document -Z allow-features

Author: jyn514

Diff for: src/doc/unstable-book/src/compiler-flags/allow-features.md

@@ -0,0 +1,14 @@
+# `allow-features`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+This flag allows limiting the features which can be enabled with `#![feature(...)]` attributes.
+By default, all features are allowed on nightly and no features are allowed on stable or beta (but see [`RUSTC_BOOTSTRAP`]).
+
+Features are comma-separated, for example `-Z allow-features=ffi_pure,f16`.
+If the flag is present, any feature listed will be allowed and any feature not listed will be disallowed.
+Any unrecognized feature is ignored.
+
+[`RUSTC_BOOTSTRAP`]: ./rustc-bootstrap.html

Diff for: src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md

@@ -11,7 +11,11 @@ Setting `RUSTC_BOOTSTRAP=1` instructs rustc to enable this for all crates.
 Setting `RUSTC_BOOTSTRAP=crate_name` instructs rustc to only apply this to crates named `crate_name`.
 Setting `RUSTC_BOOTSTRAP=-1` instructs rustc to act as if it is a stable compiler, even on the nightly release channel.
 Cargo disallows setting `cargo::rustc-env=RUSTC_BOOTSTRAP` in build scripts.
-Crates can fully opt out of unstable features by using `#![forbid(unstable_features)]` at the crate root (or any other way of enabling lints, such as `-F unstable-features`).
+Build systems can limit the features they enable with [`-Z allow-features=feature1,feature2`][Z-allow-features].
+Crates can fully opt out of unstable features by using [`#![forbid(unstable_features)]`][unstable-features] at the crate root (or any other way of enabling lints, such as `-F unstable-features`).
+
+[Z-allow-features]: ./allow-features.html
+[unstable-features]: ../../rustc/lints/listing/allowed-by-default.html#unstable-features
 
 ## Why does this environment variable exist?