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  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features.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_session/src/config.rs

@@ -1247,7 +1247,7 @@ fn parse_crate_edition(matches: &getopts::Matches) -> Edition {
         None => DEFAULT_EDITION,
     };
 
-    if !edition.is_stable() && !nightly_options::is_nightly_build() {
+    if !edition.is_stable() && !nightly_options::match_is_nightly_build(matches) {
         early_error(
             ErrorOutputType::default(),
             &format!(
@@ -1544,7 +1544,9 @@ fn parse_libs(
                     );
                 }
             };
-            if kind == NativeLibKind::StaticNoBundle && !nightly_options::is_nightly_build() {
+            if kind == NativeLibKind::StaticNoBundle
+                && !nightly_options::match_is_nightly_build(matches)
+            {
                 early_error(
                     error_format,
                     "the library kind 'static-nobundle' is only \
@@ -1833,10 +1835,10 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         cg,
         error_format,
         externs,
+        unstable_features: UnstableFeatures::from_environment(crate_name.as_deref()),
         crate_name,
         alt_std_name: None,
         libs,
-        unstable_features: UnstableFeatures::from_environment(),
         debug_assertions,
         actually_rustdoc: false,
         trimmed_def_paths: TrimmedDefPaths::default(),
@@ -1957,17 +1959,21 @@ pub mod nightly_options {
     use rustc_feature::UnstableFeatures;
 
     pub fn is_unstable_enabled(matches: &getopts::Matches) -> bool {
-        is_nightly_build() && matches.opt_strs("Z").iter().any(|x| *x == "unstable-options")
+        match_is_nightly_build(matches)
+            && matches.opt_strs("Z").iter().any(|x| *x == "unstable-options")
     }
 
-    pub fn is_nightly_build() -> bool {
-        UnstableFeatures::from_environment().is_nightly_build()
+    pub fn match_is_nightly_build(matches: &getopts::Matches) -> bool {
+        is_nightly_build(matches.opt_str("crate-name").as_deref())
+    }
+
+    pub fn is_nightly_build(krate: Option<&str>) -> bool {
+        UnstableFeatures::from_environment(krate).is_nightly_build()
     }
 
     pub fn check_nightly_options(matches: &getopts::Matches, flags: &[RustcOptGroup]) {
         let has_z_unstable_option = matches.opt_strs("Z").iter().any(|x| *x == "unstable-options");
-        let really_allows_unstable_options =
-            UnstableFeatures::from_environment().is_nightly_build();
+        let really_allows_unstable_options = match_is_nightly_build(matches);
 
         for opt in flags.iter() {
             if opt.stability == OptionStability::Stable {

compiler/rustc_session/src/parse.rs

@@ -150,7 +150,7 @@ impl ParseSess {
     pub fn with_span_handler(handler: Handler, source_map: Lrc<SourceMap>) -> Self {
         Self {
             span_diagnostic: handler,
-            unstable_features: UnstableFeatures::from_environment(),
+            unstable_features: UnstableFeatures::from_environment(None),
             config: FxHashSet::default(),
             edition: ExpnId::root().expn_data().edition,
             raw_identifier_spans: Lock::new(Vec::new()),

compiler/rustc_session/src/session.rs

@@ -745,6 +745,9 @@ impl Session {
     pub fn unstable_options(&self) -> bool {
         self.opts.debugging_opts.unstable_options
     }
+    pub fn is_nightly_build(&self) -> bool {
+        self.opts.unstable_features.is_nightly_build()
+    }
     pub fn overflow_checks(&self) -> bool {
         self.opts
             .cg