tamer: Switch back to nightly toolchain

This is to support two things:
  1. Early switch to 2021 Edition, which is stable Oct 21; and
  2. To make use of unstable const features.

The rationale is that switching to nightly does not really have any
significant downside for us, given that TAMER is used only by us and
the only risk is that unstable features may change a bit, which can be
mitigated with certain precautions.

The rationale for each unstable feature will be documented as they are used,
including documentation on what would be required to remove it and what
functionality would be lost / need to change in doing so.

tamer/Makefile.am

@@ -18,6 +18,13 @@
 #  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 ##
 
+# `Makefile.am` is processed by Automake into `Makefile.in`, which is then
+# processed by Autoconf into the final `Makefile`.
+#
+# The `@`-delimited variables are defined dynamically in `configure.ac`,
+# which is processed by Autoconf into `configure`; they are populated based
+# on the environment in which the `configure` script is invoked.
+
 .DELETE_ON_ERROR:
 
 .PHONY: all fix fmt check-fmt bench
@@ -30,27 +37,27 @@ all: bin doc
 
 .PHONY: bin
 bin:
-	@CARGO@ build $(CARGO_BUILD_FLAGS) @FEATURES@
+	@CARGO@ +@RUST_TC@ build $(CARGO_BUILD_FLAGS) @FEATURES@
 
 doc: html
 html-am:
-	@CARGO@ test --doc @FEATURES@
+	@CARGO@ +@RUST_TC@ test --doc @FEATURES@
-	@CARGO@ @CARGO_DOC_FLAGS@ doc --document-private-items @FEATURES@
+	@CARGO@ +@RUST_TC@ @CARGO_DOC_FLAGS@ doc --document-private-items @FEATURES@
 
 # note that 'cargo check' is something else; see 'cargo --help'
 test: check
 check-am: check-fmt
-	@CARGO@ test @FEATURES@
+	@CARGO@ +@RUST_TC@ test @FEATURES@
 
 check-fmt:
-	@CARGO@ fmt -- --check
+	@CARGO@ +@RUST_TC@ fmt -- --check
 
 bench:
-	@CARGO@ @CARGO_BENCH_PRE_FLAGS@ bench $(CARGO_BENCH_FLAGS) @FEATURES@
+	@CARGO@ +@RUST_TC@ @CARGO_BENCH_PRE_FLAGS@ bench $(CARGO_BENCH_FLAGS) @FEATURES@
 
 fix: fmt
 fmt:
-	@CARGO@ fmt
+	@CARGO@ +@RUST_TC@ fmt
 
 clean-am:
-	@CARGO@ clean
+	@CARGO@ +@RUST_TC@ clean

tamer/configure.ac

@@ -43,13 +43,18 @@ AC_SUBST(SUFFIX, m4_argn(4, ver_split))
 AC_ARG_VAR([CARGO], [Rust Cargo executable])
 AC_CHECK_PROGS(CARGO, [cargo])
 
+# Rust toolchain (stable/nightly) is hard-coded for now, since decisions
+# regarding whether to use it are nuanced.
+AC_SUBST([RUST_TC], nightly)
+
 test -n "$CARGO" || AC_MSG_ERROR([cargo not found])
 
-rustc_ver_req=1.54.0
+# This is a nightly version at the time of writing
+rustc_ver_req=1.57.0
 
 AC_CHECK_PROGS(RUSTC, [rustc])
-AC_MSG_CHECKING([rustc version >= $rustc_ver_req])
+AC_MSG_CHECKING([rustc $RUST_TC version >= $rustc_ver_req])
-rustc_version=$("$RUSTC" --version | cut -d' ' -f2)
+rustc_version=$("$RUSTC" "+$RUST_TC" --version | cut -d' ' -f2)
 AX_COMPARE_VERSION([$rustc_version], [ge], [$rustc_ver_req],
                    [AC_MSG_RESULT([yes ($rustc_version)])],
                    [AC_MSG_RESULT([no ($rustc_version)])
@@ -66,30 +71,30 @@ AC_SUBST([CARGO_DOC_FLAGS], [])
 # it's still an unstable feature and we'll need to use nightly.  We don't
 # check for nightly here, though---if it's missing, then cargo will tell the
 # user what to do.
-AC_MSG_CHECKING([`test` feature support])
+AC_MSG_CHECKING([`test` feature support on $RUST_TC])
-AS_IF(["$RUSTC" --crate-type lib build_aux/bench_check.rs &>/dev/null],
+AS_IF(["$RUSTC" "+$RUST_TC" --crate-type lib build-aux/bench_check.rs &>/dev/null],
       [AC_MSG_RESULT(available)],
       [AC_MSG_RESULT([no (nightly required)])
        AC_SUBST([CARGO_BENCH_PRE_FLAGS], [+nightly])])
 
 # Cargo commands may be available but not necessarily installed for the
 # active toolchain.  Let's check that.
-AC_MSG_CHECKING([whether cargo-fmt is available for active toolchain])
+AC_MSG_CHECKING([whether cargo-fmt is available on $RUST_TC])
-AS_IF([cargo fmt --help &>/dev/null],
+AS_IF([cargo "+$RUST_TC" fmt --help &>/dev/null],
       [AC_MSG_RESULT(yes)],
       [AC_MSG_RESULT(no)
-       cargo fmt --help  # run again so user can see output
+       cargo "+$RUST_TC" fmt --help  # run again so user can see output
-       AC_MSG_ERROR([missing cargo-fmt for active toolchain])])
+       AC_MSG_ERROR([missing cargo-fmt on $RUST_TC])])
 
 # Cargo commands may be available but not necessarily installed for the
 # active toolchain.  Let's check that.
-AC_MSG_CHECKING([whether cargo-doc is available for toolchain])
+AC_MSG_CHECKING([whether cargo-doc is available on $RUST_TC])
-AS_IF([cargo $CARGO_DOC_FLAGS doc --help &>/dev/null],
+AS_IF([cargo "+$RUST_TC" $CARGO_DOC_FLAGS doc --help &>/dev/null],
       [AC_MSG_RESULT(yes)],
       [AC_MSG_RESULT(no)
        # run again so user can see output
-       cargo $CARGO_DOC_FLAGS doc --help 2>&1 | sed 's/^.*: //'
+       cargo "+$RUST_TC" $CARGO_DOC_FLAGS doc --help 2>&1 | sed 's/^.*: //'
-       AC_MSG_WARN([missing cargo-doc for toolchain])
+       AC_MSG_WARN([missing cargo-doc on $RUST_TC])
        AC_MSG_WARN([`make html` will not work])])
 
 AC_ARG_VAR([FEATURES],