Something wrong in pg_hba.conf this my configuration:

#postgres.conf:

listen_addresses = 'localhost'

#pg_hba.conf:

local all postgres md5
local all appuser trust
local all devuser trust
local all all peer


i'm connecting in local using the C-Library libpq. 
My connection string is: 

host=localhost user=appuser dbname=mydbname port='5432'

However, unless I have a .pgpass file in the home of my developer user (devuser), (the one who launches the application that is linked with libpq), my executable cannot connect, I receive the error fe_sendauth : no password supplied

As I said, if the .pgpass file is here, then it connects ok. 

What I don't understand: I configured local all appuser trust. 
Why do I need a .pgpass file all the same?

(It's not a serious problem in itself, because later on for security measure, it will no longer be at trust, but md5, so I'll have the .pgpass file anyway. But I'd like to know)
I can see how one might think listen_addresses = 'localhost' is somehow related to the authentication method local. But it is not. I quote the manual on listen_addresses:


  listen_addresses (string)
  
  Specifies the TCP/IP address(es) on which the server is to listen for connections from client applications.


And the chapter The pg_hba.conf File in the manual:


  local
  
  This record matches connection attempts using Unix-domain sockets. Without a record of this type, Unix-domain socket connections are
  disallowed.


Bold emphasis mine. 

For connections via localhost (TCP/IP, local loopback) you actually need an entry for the authentication method host in your pg_hba.conf file (not for local).