Add builtins.warn

src/libexpr/eval-settings.cc

@@ -48,6 +48,10 @@ EvalSettings::EvalSettings()
 {
     auto var = getEnv("NIX_PATH");
     if (var) nixPath = parseNixPath(*var);
+
+    var = getEnv("NIX_ABORT_ON_WARN");
+    if (var && (var == "1" || var == "yes" || var == "true"))
+        builtinsAbortOnWarn = true;
 }
 
 Strings EvalSettings::getDefaultNixPath()

src/libexpr/eval-settings.hh

@@ -158,13 +158,39 @@ struct EvalSettings : Config
 
     Setting<bool> builtinsTraceDebugger{this, false, "debugger-on-trace",
         R"(
-          If set to true and the `--debugger` flag is given,
-          [`builtins.trace`](@docroot@/language/builtins.md#builtins-trace) will
-          enter the debugger like
-          [`builtins.break`](@docroot@/language/builtins.md#builtins-break).
+          If set to true and the `--debugger` flag is given, the following functions
+          will enter the debugger like [`builtins.break`](@docroot@/language/builtins.md#builtins-break).
+
+          * [`builtins.trace`](@docroot@/language/builtins.md#builtins-trace)
+          * [`builtins.traceVerbose`](@docroot@/language/builtins.md#builtins-traceVerbose)
+            if [`trace-verbose`](#conf-trace-verbose) is set to true.
+          * [`builtins.warn`](@docroot@/language/builtins.md#builtins-warn)
 
           This is useful for debugging warnings in third-party Nix code.
         )"};
+
+    Setting<bool> builtinsDebuggerOnWarn{this, false, "debugger-on-warn",
+        R"(
+          If set to true and the `--debugger` flag is given, [`builtins.warn`](@docroot@/language/builtins.md#builtins-warn)
+          will enter the debugger like [`builtins.break`](@docroot@/language/builtins.md#builtins-break).
+
+          This is useful for debugging warnings in third-party Nix code.
+
+          Use [`debugger-on-trace`](#conf-debugger-on-trace) to also enter the debugger on legacy warnings that are logged with [`builtins.trace`](@docroot@/language/builtins.md#builtins-trace).
+        )"};
+
+    Setting<bool> builtinsAbortOnWarn{this, false, "abort-on-warn",
+        R"(
+          If set to true, [`builtins.warn`](@docroot@/language/builtins.md#builtins-warn) will throw an error when logging a warning.
+
+          This will give you a stack trace that leads to the location of the warning.
+
+          This is useful for finding information about warnings in third-party Nix code when you can not start the interactive debugger, such as when Nix is called from a non-interactive script. See [`debugger-on-warn`](#conf-debugger-on-warn).
+
+          Currently, a stack trace can only be produced when the debugger is enabled, or when evaluation is aborted.
+
+          This option can be enabled by setting `NIX_ABORT_ON_WARN=1` in the environment.
+        )"};
 };
 
 extern EvalSettings evalSettings;

src/libexpr/primops.cc

@@ -1043,6 +1043,43 @@ static RegisterPrimOp primop_trace({
     .fun = prim_trace,
 });
 
+static void prim_warn(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0], pos);
+    if (args[0]->type() == nString)
+        printMsg(lvlWarn, ANSI_WARNING "warning:" ANSI_NORMAL " %1%", args[0]->string_view());
+    else
+        printMsg(lvlWarn, ANSI_WARNING "warning:" ANSI_NORMAL " %1%", ValuePrinter(state, *args[0]));
+
+    if (evalSettings.builtinsAbortOnWarn) {
+        state.error<Abort>("aborting to reveal stack trace of warning, as abort-on-warn is set").debugThrow();
+    }
+    if ((evalSettings.builtinsTraceDebugger || evalSettings.builtinsDebuggerOnWarn) && state.debugRepl && !state.debugTraces.empty()) {
+        const DebugTrace & last = state.debugTraces.front();
+        state.runDebugRepl(nullptr, last.env, last.expr);
+    }
+    state.forceValue(*args[1], pos);
+    v = *args[1];
+}
+
+static RegisterPrimOp primop_warn({
+    .name = "__warn",
+    .args = {"e1", "e2"},
+    .doc = R"(
+      Evaluate *e1*, which must be a string and print iton standard error as a warning.
+      Then return *e2*.
+      This function is useful for non-critical situations where attention is advisable.
+
+      If the
+      [`debugger-on-trace`](@docroot@/command-ref/conf-file.md#conf-debugger-on-trace)
+      or [`debugger-on-warn`](@docroot@/command-ref/conf-file.md#conf-debugger-on-warn)
+      option is set to `true` and the `--debugger` flag is given, the
+      interactive debugger will be started when `warn` is called (like
+      [`break`](@docroot@/language/builtins.md#builtins-break)).
+    )",
+    .fun = prim_warn,
+});
+
 
 /* Takes two arguments and evaluates to the second one. Used as the
  * builtins.traceVerbose implementation when --trace-verbose is not enabled

tests/functional/lang.sh

@@ -36,6 +36,13 @@ nix-instantiate --eval -E 'let x = builtins.trace { x = x; } true; in x' \
 nix-instantiate --eval -E 'let x = { repeating = x; tracing = builtins.trace x true; }; in x.tracing'\
   2>&1 | grepQuiet -F 'trace: { repeating = «repeated»; tracing = «potential infinite recursion»; }'
 
+nix-instantiate --eval -E 'builtins.warn "Hello" 123' 2>&1 | grepQuiet 'warning: Hello'
+nix-instantiate --eval -E 'builtins.addErrorContext "while doing ${"something"} interesting" (builtins.warn "Hello" 123)' 2>/dev/null | grepQuiet 123
+nix-instantiate --eval -E 'let x = builtins.warn { x = x; } true; in x' \
+  2>&1 | grepQuiet -E 'warning: { x = «potential infinite recursion»; }'
+expectStderr 1 nix-instantiate --eval --abort-on-warn -E 'builtins.warn "Hello" 123' | grepQuiet Hello
+NIX_ABORT_ON_WARN=1 expectStderr 1 nix-instantiate --eval -E 'builtins.addErrorContext "while doing ${"something"} interesting" (builtins.warn "Hello" 123)' | grepQuiet "while doing something interesting"
+
 set +x
 
 badDiff=0