Allow making RUSTC_BOOTSTRAP conditional on the crate name

The main change is that `UnstableOptions::from_environment` now requires
an (optional) crate name. If the crate name is unknown (`None`), then the new feature is not available and you still have to use `RUSTC_BOOTSTRAP=1`. In practice this means the feature is only available for `--crate-name`, not for `#![crate_name]`; I'm interested in supporting the second but I'm not sure how.

Other major changes:

- Added `Session::is_nightly_build()`, which uses the `crate_name` of
the session
- Added `nightly_options::match_is_nightly_build`, a convenience method
for looking up `--crate-name` from CLI arguments.
`Session::is_nightly_build()`should be preferred where possible, since
it will take into account `#![crate_name]` (I think).
- Added `unstable_features` to `rustdoc::RenderOptions`

  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features. In practice this shouldn't be a big
  deal, since `RUSTC_BOOTSTRAP` is the opposite of stable and everyone
  uses `RUSTC_BOOTSTRAP=1` anyway.

- Add tests

  Check against `Cheat`, not whether nightly features are allowed.
  Nightly features are always allowed on the nightly channel.

- Only call `is_nightly_build()` once within a function

- Use booleans consistently for rustc_incremental

  Sessions can't be passed through threads, so `read_file` couldn't take a
  session. To be consistent, also take a boolean in `write_file_header`.

compiler/rustc_incremental/src/persist/file_format.rs

@@ -15,7 +15,6 @@ use std::io::{self, Read};
 use std::path::Path;
 
 use rustc_serialize::opaque::Encoder;
-use rustc_session::config::nightly_options;
 
 /// The first few bytes of files generated by incremental compilation.
 const FILE_MAGIC: &[u8] = b"RSIC";
@@ -28,12 +27,12 @@ const HEADER_FORMAT_VERSION: u16 = 0;
 /// the Git commit hash.
 const RUSTC_VERSION: Option<&str> = option_env!("CFG_VERSION");
 
-pub fn write_file_header(stream: &mut Encoder) {
+pub fn write_file_header(stream: &mut Encoder, nightly_build: bool) {
     stream.emit_raw_bytes(FILE_MAGIC);
     stream
         .emit_raw_bytes(&[(HEADER_FORMAT_VERSION >> 0) as u8, (HEADER_FORMAT_VERSION >> 8) as u8]);
 
-    let rustc_version = rustc_version();
+    let rustc_version = rustc_version(nightly_build);
     assert_eq!(rustc_version.len(), (rustc_version.len() as u8) as usize);
     stream.emit_raw_bytes(&[rustc_version.len() as u8]);
     stream.emit_raw_bytes(rustc_version.as_bytes());
@@ -51,6 +50,7 @@ pub fn write_file_header(stream: &mut Encoder) {
 pub fn read_file(
     report_incremental_info: bool,
     path: &Path,
+    nightly_build: bool,
 ) -> io::Result<Option<(Vec<u8>, usize)>> {
     if !path.exists() {
         return Ok(None);
@@ -93,7 +93,7 @@ pub fn read_file(
         let mut buffer = vec![0; rustc_version_str_len];
         file.read_exact(&mut buffer)?;
 
-        if buffer != rustc_version().as_bytes() {
+        if buffer != rustc_version(nightly_build).as_bytes() {
             report_format_mismatch(report_incremental_info, path, "Different compiler version");
             return Ok(None);
         }
@@ -115,8 +115,8 @@ fn report_format_mismatch(report_incremental_info: bool, file: &Path, message: &
     }
 }
 
-fn rustc_version() -> String {
-    if nightly_options::is_nightly_build() {
+fn rustc_version(nightly_build: bool) -> String {
+    if nightly_build {
         if let Some(val) = env::var_os("RUSTC_FORCE_INCR_COMP_ARTIFACT_HEADER") {
             return val.to_string_lossy().into_owned();
         }

compiler/rustc_incremental/src/persist/load.rs

@@ -53,8 +53,12 @@ impl LoadResult<(PreviousDepGraph, WorkProductMap)> {
     }
 }
 
-fn load_data(report_incremental_info: bool, path: &Path) -> LoadResult<(Vec<u8>, usize)> {
-    match file_format::read_file(report_incremental_info, path) {
+fn load_data(
+    report_incremental_info: bool,
+    path: &Path,
+    nightly_build: bool,
+) -> LoadResult<(Vec<u8>, usize)> {
+    match file_format::read_file(report_incremental_info, path, nightly_build) {
         Ok(Some(data_and_pos)) => LoadResult::Ok { data: data_and_pos },
         Ok(None) => {
             // The file either didn't exist or was produced by an incompatible
@@ -111,13 +115,14 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
     let expected_hash = sess.opts.dep_tracking_hash();
 
     let mut prev_work_products = FxHashMap::default();
+    let nightly_build = sess.is_nightly_build();
 
     // If we are only building with -Zquery-dep-graph but without an actual
     // incr. comp. session directory, we skip this. Otherwise we'd fail
     // when trying to load work products.
     if sess.incr_comp_session_dir_opt().is_some() {
         let work_products_path = work_products_path(sess);
-        let load_result = load_data(report_incremental_info, &work_products_path);
+        let load_result = load_data(report_incremental_info, &work_products_path, nightly_build);
 
         if let LoadResult::Ok { data: (work_products_data, start_pos) } = load_result {
             // Decode the list of work_products
@@ -163,7 +168,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
     MaybeAsync::Async(std::thread::spawn(move || {
         let _prof_timer = prof.generic_activity("incr_comp_load_dep_graph");
 
-        match load_data(report_incremental_info, &path) {
+        match load_data(report_incremental_info, &path, nightly_build) {
             LoadResult::DataOutOfDate => LoadResult::DataOutOfDate,
             LoadResult::Error { message } => LoadResult::Error { message },
             LoadResult::Ok { data: (bytes, start_pos) } => {
@@ -201,7 +206,11 @@ pub fn load_query_result_cache(sess: &Session) -> OnDiskCache<'_> {
 
     let _prof_timer = sess.prof.generic_activity("incr_comp_load_query_result_cache");
 
-    match load_data(sess.opts.debugging_opts.incremental_info, &query_cache_path(sess)) {
+    match load_data(
+        sess.opts.debugging_opts.incremental_info,
+        &query_cache_path(sess),
+        sess.is_nightly_build(),
+    ) {
         LoadResult::Ok { data: (bytes, start_pos) } => OnDiskCache::new(sess, bytes, start_pos),
         _ => OnDiskCache::new_empty(sess.source_map()),
     }

compiler/rustc_incremental/src/persist/save.rs

@@ -119,7 +119,7 @@ where
 
     // generate the data in a memory buffer
     let mut encoder = Encoder::new(Vec::new());
-    file_format::write_file_header(&mut encoder);
+    file_format::write_file_header(&mut encoder, sess.is_nightly_build());
     encode(&mut encoder);
 
     // write the data out