mk: Hardcode the bootstrap key for each release

Starting with the 1.10.0 release we would like to bootstrap all compilers from
the previous stable release. For example the 1.10.0 compiler should bootstrap
from the literal 1.9.0 release artifacts. To do this, however, we need a way to
enable unstable features temporarily in a stable compiler (as the released
compiler is stable), but it turns out we already have a way to do that!

At compile time the configure script selects a `CFG_BOOTSTRAP_KEY` variable
value and then exports it into the makefiles. If the `RUSTC_BOOTSTRAP_KEY`
environment variable is set to this value, then the compiler is allowed to
"cheat" and use unstable features.

This method of choosing the bootstrap key, however, is problematic for the
intention of bootstrapping from the previous release. Each time a 1.9.0 compiler
is created, a new bootstrap key will be selected. That means that the 1.10.0
compiler will only compile from *our* literal release artifacts. Instead
distributions would like to bootstrap from their own compilers, so instead we
simply hardcode the bootstrap key for each release.

This patch uses the same `CFG_FILENAME_EXTRA` value (a hash of the release
string) as the bootstrap key. Consequently all 1.9.0 compilers, no matter where
they are compiled, will have the same bootstrap key. Additionally we won't need
to keep updating this as it'll be based on the release number anyway.

Once the 1.9.0 beta has been created, we can update the 1.10.0 nightly sources
(the `master` branch at that time) to bootstrap from that release using this
hard-coded bootstrap key. We will likely just hardcode into the makefiles what
the previous bootstrap key was and we'll change that whenever the stage0
compiler is updated.

configure

@@ -716,18 +716,6 @@ if [ -n "$CFG_ENABLE_DEBUG_JEMALLOC" ]; then putvar CFG_ENABLE_DEBUG_JEMALLOC; f
 
 if [ -n "$CFG_ENABLE_ORBIT" ]; then putvar CFG_ENABLE_ORBIT; fi
 
-# A magic value that allows the compiler to use unstable features
-# during the bootstrap even when doing so would normally be an error
-# because of feature staging or because the build turns on
-# warnings-as-errors and unstable features default to warnings.  The
-# build has to match this key in an env var. Meant to be a mild
-# deterrent from users just turning on unstable features on the stable
-# channel.
-# Basing CFG_BOOTSTRAP_KEY on CFG_BOOTSTRAP_KEY lets it get picked up
-# during a Makefile reconfig.
-CFG_BOOTSTRAP_KEY="${CFG_BOOTSTRAP_KEY-`date +%H:%M:%S`}"
-putvar CFG_BOOTSTRAP_KEY
-
 step_msg "looking for build programs"
 
 probe_need CFG_CURLORWGET  curl wget

mk/main.mk

@@ -24,6 +24,17 @@ CFG_PRERELEASE_VERSION=.1
 # versions in the same place
 CFG_FILENAME_EXTRA=$(shell printf '%s' $(CFG_RELEASE)$(CFG_EXTRA_FILENAME) | $(CFG_HASH_COMMAND))
 
+# A magic value that allows the compiler to use unstable features during the
+# bootstrap even when doing so would normally be an error because of feature
+# staging or because the build turns on warnings-as-errors and unstable features
+# default to warnings. The build has to match this key in an env var.
+#
+# This value is keyed off the release to ensure that all compilers for one
+# particular release have the same bootstrap key. Note that this is
+# intentionally not "secure" by any definition, this is largely just a deterrent
+# from users enabling unstable features on the stable compiler.
+CFG_BOOTSTRAP_KEY=$(CFG_FILENAME_EXTRA)
+
 ifeq ($(CFG_RELEASE_CHANNEL),stable)
 # This is the normal semver version string, e.g. "0.12.0", "0.12.0-nightly"
 CFG_RELEASE=$(CFG_RELEASE_NUM)