new option: debugger-on-warn

Co-authored-by: Emilia Bopp <contact@ebopp.de>

Change-Id: I5e20304538dd3e27fb36c32c0a6133cd6a6a6964

doc/manual/rl-next/builtins-warn.md

@@ -9,3 +9,5 @@ Lix now has a builtin function for emitting warnings.
 Like `builtins.trace`, it takes two arguments: the message to emit, and the expression to return.
 _Unlike_ `builtins.trace`, `builtins.warn` requires the first argument — the message — to be a string.
 In the future we may extend `builtins.warn` to accept a more structured API.
+
+To go along with this, we also have a new config setting [`debugger-on-warn`](@docroot@/command-ref/conf-file.md#conf-debugger-on-warn), which, when used with `--debugger`, makes `builtins.warn` also function like [`builtins.break`](@docroot@/language/builtins.md#builtins-break).

lix/libexpr/builtins/warn.md

@@ -4,3 +4,4 @@ args: [msg, e2]
 ---
 Evaluate string *msg* and print it on standard error. Then return *e2*.
 This function is useful for warning about unexpected conditions without aborting evaluation.
+If the [`debugger-on-warn`](@docroot@/command-ref/conf-file.md#conf-debugger-on-trace) 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)).

lix/libexpr/meson.build

@@ -19,6 +19,7 @@ libexpr_setting_definitions = files(
   'settings/allow-unsafe-native-code-during-evaluation.md',
   'settings/allowed-uris.md',
   'settings/debugger-on-trace.md',
+  'settings/debugger-on-warn.md',
   'settings/eval-cache.md',
   'settings/eval-system.md',
   'settings/ignore-try.md',

lix/libexpr/primops.cc

@@ -779,6 +779,17 @@ static void prim_warn(EvalState & state, Value ** args, Value & v)
 
     printTaggedWarning("%s", Uncolored(msg));
 
+    if (evalSettings.debuggerOnWarn) {
+        if (auto const trace = state.ctx.nextDebugTrace()) {
+            auto const error = EvalError(ErrorInfo{
+                .level = lvlWarn,
+                .msg = HintFmt("builtins.warn reached"),
+            });
+
+            state.ctx.debug->onEvalError(&error, (*trace)->env, (*trace)->expr);
+        }
+    }
+
     state.forceValue(*args[1], noPos);
     v = *args[1];
 }

lix/libexpr/settings/debugger-on-warn.md

@@ -0,0 +1,9 @@
+---
+name: debugger-on-warn
+internalName: debuggerOnWarn
+type: bool
+default: false
+---
+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).

tests/functional/repl_characterization/data/builtins_warn.test

@@ -0,0 +1,12 @@
+@args --debugger --option debugger-on-warn true
+Enter debugger with debugger-on-warn set.
+
+  nix-repl> let inspect = v: builtins.warn "inspect is deprecated" (throw "this happens after"); in inspect { }
+
+  warning: inspect is deprecated!
+  warning: builtins.warn reached!
+
+  nix-repl> :c
+
+  { }
+  error: this happens after

tests/functional/repl_characterization/repl_characterization.cc

@@ -177,6 +177,7 @@ TEST_F(ReplSessionTest, tidy)
     }
 
 REPL_TEST(basic_repl);
+REPL_TEST(builtins_warn)
 REPL_TEST(no_nested_debuggers);
 REPL_TEST(regression_9917);
 REPL_TEST(regression_9918);