added ui test, clarified error message

Author: nebulark

compiler/rustc_session/src/options.rs

@@ -380,7 +380,7 @@ mod desc {
     pub const parse_panic_strategy: &str = "either `unwind` or `abort`";
     pub const parse_on_broken_pipe: &str = "either `kill`, `error`, or `inherit`";
     pub const parse_patchable_function_entry: &str =
-        "either two comma separated integers (total_nops,prefix_nops) or one integer (total_nops)";
+        "either two comma separated integers (total_nops,prefix_nops), with prefix_nops <= total_nops, or one integer (total_nops)";
     pub const parse_opt_panic_strategy: &str = parse_panic_strategy;
     pub const parse_oom_strategy: &str = "either `panic` or `abort`";
     pub const parse_relro_level: &str = "one of: `full`, `partial`, or `off`";

tests/ui/patchable-function-entry/patchable-function-entry-flags.rs

@@ -0,0 +1,2 @@
+//@ compile-flags: -Z patchable-function-entry=1,2
+fn main() {}

tests/ui/patchable-function-entry/patchable-function-entry-flags.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `1,2` for unstable option `patchable-function-entry` - either two comma separated integers (total_nops,prefix_nops), with prefix_nops <= total_nops, or one integer (total_nops) was expected
+