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  There is a user-facing change here: things like `RUSTC_BOOTSTRAP=0` no
  longer active nightly features.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

10 files changed, 26 insertions, 22 deletions

diff --git a/src/librustdoc/clean/types.rs b/src/librustdoc/clean/types.rs
index 32b3f69ecd4..5f5deebca65 100644
--- a/src/librustdoc/clean/types.rs
+++ b/src/librustdoc/clean/types.rs
@@ -681,7 +681,9 @@ impl Attributes {
                                 }
                                 Some(&(_, _, ExternalLocation::Remote(ref s))) => s.to_string(),
                                 Some(&(_, _, ExternalLocation::Unknown)) | None => String::from(
-                                    if UnstableFeatures::from_environment().is_nightly_build() {
+                                    // NOTE: intentionally doesn't pass crate name to avoid having
+                                    // different primitive links between crates
+                                    if UnstableFeatures::from_environment(None).is_nightly_build() {
                                         "https://doc.rust-lang.org/nightly"
                                     } else {
                                         "https://doc.rust-lang.org"
diff --git a/src/librustdoc/config.rs b/src/librustdoc/config.rs
index 02885f51936..b7db665147e 100644
--- a/src/librustdoc/config.rs
+++ b/src/librustdoc/config.rs
@@ -253,6 +253,7 @@ pub struct RenderOptions {
     pub document_private: bool,
     /// Document items that have `doc(hidden)`.
     pub document_hidden: bool,
+    pub unstable_features: rustc_feature::UnstableFeatures,
 }
 
 /// Temporary storage for data obtained during `RustdocVisitor::clean()`.
@@ -295,7 +296,7 @@ impl Options {
                 println_condition(p.condition);
             }
 
-            if nightly_options::is_nightly_build() {
+            if nightly_options::match_is_nightly_build(matches) {
                 println!("\nPasses run with `--show-coverage`:");
                 for p in passes::COVERAGE_PASSES {
                     print!("{:>20}", p.pass.name);
@@ -479,6 +480,7 @@ impl Options {
             &matches.opt_strs("html-after-content"),
             &matches.opt_strs("markdown-before-content"),
             &matches.opt_strs("markdown-after-content"),
+            nightly_options::match_is_nightly_build(&matches),
             &diag,
             &mut id_map,
             edition,
@@ -535,7 +537,9 @@ impl Options {
         let output_format = match matches.opt_str("output-format") {
             Some(s) => match OutputFormat::try_from(s.as_str()) {
                 Ok(o) => {
-                    if o.is_json() && !(show_coverage || nightly_options::is_nightly_build()) {
+                    if o.is_json()
+                        && !(show_coverage || nightly_options::match_is_nightly_build(matches))
+                    {
                         diag.struct_err("json output format isn't supported for doc generation")
                             .emit();
                         return Err(1);
@@ -586,7 +590,6 @@ impl Options {
 
