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](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_ast_lowering/src/lib.rs

@@ -53,7 +53,6 @@ use rustc_hir::definitions::{DefKey, DefPathData, Definitions};
 use rustc_hir::intravisit;
 use rustc_hir::{ConstArg, GenericArg, ParamName};
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_session::config::nightly_options;
 use rustc_session::lint::{builtin::BARE_TRAIT_OBJECTS, BuiltinLintDiagnostics, LintBuffer};
 use rustc_session::parse::ParseSess;
 use rustc_session::Session;
@@ -1398,8 +1397,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             "`impl Trait` not allowed outside of {}",
                             allowed_in,
                         );
-                        if pos == ImplTraitPosition::Binding && nightly_options::is_nightly_build()
-                        {
+                        if pos == ImplTraitPosition::Binding && self.sess.is_nightly_build() {
                             err.help(
                                 "add `#![feature(impl_trait_in_bindings)]` to the crate \
                                    attributes to enable",