Validate attrs init aliases before code generation

docs/init.md

@@ -90,7 +90,8 @@ Even if you're not using this feature, it's important to be aware of it because
 ...    _1: int
 Traceback (most recent call last):
    ...
-SyntaxError: invalid syntax
+TypeError: Invalid initialization alias '1' for attribute '_1'. Aliases must be valid Python identifiers.
+
 ```
 
 In this case a valid attribute name `_1` got transformed into an invalid argument name `1`.
@@ -251,10 +252,7 @@ ValueError: 'x' has to be smaller than 'y'!
 ...     x = field(validator=attrs.validators.instance_of(int))
 >>> C(42)
 C(x=42)
->>> C("42")
-Traceback (most recent call last):
-   ...
-TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, factory=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None), <type 'int'>, '42')
+TypeError: ("'x' must be <class 'int'> (got '42' that is a <class 'str'>).", Attribute(name='x', ..., alias='x'), <class 'int'>, '42')
 ```
 
 Of course you can mix and match the two approaches at your convenience.
@@ -270,10 +268,7 @@ If you use both ways to define validators for an attribute, they are both ran:
 ...             raise ValueError("value out of bounds")
 >>> C(128)
 C(x=128)
->>> C("128")
-Traceback (most recent call last):
-   ...
-TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=None), <class 'int'>, '128')
+TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', ..., alias='x'), <class 'int'>, '128')
 >>> C(256)
 Traceback (most recent call last):
    ...
@@ -290,7 +285,7 @@ C(x='128')
 >>> C("128")
 Traceback (most recent call last):
    ...
-TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=None), <class 'int'>, '128')
+TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', ..., alias='x'), <class 'int'>, '128')
 ```
 
 ... or within a context manager:
@@ -302,7 +297,7 @@ C(x='128')
 >>> C("128")
 Traceback (most recent call last):
    ...
-TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=None), <class 'int'>, '128')
+TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', ..., alias='x'), <class 'int'>, '128')
 ```
 
 (converters)=

src/attr/_make.py

@@ -8,6 +8,7 @@
 import enum
 import inspect
 import itertools
+import keyword
 import linecache
 import sys
 import types
@@ -496,6 +497,8 @@ def _transform_attrs(
             _OBJ_SETATTR.__get__(a)("alias", _default_init_alias_for(a.name))
             _OBJ_SETATTR.__get__(a)("alias_is_default", True)
 
+    _validate_init_aliases(attrs)
+
     # Create AttrsClass *after* applying the field_transformer since it may
     # add or remove attributes!
     attr_names = [a.name for a in attrs]
@@ -2426,6 +2429,27 @@ def _default_init_alias_for(name: str) -> str:
     return name.lstrip("_")
 
 
+def _validate_init_aliases(attrs: tuple[Attribute, ...]) -> None:
+    """
+    Ensure init aliases are valid Python parameter names.
+    """
+    for a in attrs:
+        if a.init is False:
+            continue
+
+        alias = a.alias
+        if (
+            not isinstance(alias, str)
+            or not alias.isidentifier()
+            or keyword.iskeyword(alias)
+        ):
+            msg = (
+                f"Invalid initialization alias {alias!r} for attribute "
+                f"{a.name!r}. Aliases must be valid Python identifiers."
+            )
+            raise TypeError(msg)
+
+
 class Attribute:
     """
     *Read-only* representation of an attribute.

tests/test_make.py

@@ -2448,6 +2448,43 @@ class EvolveCase:
             dunder__=5,
         ) == EvolveCase(1, 4, 5)
 
+    @pytest.mark.parametrize("alias", ["x=1", "class", "not valid", 1])
+    def test_invalid_alias(self, alias):
+        """
+        Invalid aliases are rejected before they can be used in generated
+        __init__ source code.
+        """
+
+        with pytest.raises(TypeError, match="Invalid initialization alias"):
+
+            @attrs.define
+            class C:
+                x: int = attrs.field(alias=alias)
+
+    def test_invalid_alias_not_executed(self, monkeypatch):
+        """
+        Aliases are parameter names, not Python source code.
+        """
+
+        marker = "_attrs_alias_executed"
+        monkeypatch.delattr("builtins." + marker, raising=False)
+
+        with pytest.raises(TypeError, match="Invalid initialization alias"):
+            attr.make_class(
+                "C",
+                {
+                    "x": attr.ib(
+                        alias=(
+                            "x=__import__('builtins').setattr("
+                            "__import__('builtins'), "
+                            f"{marker!r}, True)"
+                        )
+                    )
+                },
+            )
+
+        assert getattr(__import__("builtins"), marker, False) is False
+
     def test_alias_is_default(self):
         """
         alias_is_default is True for auto-generated aliases and False for