--show-iface: Qualify all non-local names

Summary:
In order to disambiguate names from different modules, qualify all names
that don't originate in the current module.

Also update docs for QueryQualifyName

Test Plan: validate

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter, tdammers

GHC Trac Issues: #15269

Differential Revision: https://phabricator.haskell.org/D4852

2 files changed, 19 insertions, 7 deletions

diff --git a/compiler/iface/LoadIface.hs b/compiler/iface/LoadIface.hs
index 4524402985..34ba1cbb7a 100644
--- a/compiler/iface/LoadIface.hs
+++ b/compiler/iface/LoadIface.hs
@@ -1047,6 +1047,15 @@ ifaceStats eps
                 Printing interfaces
 *                                                                      *
 ************************************************************************
+
+Note [Name qualification with --show-iface]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to disambiguate between identifiers from different modules, we qualify
+all names that don't originate in the current module. In order to keep visual
+noise as low as possible, we keep local names unqualified.
+
+For some background on this choice see trac #15269.
 -}
 
 -- | Read binary interface, and print it out
@@ -1057,8 +1066,15 @@ showIface hsc_env filename = do
    iface <- initTcRnIf 's' hsc_env () () $
        readBinIface IgnoreHiWay TraceBinIFaceReading filename
    let dflags = hsc_dflags hsc_env
+       -- See Note [Name qualification with --show-iface]
+       qualifyImportedNames mod _
+           | mod == mi_module iface = NameUnqual
+           | otherwise              = NameNotInScope1
+       print_unqual = QueryQualify qualifyImportedNames
+                                   neverQualifyModules
+                                   neverQualifyPackages
    putLogMsg dflags NoReason SevDump noSrcSpan
-      (defaultDumpStyle dflags) (pprModIface iface)
+      (mkDumpStyle dflags print_unqual) (pprModIface iface)
 
 -- Show a ModIface but don't display details; suitable for ModIfaces stored in
 -- the EPT.
diff --git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index 4df9adfe84..929c7f3d58 100644
--- a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ -181,12 +181,8 @@ data PrintUnqualified = QueryQualify {
     queryQualifyPackage :: QueryQualifyPackage
 }
 
--- | given an /original/ name, this function tells you which module
--- name it should be qualified with when printing for the user, if
--- any.  For example, given @Control.Exception.catch@, which is in scope
--- as @Exception.catch@, this function will return @Just "Exception"@.
--- Note that the return value is a ModuleName, not a Module, because
--- in source code, names are qualified by ModuleNames.
+-- | Given a `Name`'s `Module` and `OccName`, decide whether and how to qualify
+-- it.
 type QueryQualifyName = Module -> OccName -> QualifyName
 
 -- | For a given module, we need to know whether to print it with