Error if an invalid field name is used with Field

pydantic/_internal/_model_construction.py

@@ -308,16 +308,23 @@ def inspect_namespace(  # noqa C901
         elif isinstance(value, ModelPrivateAttr):
             if var_name.startswith('__'):
                 raise NameError(
-                    f'Private attributes "{var_name}" must not have dunder names; '
-                    'use a single underscore prefix instead.'
+                    'Private attributes must not use dunder names;'
+                    f' instead of {var_name!r} use a single underscore prefix instead.'
                 )
             elif is_valid_field_name(var_name):
                 raise NameError(
-                    f'Private attributes "{var_name}" must not be a valid field name; '
-                    f'use sunder names, e.g. "_{var_name}"'
+                    'Private attributes must not use valid field names;'
+                    f' instead of {var_name!r} use sunder names, e.g. {"_" + var_name!r}'
                 )
             private_attributes[var_name] = value
             del namespace[var_name]
+        elif isinstance(value, FieldInfo) and not is_valid_field_name(var_name):
+            suggested_name = var_name.lstrip('_') or 'my_field'  # don't suggest '' for all-underscore name
+            raise NameError(
+                f'Fields must not use names with leading underscores;'
+                f' e.g., instead of {var_name!r} use {suggested_name!r}'
+            )
+
         elif var_name.startswith('__'):
             continue
         elif is_valid_privateattr_name(var_name):

tests/test_main.py

@@ -2746,3 +2746,26 @@ class Model(BaseModel):
         )
 
     assert 'class Model' in module.help_result_string
+
+
+def test_cannot_use_leading_underscore_field_names():
+    with pytest.raises(
+        NameError, match="Fields must not use names with leading underscores; e.g., instead of '_x' use 'x'"
+    ):
+
+        class Model1(BaseModel):
+            _x: int = Field(alias='x')
+
+    with pytest.raises(
+        NameError, match="Fields must not use names with leading underscores; e.g., instead of '__x__' use 'x__'"
+    ):
+
+        class Model2(BaseModel):
+            __x__: int = Field()
+
+    with pytest.raises(
+        NameError, match="Fields must not use names with leading underscores; e.g., instead of '___' use 'my_field'"
+    ):
+
+        class Model3(BaseModel):
+            ___: int = Field(default=1)

tests/test_private_attributes.py

@@ -140,7 +140,7 @@ class Model(BaseModel):
 def test_private_attribute_invalid_name():
     with pytest.raises(
         NameError,
-        match='Private attributes "foo" must not be a valid field name; use sunder names, e.g. "_foo"',
+        match="Private attributes must not use valid field names; instead of 'foo' use sunder names, e.g. '_foo'",
     ):
 
         class Model(BaseModel):
@@ -266,7 +266,8 @@ class Model(ParentAModel, ParentBModel):
 def test_private_attributes_not_dunder() -> None:
     with pytest.raises(
         NameError,
-        match='Private attributes "__foo__" must not have dunder names; use a single underscore prefix instead.',
+        match="Private attributes must not use dunder names;"
+        " instead of '__foo__' use a single underscore prefix instead.",
     ):
 
         class MyModel(BaseModel):