Auto merge of #39851 - alexcrichton:verify-unstable, r=brson

test: Verify all sysroot crates are unstable

As we continue to add more crates to the compiler and use them to implement
various features we want to be sure we're not accidentally expanding the API
surface area of the compiler! To that end this commit adds a new `run-make` test
which will attempt to `extern crate foo` all crates in the sysroot, verifying
that they're all unstable.

This commit discovered that the `std_shim` and `test_shim` crates were
accidentally stable and fixes the situation by deleting those shims. The shims
are no longer necessary due to changes in Cargo that have happened since they
were originally incepted.

src/test/run-make/sysroot-crates-are-unstable/Makefile

@@ -0,0 +1,34 @@
+-include ../tools.mk
+
+# This is a whitelist of crates which are stable, we don't check for the
+# instability of these crates as they're all stable!
+STABLE_CRATES := \
+	std \
+	core \
+	proc_macro \
+	rsbegin.o \
+	rsend.o \
+	dllcrt2.o \
+	crt2.o
+
+# Generate a list of all crates in the sysroot. To do this we list all files in
+# rustc's sysroot, look at the filename, strip everything after the `-`, and
+# strip the leading `lib` (if present)
+SYSROOT := $(shell $(RUSTC) --print sysroot)
+LIBS := $(wildcard $(SYSROOT)/lib/rustlib/$(TARGET)/lib/*)
+LIBS := $(foreach lib,$(LIBS),$(notdir $(lib)))
+LIBS := $(foreach lib,$(LIBS),$(word 1,$(subst -, ,$(lib))))
+LIBS := $(foreach lib,$(LIBS),$(patsubst lib%,%,$(lib)))
+LIBS := $(filter-out $(STABLE_CRATES),$(LIBS))
+
+all: $(foreach lib,$(LIBS),check-crate-$(lib)-is-unstable)
+
+check-crate-%-is-unstable:
+	@echo verifying $* is an unstable crate
+	@echo 'extern crate $*;' | \
+		$(RUSTC) - --crate-type rlib 2>&1 | cat > $(TMPDIR)/$*; \
+		true
+	@grep -q 'use of unstable library feature' $(TMPDIR)/$* || \
+		(echo crate $* is not unstable && \
+		cat $(TMPDIR)/$* && \
+		false)