rustc-dev-guide: document `needs-crate-type`
diff --git a/src/doc/rustc-dev-guide/src/tests/directives.md b/src/doc/rustc-dev-guide/src/tests/directives.md
index 8e4a710..7ed583c 100644
--- a/src/doc/rustc-dev-guide/src/tests/directives.md
+++ b/src/doc/rustc-dev-guide/src/tests/directives.md
@@ -191,8 +191,13 @@
specified atomic widths, e.g. the test with `//@ needs-target-has-atomic: 8,
16, ptr` will only run if it supports the comma-separated list of atomic
widths.
-- `needs-dynamic-linking` - ignores if target does not support dynamic linking
+- `needs-dynamic-linking` — ignores if target does not support dynamic linking
(which is orthogonal to it being unable to create `dylib` and `cdylib` crate types)
+- `needs-crate-type` — ignores if target platform does not support one or more
+ of the comma-delimited list of specified crate types. For example,
+ `//@ needs-crate-type: cdylib, proc-macro` will cause the test to be ignored
+ on `wasm32-unknown-unknown` target because the target does not support the
+ `proc-macro` crate type.
The following directives will check LLVM support:
