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

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)
     });
 }