Add cfg = "data" to incompatible changes docs
Closes #6183.
PiperOrigin-RevId: 215374940
diff --git a/site/docs/skylark/backward-compatibility.md b/site/docs/skylark/backward-compatibility.md
index a66785d..a5ecefd 100644
--- a/site/docs/skylark/backward-compatibility.md
+++ b/site/docs/skylark/backward-compatibility.md
@@ -56,6 +56,7 @@
* [Disallow using C++ Specific Make Variables from the configuration](#disallow-using-c-specific-make-variables-from-the-configuration)
* [Disable legacy C++ configuration API](#disable-legacy-c-configuration-api)
* [Disable legacy C++ toolchain API](#disable-legacy-c-toolchain-api)
+* [Disallow `cfg = "data"`](#disallow-cfg--data)
### Dictionary concatenation
@@ -882,4 +883,34 @@
* Default: `false`
* Introduced in: `0.19.0`
+### Disallow `cfg = "data"`
+
+`cfg = "data"` is a no-op that incorrectly gives the impression dependencies under
+it are built in a distinct "data" mode:
+
+```python
+my_rule = rule(
+ ...
+ "some_attr": attr.label_list(
+ cfg = "data" # This line does nothing
+ )
+)
+```
+
+The original semantics were unclear and were
+[removed](https://github.com/bazelbuild/bazel/commit/8820d3ae601f229b72c61d2eb601b0e8e9b0111a#diff-ffd6930edbe7f2529b608c400fd19456)
+in 0.16.0.
+
+Because this syntax is non-functional and confusing, it's being removed outright
+([#6153](https://github.com/bazelbuild/bazel/issues/6153)). The functionality it
+implies will be provided by
+[Starlark build configuration](https://github.com/bazelbuild/bazel/issues/5574).
+
+When `--incompatible_disallow_data_transition=true`, builds using this syntax
+fail with an error.
+
+* Flag: `--incompatible_disallow_data_transition`
+* Default: `false`
+* Introduced in: `0.16.0`
+
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