Add --arg-from-file for reading a string from a file

src/libcmd/common-eval-args.cc

@@ -31,6 +31,15 @@ MixEvalArgs::MixEvalArgs()
         .handler = {[&](std::string name, std::string s) { autoArgs.insert_or_assign(name, AutoArg{AutoArgString(s)}); }},
     });
 
+    addFlag({
+        .longName = "arg-from-file",
+        .description = "Pass the contents of file *path* as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name", "path"},
+        .handler = {[&](std::string name, std::string path) { autoArgs.insert_or_assign(name, AutoArg{AutoArgFile(path)}); }},
+        .completer = completePath
+    });
+
     addFlag({
         .longName = "include",
         .shortName = 'I',
@@ -162,6 +171,9 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
             },
             [&](const AutoArgString & arg) {
                 v->mkString(arg.s);
+            },
+            [&](const AutoArgFile & arg) {
+                v->mkString(readFile(arg.path));
             }
         }, arg);
         res.insert(state.symbols.create(name), v);

src/libcmd/common-eval-args.hh

@@ -28,8 +28,9 @@ struct MixEvalArgs : virtual Args, virtual MixRepair
 private:
     struct AutoArgExpr { std::string expr; };
     struct AutoArgString { std::string s; };
+    struct AutoArgFile { std::filesystem::path path; };
 
-    using AutoArg = std::variant<AutoArgExpr, AutoArgString>;
+    using AutoArg = std::variant<AutoArgExpr, AutoArgString, AutoArgFile>;
 
     std::map<std::string, AutoArg> autoArgs;
 };

tests/functional/eval.sh

@@ -41,3 +41,11 @@ mkdir -p $TEST_ROOT/xyzzy $TEST_ROOT/foo
 ln -sfn ../xyzzy $TEST_ROOT/foo/bar
 printf 123 > $TEST_ROOT/xyzzy/default.nix
 [[ $(nix eval --impure --expr "import $TEST_ROOT/foo/bar") = 123 ]]
+
+# Test --arg-from-file.
+[[ "$(nix eval --raw --arg-from-file foo config.nix --expr '{ foo }: { inherit foo; }' foo)" = "$(cat config.nix)" ]]
+
+# Check that special(-ish) files are drained.
+if [[ -e /proc/version ]]; then
+    [[ "$(nix eval --raw --arg-from-file foo /proc/version --expr '{ foo }: { inherit foo; }' foo)" = "$(cat /proc/version)" ]]
+fi