Index: head/lang/rust-nightly/Makefile
===================================================================
--- head/lang/rust-nightly/Makefile
+++ head/lang/rust-nightly/Makefile
@@ -23,6 +23,8 @@
 CONFLICTS_BUILD=	rust # see FIXME in `pre-fetch` in lang/rust/Makefile
 CONFLICTS_INSTALL=	rust
 
+PKGMESSAGE=	${.CURDIR}/../rust/pkg-message
+
 RUST_BOOTSTRAP_DIR=		2017-02-01
 RUST_BOOTSTRAP_VERSION=		beta
 
Index: head/lang/rust/pkg-message
===================================================================
--- head/lang/rust/pkg-message
+++ head/lang/rust/pkg-message
@@ -0,0 +1,12 @@
+======================================================================
+
+Printing Rust backtraces requires procfs(5) mounted on /proc .
+If you have not already done so, please do the following:
+
+	mount -t procfs proc /proc
+
+To make it permanent, you need the following lines in /etc/fstab:
+
+	proc	/proc		procfs		rw	0	0
+
+======================================================================