Rollup merge of #77802 - jyn514:bootstrap-specific, r=nikomatsakis

Allow making `RUSTC_BOOTSTRAP` conditional on the crate name

Motivation: This came up in the [Zulip stream](https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/Require.20users.20to.20confirm.20they.20know.20RUSTC_.E2.80.A6.20compiler-team.23350/near/208403962) for https://github.com/rust-lang/compiler-team/issues/350.
See also https://github.com/rust-lang/cargo/pull/6608#issuecomment-458546258; this implements https://github.com/rust-lang/cargo/issues/6627.
The goal is for this to eventually allow prohibiting setting `RUSTC_BOOTSTRAP` in build.rs (https://github.com/rust-lang/cargo/issues/7088).

## User-facing changes

- `RUSTC_BOOTSTRAP=1` still works; there is no current plan to remove this.
- Things like `RUSTC_BOOTSTRAP=0` no longer activate 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.
- `RUSTC_BOOTSTRAP=x` will enable nightly features only for crate `x`.
- `RUSTC_BOOTSTRAP=x,y` will enable nightly features only for crates `x` and `y`.

## Implementation changes

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`

I'm not sure whether this counts as T-compiler or T-lang; _technically_ RUSTC_BOOTSTRAP is an implementation detail, but it's been used so much it seems like this counts as a language change too.

r? `@joshtriplett`
cc `@Mark-Simulacrum` `@hsivonen`

compiler/rustc_session/src/config.rs

@@ -1250,7 +1250,7 @@ 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!(
@@ -1547,7 +1547,9 @@ 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 \
@@ -1836,10 +1838,10 @@ 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(),
@@ -1960,17 +1962,21 @@ 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 {