document RUSTC_BOOTSTRAP in the unstable book

Author: jyn514

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

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+# `RUSTC_BOOTSTRAP`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+The `RUSTC_BOOTSTRAP` environment variable tells rustc to act as if it is a nightly compiler;
+in particular, it allows `#![feature(...)]` attributes and `-Z` flags even on the stable release channel.
+
+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`).
+
+## Why does this environment variable exist?
+
+`RUSTC_BOOTSTRAP`, as the name suggests, is used for bootstrapping the compiler from an earlier version.
+In particular, nightly is built with beta, and beta is built with stable.
+Since the standard library and compiler both use unstable features, `RUSTC_BOOTSTRAP` is required so that we can use the previous version to build them.
+
+## Why is this environment variable so easy to use for people not in the rust project?
+
+Originally, `RUSTC_BOOTSTRAP` required passing in a hash of the previous compiler version, to discourage using it for any purpose other than bootstrapping.
+That constraint was later relaxed; see <https://github.com/rust-lang/rust/issues/36548> for the discussion that happened at that time.
+
+People have at various times proposed re-adding the technical constraints.
+However, doing so is extremely disruptive for several major projects that we very much want to keep using the latest toolchain versions, such as Firefox, Rust for Linux, and Chromium.
+We continue to allow `RUSTC_BOOTSTRAP` until we can come up with an alternative that does not disrupt our largest constituents.
+
+## History
+
+- [Allowed without a hash](https://github.com/rust-lang/rust/pull/37265) ([discussion](https://github.com/rust-lang/rust/issues/36548))
+- [Extended to crate names](https://github.com/rust-lang/rust/pull/77802) ([discussion](https://github.com/rust-lang/cargo/issues/7088))
+- [Disallowed for build scripts](https://github.com/rust-lang/cargo/pull/9181) ([discussion](https://github.com/rust-lang/compiler-team/issues/350))
+- [Extended to emulate stable](https://github.com/rust-lang/rust/pull/132993) ([discussion](https://github.com/rust-lang/rust/issues/123404))