated to s_gmail of prefs.js etc.

As for "forcing [Gmail]/Trash as trash folder", it look done by nsImapIncomingServer::DiscoveryDone() (see bug 829185 comment #11).
Logic of DiscoveryDone() is very simple;
- If Gmail IMAP, force "trash in XLIST reponse" as trash, and clear SpecialUse flag
- If not Gmail IMAP account, respect mail.server.serverN.trash_folder_name.

One of biggest reasons of "forcing [Gmail]/<trash-in-XLIST>" was localized <trash-in-XLIST> by Gmail. Because Gmail/Gmail IMAP changes actual Mbox name to localized name when Display Languge: is changed, and the Display Language: change can be done by user any time, it's very hard to process following variants.
  Trash in en-US Tb,                    Trash in localized Tb,
  [Gmail]/Trash in en-US Tb,            [Gmail]/Trash in localized Tb,  
  localized-Trash in en-US Tb,          localized-Trash in localized Tb,
  localized-[Gmail]/Trash in en-US Tb,  localized-[Gmail]/Trash in localized Tb
Simplest & certain way is "always use trash in X-LIST".

So, "forcing trash in X-LIST" is still mandatory feature.

But, "prohibiting other than trash in X-LIST as trash of Tb" is never
proper action of an mailer who supports IMAP.

So, "forcing trash in X-LIST if Gmail IMAP" should be optional, although
"forcing trash in X-LIST if Gmail IMAP" should be default.

A possible and simple circumvention of this bug is as follows, altouh it can't be final, absolutely correct, sufficient solution.
  new prefs : mail.server.serverN.force_Gmail_Trash_if_is_gmail(deault=true)
  DiscoveryDone() :
    If Gmail IMAP && mail.server.serverN.force_Gmail_Trash_if_is_gmail_true
       Force trash in X-LIST, clear Trash flag of all other folders,
       as currently Tb does
    Else If Gmail IMAP
       Respect mail.server.serverN.trash_folder_name, 
       and clear Trash flag of all other folders
    Else (non Gmail IMAP)
       Do as currently Tb does

However, as I wrote in bug 829185 comment #14, some issues was found in
incomingServer.isGMailServer access and incomingServer.trashFolderName
access from JavaScript. These may produce new problems if
mail.server.serverN.trash_folder_name setting will be used when
is_gmail/isGMailServer is true.

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https://bugs.launchpad.net/bugs/885525

Title:
  Deleting a Gmail Message Always Sends Item to [Gmail]/Trash

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