Print the value in error: cannot coerce messages

This extends the `error: cannot coerce a TYPE to a string` message
to print the value that could not be coerced. This helps with debugging
by making it easier to track down where the value is being produced
from, especially in errors with deep or unhelpful stack traces.

src/libexpr/eval.cc

@@ -2255,7 +2255,9 @@ BackedStringView EvalState::coerceToString(
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
         if (i == v.attrs->end()) {
-            error("cannot coerce %1% to a string", showType(v))
+            error("cannot coerce %1% to a string: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
                 .withTrace(pos, errorCtx)
                 .debugThrow<TypeError>();
         }
@@ -2301,7 +2303,9 @@ BackedStringView EvalState::coerceToString(
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v))
+    error("cannot coerce %1% to a string: %2%",
+          showType(v),
+          ValuePrinter(*this, v, errorPrintOptions))
         .withTrace(pos, errorCtx)
         .debugThrow<TypeError>();
 }
@@ -2661,7 +2665,7 @@ void EvalState::printStatistics()
 std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType())
+        .msg = hintfmt("cannot coerce %1% to a string: %2%", showType(), *this)
     });
 }
 

src/libexpr/print-options.hh

@@ -36,11 +36,17 @@ struct PrintOptions
      */
     size_t maxDepth = std::numeric_limits<size_t>::max();
     /**
-     * Maximum number of attributes in an attribute set to print.
+     * Maximum number of attributes in attribute sets to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * attribute set encountered.
      */
     size_t maxAttrs = std::numeric_limits<size_t>::max();
     /**
      * Maximum number of list items to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * list encountered.
      */
     size_t maxListItems = std::numeric_limits<size_t>::max();
     /**

src/libexpr/print.cc

@@ -20,7 +20,7 @@ void printElided(
 {
     if (ansiColors)
         output << ANSI_FAINT;
-    output << " «";
+    output << "«";
     pluralize(output, value, single, plural);
     output << " elided»";
     if (ansiColors)
@@ -37,7 +37,7 @@ printLiteralString(std::ostream & str, const std::string_view string, size_t max
     str << "\"";
     for (auto i = string.begin(); i != string.end(); ++i) {
         if (charsPrinted >= maxLength) {
-            str << "\"";
+            str << "\" ";
             printElided(str, string.length() - charsPrinted, "byte", "bytes", ansiColors);
             return str;
         }
@@ -161,6 +161,8 @@ private:
     EvalState & state;
     PrintOptions options;
     std::optional<ValuesSeen> seen;
+    size_t attrsPrinted = 0;
+    size_t listItemsPrinted = 0;
 
     void printRepeated()
     {
@@ -279,7 +281,6 @@ private:
             else
                 std::sort(sorted.begin(), sorted.end(), ImportantFirstAttrNameCmp());
 
-            size_t attrsPrinted = 0;
             for (auto & i : sorted) {
                 if (attrsPrinted >= options.maxAttrs) {
                     printElided(sorted.size() - attrsPrinted, "attribute", "attributes");
@@ -307,7 +308,6 @@ private:
 
         output << "[ ";
         if (depth < options.maxDepth) {
-            size_t listItemsPrinted = 0;
             for (auto elem : v.listItems()) {
                 if (listItemsPrinted >= options.maxListItems) {
                     printElided(v.listSize() - listItemsPrinted, "item", "items");
@@ -486,6 +486,9 @@ public:
 
     void print(Value & v)
     {
+        attrsPrinted = 0;
+        listItemsPrinted = 0;
+
         if (options.trackRepeated) {
             seen.emplace();
         } else {