Display tweak and docs

Author: Tinche

docs/index.rst

@@ -17,6 +17,7 @@ Contents:
    converters
    structuring
    unstructuring
+   validation
    preconf
    customizing
    unions

docs/validation.rst

@@ -0,0 +1,74 @@
+==========
+Validation
+==========
+
+`cattrs` has a detailed validation mode since version 2022.1.0, and this mode is enabled by default.
+When running under detailed validation, the un/structuring hooks are slightly slower but produce more precise and exhaustive error messages.
+
+Detailed validation
+-------------------
+In detailed validation mode, any un/structuring errors will be grouped and raised together as a ``cattrs.BaseValidationError``, which is a `PEP 654 ExceptionGroup`_.
+ExceptionGroups are special exceptions which contain lists of other exceptions, which may themselves be other ExceptionGroups.
+In essence, ExceptionGroups are trees of exceptions.
+
+When un/structuring a class, `cattrs` will gather any exceptions on a field-by-field basis and raise them as a ``cattrs.ClassValidationError``, which is a subclass of ``BaseValidationError``.
+When structuring sequences and mappings, `cattrs` will gather any exceptions on a key- or index-basis and raise them as a ``cattrs.IterableValidationError``, which is a subclass of ``BaseValidationError``.
+
+The exceptions will also have their ``__note__`` attributes set, as per `PEP 678`_, showing the field, key or index for each inner exception.
+
+A simple example involving a class containing a list and a dictionary:
+
+.. code-block:: python
+
+    @define
+    class Class:
+        a_list: list[int]
+        a_dict: dict[str, int]
+
+    >>> structure({"a_list": ["a"], "a_dict": {"str": "a"}})
+      + Exception Group Traceback (most recent call last):
+      |   File "<stdin>", line 1, in <module>
+      |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 276, in structure
+      |     return self._structure_func.dispatch(cl)(obj, cl)
+      |   File "<cattrs generated structure __main__.Class>", line 14, in structure_Class
+      |     if errors: raise __c_cve('While structuring Class', errors, __cl)
+      | cattrs.errors.ClassValidationError: While structuring Class
+      +-+---------------- 1 ----------------
+        | Exception Group Traceback (most recent call last):
+        |   File "<cattrs generated structure __main__.Class>", line 5, in structure_Class
+        |     res['a_list'] = __c_structure_a_list(o['a_list'], __c_type_a_list)
+        |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 457, in _structure_list
+        |     raise IterableValidationError(
+        | cattrs.errors.IterableValidationError: While structuring list[int]
+        | Structuring class Class @ attribute a_list
+        +-+---------------- 1 ----------------
+          | Traceback (most recent call last):
+          |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 450, in _structure_list
+          |     res.append(handler(e, elem_type))
+          |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 375, in _structure_call
+          |     return cl(obj)
+          | ValueError: invalid literal for int() with base 10: 'a'
+          | Structuring list[int] @ index 0
+          +------------------------------------
+        +---------------- 2 ----------------
+        | Exception Group Traceback (most recent call last):
+        |   File "<cattrs generated structure __main__.Class>", line 10, in structure_Class
+        |     res['a_dict'] = __c_structure_a_dict(o['a_dict'], __c_type_a_dict)
+        |   File "", line 17, in structure_mapping
+        | cattrs.errors.IterableValidationError: While structuring dict
+        | Structuring class Class @ attribute a_dict
+        +-+---------------- 1 ----------------
+          | Traceback (most recent call last):
+          |   File "", line 5, in structure_mapping
+          | ValueError: invalid literal for int() with base 10: 'a'
+          | Structuring mapping value @ key 'str'
+          +------------------------------------
+
+.. _`PEP 654 ExceptionGroup`: https://www.python.org/dev/peps/pep-0654/
+.. _`PEP 678`: https://www.python.org/dev/peps/pep-0678/
+
+Non-detailed validation
+-----------------------
+
+Non-detailed validation can be enabled by initializing any of the converters with ``detailed_validation=False``.
+In this mode, any errors during un/structuring will bubble up directly as soon as they happen

src/cattrs/gen.py

@@ -679,7 +679,7 @@ def make_mapping_structure_fn(
             lines.append("      errors.append(e)")
             lines.append("  if errors:")
             lines.append(
-                f"    raise IterableValidationError('While structuring {structure_to.__name__}', errors, __cattr_mapping_cl)"
+                f"    raise IterableValidationError('While structuring {cl!r}', errors, __cattr_mapping_cl)"
             )
         else:
             lines.append(f"  res = {{{k_s}: {v_s} for k, v in mapping.items()}}")