fix: improve error messages in deep_iterable and deep_mapping validators

When inner validators fail, the error message now includes the index/key
of the failing element instead of just the parent attribute name.

Before: "Length of 'x' must be >= 1: 0"
After:  "Length of 'x[1]' must be >= 1: 0"

This makes it clear which element in the iterable/mapping failed validation.

Fixes #1245

Author: JasonOA888

src/attr/validators.py

@@ -343,8 +343,13 @@ def __call__(self, inst, attr, value):
         if self.iterable_validator is not None:
             self.iterable_validator(inst, attr, value)
 
-        for member in value:
-            self.member_validator(inst, attr, member)
+        for idx, member in enumerate(value):
+            try:
+                self.member_validator(inst, attr, member)
+            except (ValueError, TypeError) as e:
+                # Re-raise with context about which member failed
+                msg = str(e).replace(f"'{attr.name}'", f"'{attr.name}[{idx}]'")
+                raise type(e)(msg) from None
 
     def __repr__(self):
         iterable_identifier = (
@@ -399,9 +404,17 @@ def __call__(self, inst, attr, value):
 
         for key in value:
             if self.key_validator is not None:
-                self.key_validator(inst, attr, key)
+                try:
+                    self.key_validator(inst, attr, key)
+                except (ValueError, TypeError) as e:
+                    msg = str(e).replace(f"'{attr.name}'", f"'{attr.name}[key:{key!r}]'")
+                    raise type(e)(msg) from None
             if self.value_validator is not None:
-                self.value_validator(inst, attr, value[key])
+                try:
+                    self.value_validator(inst, attr, value[key])
+                except (ValueError, TypeError) as e:
+                    msg = str(e).replace(f"'{attr.name}'", f"'{attr.name}[{key!r}]'")
+                    raise type(e)(msg) from None
 
     def __repr__(self):
         return f"<deep_mapping validator for objects mapping {self.key_validator!r} to {self.value_validator!r}>"