         Ok(Options {
             input,
-            crate_name,
             proc_macro_crate,
             error_format,
             libs,
@@ -637,7 +640,11 @@ impl Options {
                 generate_search_filter,
                 document_private,
                 document_hidden,
+                unstable_features: rustc_feature::UnstableFeatures::from_environment(
+                    crate_name.as_deref(),
+                ),
             },
+            crate_name,
             output_format,
         })
     }
@@ -655,7 +662,8 @@ fn check_deprecated_options(matches: &getopts::Matches, diag: &rustc_errors::Han
     for flag in deprecated_flags.iter() {
         if matches.opt_present(flag) {
             if *flag == "output-format"
-                && (matches.opt_present("show-coverage") || nightly_options::is_nightly_build())
+                && (matches.opt_present("show-coverage")
+                    || nightly_options::match_is_nightly_build(matches))
             {
                 continue;
             }
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index 5eca54199d6..4cfc0a10c00 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -370,7 +370,7 @@ pub fn run_core(
         cg: codegen_options,
         externs,
         target_triple: target,
-        unstable_features: UnstableFeatures::from_environment(),
+        unstable_features: UnstableFeatures::from_environment(crate_name.as_deref()),
         actually_rustdoc: true,
         debugging_opts,
         error_format,
diff --git a/src/librustdoc/doctest.rs b/src/librustdoc/doctest.rs
index eb33890fb5f..5e40e6b151d 100644
--- a/src/librustdoc/doctest.rs
+++ b/src/librustdoc/doctest.rs
@@ -1,7 +1,6 @@
 use rustc_ast as ast;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorReported;
-use rustc_feature::UnstableFeatures;
 use rustc_hir as hir;
 use rustc_hir::intravisit;
 use rustc_hir::{HirId, CRATE_HIR_ID};
@@ -70,7 +69,7 @@ pub fn run(options: Options) -> Result<(), ErrorReported> {
         lint_cap: Some(options.lint_cap.clone().unwrap_or_else(|| lint::Forbid)),
         cg: options.codegen_options.clone(),
         externs: options.externs.clone(),
-        unstable_features: UnstableFeatures::from_environment(),
+        unstable_features: options.render_options.unstable_features,
         actually_rustdoc: true,
         debugging_opts: config::DebuggingOptions { ..config::basic_debugging_options() },
         edition: options.edition,
diff --git a/src/librustdoc/externalfiles.rs b/src/librustdoc/externalfiles.rs
index c8121d39d0f..900821dbf4a 100644
--- a/src/librustdoc/externalfiles.rs
+++ b/src/librustdoc/externalfiles.rs
@@ -1,6 +1,5 @@
 use crate::html::markdown::{ErrorCodes, IdMap, Markdown, Playground};
 use crate::rustc_span::edition::Edition;
-use rustc_feature::UnstableFeatures;
 use std::fs;
 use std::path::Path;
 use std::str;
@@ -25,12 +24,13 @@ impl ExternalHtml {
         after_content: &[String],
         md_before_content: &[String],
         md_after_content: &[String],
+        nightly_build: bool,
         diag: &rustc_errors::Handler,
         id_map: &mut IdMap,
         edition: Edition,
         playground: &Option<Playground>,
     ) -> Option<ExternalHtml> {
-        let codes = ErrorCodes::from(UnstableFeatures::from_environment().is_nightly_build());
+        let codes = ErrorCodes::from(nightly_build);
         let ih = load_external_files(in_header, diag)?;
         let bc = load_external_files(before_content, diag)?;
         let m_bc = load_external_files(md_before_content, diag)?;
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 5ac0ffcfbf1..b3bfa7138a0 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -52,7 +52,6 @@ use rustc_ast_pretty::pprust;
 use rustc_attr::StabilityLevel;
 use rustc_data_structures::flock;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_feature::UnstableFeatures;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::Mutability;
@@ -397,6 +396,7 @@ impl FormatRenderer for Context {
             resource_suffix,
             static_root_path,
             generate_search_filter,
+            unstable_features,
             ..
         } = options;
 
@@ -466,7 +466,7 @@ impl FormatRenderer for Context {
             static_root_path,
             fs: DocFS::new(sender),
             edition,
-            codes: ErrorCodes::from(UnstableFeatures::from_environment().is_nightly_build()),
+            codes: ErrorCodes::from(unstable_features.is_nightly_build()),
             playground,
         };
 
diff --git a/src/librustdoc/markdown.rs b/src/librustdoc/markdown.rs
index 3a87e1c46a6..33bd57223b8 100644
--- a/src/librustdoc/markdown.rs
+++ b/src/librustdoc/markdown.rs
@@ -2,7 +2,6 @@ use std::fs::{create_dir_all, read_to_string, File};
 use std::io::prelude::*;
 use std::path::Path;
 
-use rustc_feature::UnstableFeatures;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::DUMMY_SP;
 
