Update check_tree to better detect errors in cargo-tree.

maint/check_tree

@@ -3,17 +3,37 @@
 # Use cargo-tree to check our dependencies for crates which we must
 # not depend on unconditionally.
 
+set -eu
 
 forbid () {
     local our_crate="$1"
     local feature="$2"
     local forbidden="$3"
-    if cargo tree -i "$forbidden" -p "$our_crate" --features "$feature" 2>/dev/null ; then
-        echo "Uh-oh: $forbidden has shown up in $our_crate/$feature."
-	exit 1
-    else
-        echo "Didn't find $forbidden in $our_crate/$feature.  Good."
-    fi
+
+    set +e
+    cargo tree --prefix=none -p "$our_crate" --features "$feature" \
+	  --format="    {p}" | grep "^    $forbidden"
+    local result="${PIPESTATUS[*]}"
+    set -e
+
+    case "$result" in
+	"0 0")
+	    # cargo-tree succeeded, and so did grep: we found the
+	    # forbidden package.
+            echo "Uh-oh: $forbidden has shown up in $our_crate/$feature."
+    	    exit 1
+	    ;;
+	"0 1")
+	    # cargo-tree succeeded, and grep failed: we didn't find the
+	    # forbidden package.
+	    echo "Didn't find $forbidden in $our_crate/$feature.  Good."
+	    ;;
+	*)
+	    # cargo-tree failed (or maybe grep is gruesomely nonstandard)
+	    echo "cargo tree failed unexpectedly when checking for $forbidden in $our_crate/$feature" >&2
+	    exit 1
+	    ;;
+    esac
 }
 
 # We can't use these crates in arti/full, since they expose us to the old