Print the value in error: cannot coerce messages (#9553)

* 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.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libexpr/eval.cc

@@ -26,9 +26,9 @@
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
-#include <iostream>
 #include <fstream>
 #include <functional>
+#include <strstream>
 
 #include <sys/resource.h>
 #include <nlohmann/json.hpp>
@@ -2286,7 +2286,7 @@ 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), printValue(*this, v))
                 .withTrace(pos, errorCtx)
                 .debugThrow<TypeError>();
         }
@@ -2332,7 +2332,7 @@ BackedStringView EvalState::coerceToString(
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v))
+    error("cannot coerce %1% to a string: %2%", showType(v), printValue(*this, v))
         .withTrace(pos, errorCtx)
         .debugThrow<TypeError>();
 }
@@ -2691,8 +2691,10 @@ void EvalState::printStatistics()
 
 std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
+    std::strstream printed;
+    print(printed);
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType())
+        .msg = hintfmt("cannot coerce %1% to a string: %2%", showType(), printed.str())
     });
 }