Add builtins.toStringDebug

Added `builtins.toStringDebug`, which formats a value as a string for
debugging purposes. Unlike `builtins.toString`, `builtins.toStringDebug`
will never error and will always produce human-readable, pretty-printed
output (including for expressions that error). This makes it ideal for
interpolation into `builtins.trace` calls and `assert` messages.

doc/manual/rl-next/to-string-debug.md

@@ -0,0 +1,10 @@
+---
+synopsis: Add `builtins.toStringDebug`
+prs:
+---
+
+Added `builtins.toStringDebug`, which formats a value as a string for debugging
+purposes. Unlike `builtins.toString`, `builtins.toStringDebug` will never error
+and will always produce human-readable, pretty-printed output (including for
+expressions that error). This makes it ideal for interpolation into
+`builtins.trace` calls and `assert` messages.

src/libexpr/primops.cc

@@ -9,6 +9,7 @@
 #include "json-to-value.hh"
 #include "names.hh"
 #include "path-references.hh"
+#include "print-options.hh"
 #include "store-api.hh"
 #include "util.hh"
 #include "processes.hh"
@@ -1000,7 +1001,7 @@ static void prim_trace(EvalState & state, const PosIdx pos, Value * * args, Valu
     if (args[0]->type() == nString)
         printError("trace: %1%", args[0]->string_view());
     else
-        printError("trace: %1%", ValuePrinter(state, *args[0]));
+        printError("trace: %1%", ValuePrinter(state, *args[0], debugPrintOptions));
     if (evalSettings.builtinsTraceDebugger && state.debugRepl && !state.debugTraces.empty()) {
         const DebugTrace & last = state.debugTraces.front();
         state.runDebugRepl(nullptr, last.env, last.expr);

src/libexpr/primops/toStringDebug.cc

@@ -0,0 +1,32 @@
+#include "primops.hh"
+#include "print-options.hh"
+
+namespace nix {
+
+static void prim_toStringDebug(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    v.mkString(printValue(state, *args[0], debugPrintOptions));
+}
+
+static RegisterPrimOp primop_toStringDebug({
+    .name = "toStringDebug",
+    .args = {"value"},
+    .doc = R"(
+      Format a value as a string for debugging purposes.
+
+      Unlike [`toString`](@docroot@/language/builtins.md#builtins-toString),
+      `toStringDebug` will never error and will always produce human-readable
+      output (including for values that throw errors). For this reason,
+      `toStringDebug` is ideal for interpolation into messages in
+      [`trace`](@docroot@/language/builtins.md#builtins-trace)
+      calls and [`assert`](@docroot@/language/constructs.html#assertions)
+      statements.
+
+      Output will be pretty-printed and include ANSI escape sequences.
+      If the value contains too many values (for instance, more than 32
+      attributes or list items), some values will be elided.
+    )",
+    .fun = prim_toStringDebug,
+});
+
+}

src/libexpr/print-options.hh

@@ -89,4 +89,21 @@ static PrintOptions errorPrintOptions = PrintOptions {
     .maxStringLength = 1024
 };
 
+/**
+ * `PrintOptions` for unknown and therefore potentially large values in
+ * debugging contexts, to avoid printing "too much" output.
+ *
+ * This is like `errorPrintOptions`, but prints more values.
+ */
+static PrintOptions debugPrintOptions = PrintOptions {
+    .ansiColors = true,
+    .force = true,
+    .derivationPaths = true,
+    .maxDepth = 15,
+    .maxAttrs = 32,
+    .maxListItems = 32,
+    .maxStringLength = 1024,
+    .prettyIndent = 2
+};
+
 }

src/libexpr/print.cc

@@ -592,11 +592,24 @@ class Printer
     }
 };
 
+/**
+ * Print the given value to `output`.
+ */
 void printValue(EvalState & state, std::ostream & output, Value & v, PrintOptions options)
 {
     Printer(output, state, options).print(v);
 }
 
+/**
+ * Print the given value to a new string.
+ */
+std::string printValue(EvalState & state, Value & v, PrintOptions options)
+{
+    std::ostringstream output;
+    printValue(state, output, v, options);
+    return output.str();
+}
+
 std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer)
 {
     printValue(printer.state, output, printer.value, printer.options);

src/libexpr/print.hh

@@ -62,6 +62,8 @@ std::ostream & printIdentifier(std::ostream & o, std::string_view s);
 
 void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
 
+std::string printValue(EvalState & state, Value & v, PrintOptions options = PrintOptions {});
+
 /**
  * A partially-applied form of `printValue` which can be formatted using `<<`
  * without allocating an intermediate string.

tests/functional/lang.sh

@@ -26,10 +26,15 @@ expectStderr 1 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext
 expectStderr 1 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello %" (throw "Foo")' | grepQuiet 'Hello %'
 
 nix-instantiate --eval -E 'let x = builtins.trace { x = x; } true; in x' \
-  2>&1 | grepQuiet -E 'trace: { x = «potential infinite recursion»; }'
+  2>&1 | grepQuiet -F "trace: {
+  x = «potential infinite recursion»;
+}"
 
 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»; }'
+  2>&1 | grepQuiet -F "trace: {
+  repeating = «repeated»;
+  tracing = «potential infinite recursion»;
+}"
 
 set +x
 

tests/functional/lang/eval-okay-print.err.exp

@@ -1 +1,3 @@
-trace: [ «thunk» ]
+trace: [
+  2
+]