Merge pull request #9754 from 9999years/print-value-when-coercion-fails

Print the value in `error: cannot coerce` messages

(cherry picked from commit 5b7bfd2d6b89d7dd5f54c1ca6c8072358d31a84e)

===

test taken from 6e8d5983143ae576e3f4b1d2954a5267f2943a49; it was added
previously (and not backported because its pr was a mostly-revert), but
it's useful to have around.

Change-Id: Icbd14b55e3610ce7b774667bf14b82e6dc717982

3 files changed, 21 insertions, 8 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 6d314463b..d3d1eb7f1 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -2279,7 +2279,9 @@ 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>();
         }
@@ -2325,7 +2327,9 @@ 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>();
 }
@@ -2657,7 +2661,7 @@ 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)
     });
 }
 
diff --git a/src/libexpr/print-options.hh b/src/libexpr/print-options.hh
index aba2eaeae..e03746ece 100644
--- a/src/libexpr/print-options.hh
+++ b/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();
     /**
diff --git a/src/libexpr/print.cc b/src/libexpr/print.cc
index 28ca8874c..139568b5e 100644
--- a/src/libexpr/print.cc
+++ b/src/libexpr/print.cc
@@ -18,7 +18,7 @@ void printElided(
 {
     if (ansiColors)
         output << ANSI_FAINT;
-    output << " «";
+    output << "«";
     pluralize(output, value, single, plural);
     output << " elided»";
     if (ansiColors)
@@ -35,7 +35,7 @@ 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;
         }
@@ -159,6 +159,8 @@ private:
     EvalState & state;
     PrintOptions options;
     std::optional<ValuesSeen> seen;
+    size_t attrsPrinted = 0;
+    size_t listItemsPrinted = 0;
 
     void printRepeated()
     {
@@ -277,7 +279,6 @@ 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");
@@ -305,7 +306,6 @@ 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");
@@ -484,6 +484,9 @@ public:
 
     void print(Value & v)
     {
+        attrsPrinted = 0;
+        listItemsPrinted = 0;
+
         if (options.trackRepeated) {
             seen.emplace();
         } else {