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_passes/src/check_const.rs

@@ -15,7 +15,6 @@ use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_middle::hir::map::Map;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
-use rustc_session::config::nightly_options;
 use rustc_session::parse::feature_err;
 use rustc_span::{sym, Span, Symbol};
 
@@ -145,7 +144,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                 //
                 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
                 // is a pretty narrow case, however.
-                if nightly_options::is_nightly_build() {
+                if tcx.sess.is_nightly_build() {
                     for gate in missing_secondary {
                         let note = format!(
                             "add `#![feature({})]` to the crate attributes to enable",