document #[cfg(bootstrap)] in dependencies

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -81,6 +81,7 @@
 - [How Bootstrap does it](./building/bootstrapping/how-bootstrap-does-it.md)
 - [Writing tools in Bootstrap](./building/bootstrapping/writing-tools-in-bootstrap.md)
 - [Debugging bootstrap](./building/bootstrapping/debugging-bootstrap.md)
+- [cfg(bootstrap) in dependencies](./building/bootstrapping/bootstrap-in-dependencies.md)
 
 # High-level Compiler Architecture
 

src/doc/rustc-dev-guide/src/building/bootstrapping/bootstrap-in-dependencies.md

@@ -0,0 +1,53 @@
+# `cfg(bootstrap)` in compiler dependencies
+
+The rust compiler uses some external crates that can run into cyclic dependencies with the compiler itself: the compiler needs an updated crate to build, but the crate needs an updated compiler. This page describes how `#[cfg(bootstrap)]` can be used to break this cycle.
+
+## Enabling `#[cfg(bootstrap)]`
+
+Usually the use of `#[cfg(bootstrap)]` in an external crate causes a warning:
+
+```
+warning: unexpected `cfg` condition name: `bootstrap`
+ --> src/main.rs:1:7
+  |
+1 | #[cfg(bootstrap)]
+  |       ^^^^^^^^^
+  |
+  = help: expected names are: `docsrs`, `feature`, and `test` and 31 more
+  = help: consider using a Cargo feature instead
+  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
+           [lints.rust]
+           unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bootstrap)'] }
+  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(bootstrap)");` to the top of the `build.rs`
+  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
+  = note: `#[warn(unexpected_cfgs)]` on by default
+```
+
+This warning can be silenced by adding these lines to the project's `Cargo.toml`:
+
+```toml
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bootstrap)'] }
+```
+
+Now `#[cfg(bootstrap)]` can be used in the crate just like it can be in the compiler: when the bootstrap compiler is used, code annotated with `#[cfg(bootstrap)]` is compiled, otherwise code annotated with `#[cfg(not(bootstrap))]` is compiled.
+
+## The update dance
+
+As a concrete example we'll use a change where the `#[naked]` attribute was made into an unsafe attribute, which caused a cyclic dependency with the `compiler-builtins` crate.
+
+### Step 1: accept the new behavior in the compiler ([#139797](https://github.com/rust-lang/rust/pull/139797))
+
+In this example it is possible to accept both the old and new behavior at the same time by disabling an error.
+
+### Step 2: update the crate ([#821](https://github.com/rust-lang/compiler-builtins/pull/821))
+
+Now in the crate, use `#[cfg(bootstrap)]` to use the old behavior, or `#[cfg(not(bootstrap))]` to use the new behavior.
+
+### Step 3: update the crate version used by the compiler ([#139934](https://github.com/rust-lang/rust/pull/139934))
+
+For `compiler-builtins` this meant a version bump, in other cases it may be a git submodule update.
+
+### Step 4: remove the old behavior from the compiler ([#139753](https://github.com/rust-lang/rust/pull/139753))
+
+The updated crate can now be used. In this example that meant that the old behavior could be removed.