Add --arg-from-stdin to read an argument from stdin

src/libcmd/common-eval-args.cc

@@ -40,6 +40,14 @@ MixEvalArgs::MixEvalArgs()
         .completer = completePath
     });
 
+    addFlag({
+        .longName = "arg-from-stdin",
+        .description = "Pass the contents of stdin as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name"},
+        .handler = {[&](std::string name) { autoArgs.insert_or_assign(name, AutoArg{AutoArgStdin{}}); }},
+    });
+
     addFlag({
         .longName = "include",
         .shortName = 'I',
@@ -174,6 +182,9 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
             },
             [&](const AutoArgFile & arg) {
                 v->mkString(readFile(arg.path));
+            },
+            [&](const AutoArgStdin & arg) {
+                v->mkString(readFile(STDIN_FILENO));
             }
         }, arg);
         res.insert(state.symbols.create(name), v);

src/libcmd/common-eval-args.hh

@@ -29,8 +29,9 @@ private:
     struct AutoArgExpr { std::string expr; };
     struct AutoArgString { std::string s; };
     struct AutoArgFile { std::filesystem::path path; };
+    struct AutoArgStdin { };
 
-    using AutoArg = std::variant<AutoArgExpr, AutoArgString, AutoArgFile>;
+    using AutoArg = std::variant<AutoArgExpr, AutoArgString, AutoArgFile, AutoArgStdin>;
 
     std::map<std::string, AutoArg> autoArgs;
 };

tests/functional/eval.sh

@@ -49,3 +49,6 @@ printf 123 > $TEST_ROOT/xyzzy/default.nix
 if [[ -e /proc/version ]]; then
     [[ "$(nix eval --raw --arg-from-file foo /proc/version --expr '{ foo }: { inherit foo; }' foo)" = "$(cat /proc/version)" ]]
 fi
+
+# Test --arg-from-stdin.
+[[ "$(echo bla | nix eval --raw --arg-from-stdin foo --expr '{ foo }: { inherit foo; }' foo)" = bla ]]