According to rust-lang/rust#40123 rust now supports LLVM 4

vaartis · vivo75 · commit 0acc41ec8359 · 2017-09-06T21:44:35.000+02:00
diff --git a/dev-lang/rust/rust-9999.ebuild b/dev-lang/rust/rust-9999.ebuild
@@ -34,7 +34,7 @@ CDEPEND="libcxx? ( sys-libs/libcxx )
 	>=app-eselect/eselect-rust-0.3_pre20150425
 	!dev-lang/rust:0
 	system-llvm? ( >=sys-devel/llvm-3.8.1-r2
-			<sys-devel/llvm-3.10.0 )
+			<sys-devel/llvm-5.0.0 )
 "
 DEPEND="${CDEPEND}
 	${PYTHON_DEPS}

Original file line number	Diff line number	Diff line change
`@@ -34,7 +34,7 @@ CDEPEND="libcxx? ( sys-libs/libcxx )`
`34`	`34`	`>=app-eselect/eselect-rust-0.3_pre20150425`
`35`	`35`	`!dev-lang/rust:0`
`36`	`36`	`system-llvm? ( >=sys-devel/llvm-3.8.1-r2`
`37`		`- <sys-devel/llvm-3.10.0 )`
	`37`	`+ <sys-devel/llvm-5.0.0 )`
`38`	`38`	`"`
`39`	`39`	`DEPEND="${CDEPEND}`
`40`	`40`	`${PYTHON_DEPS}`