Add documentation, rename to debugger-on-trace

doc/manual/rl-next/debugger-on-trace.md

@@ -1,9 +1,9 @@
 ---
-synopsis: Enter the `--debugger` when `builtins.trace` is called if `builtins-trace-debugger` is set
+synopsis: Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set
 prs: 9914
 ---
 
-If the `builtins-trace-debugger` option is set and `--debugger` is given,
+If the `debugger-on-trace` option is set and `--debugger` is given,
 `builtins.trace` calls will behave similarly to `builtins.break` and will enter
 the debug REPL. This is useful for determining where warnings are being emitted
 from.

src/libexpr/eval-settings.hh

@@ -128,8 +128,15 @@ struct EvalSettings : Config
     Setting<unsigned int> maxCallDepth{this, 10000, "max-call-depth",
         "The maximum function call depth to allow before erroring."};
 
-    Setting<bool> builtinsTraceDebugger{this, false, "builtins-trace-debugger",
+    Setting<bool> builtinsTraceDebugger{this, false, "debugger-on-trace",
-        "Whether to enter the debugger on `builtins.trace` calls."};
+        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).
+
+          This is useful for debugging warnings in third-party Nix code.
+        )"};
 };
 
 extern EvalSettings evalSettings;

src/libexpr/primops.cc

@@ -1010,6 +1010,12 @@ static RegisterPrimOp primop_trace({
       Evaluate *e1* and print its abstract syntax representation on
       standard error. Then return *e2*. This function is useful for
       debugging.
+
+      If the
+      [`debugger-on-trace`](@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 `trace` is called (like
+      [`break`](@docroot@/language/builtins.md#builtins-break)).
     )",
     .fun = prim_trace,
 });