From 70cb2ffaccb91eff6f4afe2552d0784e279f1fe9 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Sat, 3 Jul 2021 14:19:10 +0200
Subject: [PATCH 1/2] libcmd/installables: implement completion for Nix
 expressions passed via `-f`

---
 src/libcmd/installables.cc | 44 ++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 7 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index fe52912cf..49f063334 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -171,14 +171,44 @@ Strings SourceExprCommand::getDefaultFlakeAttrPathPrefixes()
 
 void SourceExprCommand::completeInstallable(std::string_view prefix)
 {
-    if (file) return; // FIXME
+    if (file) {
+        evalSettings.pureEval = false;
+        auto state = getEvalState();
+        Expr *e = state->parseExprFromFile(
+            resolveExprPath(state->checkSourcePath(lookupFileArg(*state, *file)))
+        );
 
-    completeFlakeRefWithFragment(
-        getEvalState(),
-        lockFlags,
-        getDefaultFlakeAttrPathPrefixes(),
-        getDefaultFlakeAttrPaths(),
-        prefix);
+        Value root;
+        state->eval(e, root);
+
+        auto autoArgs = getAutoArgs(*state);
+
+        std::string prefix_ = std::string(prefix);
+        auto sep = prefix_.rfind('.');
+        if (sep != std::string::npos) {
+            prefix_.erase(sep);
+        } else {
+            prefix_ = "";
+        }
+
+        Value &v1(*findAlongAttrPath(*state, prefix_, *autoArgs, root).first);
+        state->forceValue(v1);
+        Value v2;
+        state->autoCallFunction(*autoArgs, v1, v2);
+
+        if (v2.type() == nAttrs) {
+            for (auto & i : *v2.attrs) {
+                completions->add(i.name);
+            }
+        }
+    } else {
+        completeFlakeRefWithFragment(
+            getEvalState(),
+            lockFlags,
+            getDefaultFlakeAttrPathPrefixes(),
+            getDefaultFlakeAttrPaths(),
+            prefix);
+    }
 }
 
 void completeFlakeRefWithFragment(

From 3c5f69bb60f858f471121adaf172edb47628188e Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Mon, 5 Jul 2021 21:37:33 +0200
Subject: [PATCH 2/2] completeInstallable: also match for already typed
 prefixes

---
 src/libcmd/installables.cc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 49f063334..5f263061b 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -185,9 +185,12 @@ void SourceExprCommand::completeInstallable(std::string_view prefix)
 
         std::string prefix_ = std::string(prefix);
         auto sep = prefix_.rfind('.');
+        std::string searchWord;
         if (sep != std::string::npos) {
-            prefix_.erase(sep);
+            searchWord = prefix_.substr(sep, std::string::npos);
+            prefix_ = prefix_.substr(0, sep);
         } else {
+            searchWord = prefix_;
             prefix_ = "";
         }
 
@@ -198,7 +201,10 @@ void SourceExprCommand::completeInstallable(std::string_view prefix)
 
         if (v2.type() == nAttrs) {
             for (auto & i : *v2.attrs) {
-                completions->add(i.name);
+                std::string name = i.name;
+                if (name.find(searchWord) == 0) {
+                    completions->add(i.name);
+                }
             }
         }
     } else {