add "Compiler environment variables" section to the unstable book

src/doc/unstable-book/src/compiler-environment-variables.md

@@ -0,0 +1 @@
+# Compiler environment variables

src/doc/unstable-book/src/compiler-environment-variables/RUSTC_BOOTSTRAP.md

@@ -14,7 +14,7 @@ Cargo disallows setting `cargo::rustc-env=RUSTC_BOOTSTRAP` in build scripts.
 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
+[Z-allow-features]: ../compiler-flags/allow-features.html
 [unstable-features]: ../../rustc/lints/listing/allowed-by-default.html#unstable-features
 
 ## Why does this environment variable exist?

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

@@ -11,4 +11,4 @@ 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
+[`RUSTC_BOOTSTRAP`]: ../compiler-environment-variables/RUSTC_BOOTSTRAP.html

src/tools/unstable-book-gen/src/SUMMARY.md

@@ -1,5 +1,7 @@
 [The Unstable Book](the-unstable-book.md)
 
+- [Compiler environment variables](compiler-environment-variables.md)
+{compiler_env_vars}
 - [Compiler flags](compiler-flags.md)
 {compiler_flags}
 - [Language features](language-features.md)

src/tools/unstable-book-gen/src/main.rs

@@ -30,8 +30,12 @@ fn set_to_summary_str(set: &BTreeSet<String>, dir: &str) -> String {
 
 fn generate_summary(path: &Path, lang_features: &Features, lib_features: &Features) {
     let compiler_flags = collect_unstable_book_section_file_names(&path.join("src/compiler-flags"));
+    let compiler_env_vars =
+        collect_unstable_book_section_file_names(&path.join("src/compiler-environment-variables"));
 
     let compiler_flags_str = set_to_summary_str(&compiler_flags, "compiler-flags");
+    let compiler_env_vars_str =
+        set_to_summary_str(&compiler_env_vars, "compiler-environment-variables");
 
     let unstable_lang_features = collect_unstable_feature_names(&lang_features);
     let unstable_lib_features = collect_unstable_feature_names(&lib_features);
@@ -42,6 +46,7 @@ fn generate_summary(path: &Path, lang_features: &Features, lib_features: &Featur
     let summary_path = path.join("src/SUMMARY.md");
     let content = format!(
         include_str!("SUMMARY.md"),
+        compiler_env_vars = compiler_env_vars_str,
         compiler_flags = compiler_flags_str,
         language_features = lang_features_str,
         library_features = lib_features_str