@@ -66,7 +65,7 @@ pub fn render<P: AsRef<Path>>(
     let title = metadata[0];
 
     let mut ids = IdMap::new();
-    let error_codes = ErrorCodes::from(UnstableFeatures::from_environment().is_nightly_build());
+    let error_codes = ErrorCodes::from(options.unstable_features.is_nightly_build());
     let text = if !options.markdown_no_toc {
         MarkdownWithToc(text, &mut ids, error_codes, edition, &playground).into_string()
     } else {
@@ -131,7 +130,7 @@ pub fn test(mut options: Options) -> Result<(), String> {
         options.enable_per_target_ignores,
     );
     collector.set_position(DUMMY_SP);
-    let codes = ErrorCodes::from(UnstableFeatures::from_environment().is_nightly_build());
+    let codes = ErrorCodes::from(options.render_options.unstable_features.is_nightly_build());
 
     find_testable_code(&input_str, &mut collector, codes, options.enable_per_target_ignores, None);
 
diff --git a/src/librustdoc/passes/doc_test_lints.rs b/src/librustdoc/passes/doc_test_lints.rs
index 686ec51fb06..5fd7f5f81f7 100644
--- a/src/librustdoc/passes/doc_test_lints.rs
+++ b/src/librustdoc/passes/doc_test_lints.rs
@@ -92,9 +92,7 @@ pub fn look_for_tests<'tcx>(cx: &DocContext<'tcx>, dox: &str, item: &Item) {
 
     find_testable_code(&dox, &mut tests, ErrorCodes::No, false, None);
 
-    if tests.found_tests == 0
-        && rustc_feature::UnstableFeatures::from_environment().is_nightly_build()
-    {
+    if tests.found_tests == 0 && cx.tcx.sess.is_nightly_build() {
         if should_have_doc_example(cx, &item) {
             debug!("reporting error for {:?} (hir_id={:?})", item, hir_id);
             let sp = span_of_attrs(&item.attrs).unwrap_or(item.source.span());
diff --git a/src/librustdoc/passes/html_tags.rs b/src/librustdoc/passes/html_tags.rs
index 1d9be619ec9..26b64b4905e 100644
--- a/src/librustdoc/passes/html_tags.rs
+++ b/src/librustdoc/passes/html_tags.rs
@@ -5,7 +5,6 @@ use crate::fold::DocFolder;
 use crate::html::markdown::opts;
 use core::ops::Range;
 use pulldown_cmark::{Event, Parser};
-use rustc_feature::UnstableFeatures;
 use rustc_session::lint;
 use std::iter::Peekable;
 use std::str::CharIndices;
@@ -27,7 +26,7 @@ impl<'a, 'tcx> InvalidHtmlTagsLinter<'a, 'tcx> {
 }
 
 pub fn check_invalid_html_tags(krate: Crate, cx: &DocContext<'_>) -> Crate {
-    if !UnstableFeatures::from_environment().is_nightly_build() {
+    if !cx.tcx.sess.is_nightly_build() {
         krate
     } else {
         let mut coll = InvalidHtmlTagsLinter::new(cx);
diff --git a/src/librustdoc/passes/non_autolinks.rs b/src/librustdoc/passes/non_autolinks.rs
index 4a8fc7fc618..964773dc055 100644
--- a/src/librustdoc/passes/non_autolinks.rs
+++ b/src/librustdoc/passes/non_autolinks.rs
@@ -7,7 +7,6 @@ use core::ops::Range;
 use pulldown_cmark::{Event, LinkType, Parser, Tag};
 use regex::Regex;
 use rustc_errors::Applicability;
-use rustc_feature::UnstableFeatures;
 use rustc_session::lint;
 
 pub const CHECK_NON_AUTOLINKS: Pass = Pass {
@@ -54,7 +53,7 @@ impl<'a, 'tcx> NonAutolinksLinter<'a, 'tcx> {
 }
 
 pub fn check_non_autolinks(krate: Crate, cx: &DocContext<'_>) -> Crate {
-    if !UnstableFeatures::from_environment().is_nightly_build() {
+    if !cx.tcx.sess.is_nightly_build() {
         krate
     } else {
         let mut coll = NonAutolinksLinter::new(cx);