Add a TOC to the unstable chapter.

Author: ehuss

src/doc/src/reference/unstable.md

@@ -58,6 +58,55 @@ Each new feature described below should explain how to use it.
 [nightly channel]: ../../book/appendix-07-nightly-rust.html
 [stabilized]: https://doc.crates.io/contrib/process/unstable.html#stabilization
 
+### List of unstable features
+
+* Unstable-specific features
+    * [-Z allow-features](#allow-features) — Provides a way to restrict which unstable features are used.
+* Build scripts and linking
+    * [extra-link-arg](#extra-link-arg) — Allows build scripts to pass extra link arguments in more cases.
+    * [Metabuild](#metabuild) — Provides declarative build scripts.
+* Resolver and features
+    * [no-index-update](#no-index-update) — Prevents cargo from updating the index cache.
+    * [avoid-dev-deps](#avoid-dev-deps) — Prevents the resolver from including dev-dependencies during resolution.
+    * [minimal-versions](#minimal-versions) — Forces the resolver to use the lowest compatible version instead of the highest.
+    * [public-dependency](#public-dependency) — Allows dependencies to be classified as either public or private.
+    * [Namespaced features](#namespaced-features) — Separates optional dependencies into a separate namespace from regular features, and allows feature names to be the same as some dependency name.
+    * [Weak dependency features](#weak-dependency-features) — Allows setting features for dependencies without enabling optional dependencies.
+* Output behavior
+    * [out-dir](#out-dir) — Adds a directory where artifacts are copied to.
+    * [terminal-width](#terminal-width) — Tells rustc the width of the terminal so that long diagnostic messages can be truncated to be more readable.
+* Compile behavior
+    * [mtime-on-use](#mtime-on-use) — Updates the last-modified timestamp on every dependency every time it is used, to provide a mechanism to delete unused artifacts.
+    * [doctest-xcompile](#doctest-xcompile) — Supports running doctests with the `--target` flag.
+    * [multitarget](#multitarget) — Supports building for multiple targets at the same time.
+    * [build-std](#build-std) — Builds the standard library instead of using pre-built binaries.
+    * [build-std-features](#build-std-features) — Sets features to use with the standard library.
+    * [binary-dep-depinfo](#binary-dep-depinfo) — Causes the dep-info file to track binary dependencies.
+    * [panic-abort-tests](#panic-abort-tests) — Allows running tests with the "abort" panic strategy.
+* rustdoc
+    * [`doctest-in-workspace`](#doctest-in-workspace) — Fixes workspace-relative paths when running doctests.
+    * [rustdoc-map](#rustdoc-map) — Provides mappings for documentation to link to external sites like [docs.rs](https://docs.rs/).
+* `Cargo.toml` extensions
+    * [Custom named profiles](#custom-named-profiles) — Adds custom named profiles in addition to the standard names.
+    * [Profile `strip` option](#profile-strip-option) — Forces the removal of debug information and symbols from executables.
+    * [per-package-target](#per-package-target) — Sets the `--target` to use for each individual package.
+    * [rust-version](#rust-version) — Allows to declare the minimum supported Rust version.
+    * [Edition 2021](#edition-2021) — Adds support for the 2021 Edition.
+* Information and metadata
+    * [Build-plan](#build-plan) — Emits JSON information on which commands will be run.
+    * [timings](#timings) — Generates a report on how long individual dependencies took to run.
+    * [unit-graph](#unit-graph) — Emits JSON for Cargo's internal graph structure.
+    * [future incompat report](#future-incompat-report) — Displays a report for future incompatibilities that may error in the future.
+* Configuration
+    * [config-cli](#config-cli) — Adds the ability to pass configuration options on the command-line.
+    * [config-include](#config-include) — Adds the ability for config files to include other files.
+    * [configurable-env](#configurable-env) — Adds support for defining environment variables that will be set when building and running.
+    * [patch-in-config](#patch-in-config) — Adds support for specifying the `[patch]` table in config files.
+    * [`cargo config`](#cargo-config) — Adds a new subcommand for viewing config files.
+* Registries
+    * [credential-process](#credential-process) — Adds support for fetching registry tokens from an external authentication program.
+    * [`cargo logout`](#cargo-logout) — Adds the `logout` command to remove the currently saved registry token.
+
 ### allow-features
 
 This permanently-unstable flag makes it so that only a listed set of