Update error message

src/libsyntax/test.rs

@@ -379,9 +379,7 @@ fn is_test_fn(cx: &TestCtxt, i: &ast::Item) -> bool {
                     BadTestSignature::NoArgumentsAllowed =>
                         diag.span_err(i.span, "functions used as tests can not have any arguments"),
                     BadTestSignature::ShouldPanicOnlyWithNoArgs =>
-                        diag.span_err(i.span,
-                                      "functions used as tests returning Result<_, _> must \
-                                      not use #[should_panic]"),
+                        diag.span_err(i.span, "functions using `#[should_panic]` must return `()`"),
                 }
                 false
             }

src/test/ui/rfc-1937-termination-trait/termination-trait-in-test-should-panic.rs

@@ -20,7 +20,7 @@ use test::Bencher;
 #[test]
 #[should_panic]
 fn not_a_num() -> Result<(), ParseIntError> {
-    //~^ ERROR functions used as tests returning Result<_, _> must not use #[should_panic]
+    //~^ ERROR functions using `#[should_panic]` must return `()`
     let _: u32 = "abc".parse()?;
     Ok(())
 }

src/test/ui/rfc-1937-termination-trait/termination-trait-in-test-should-panic.stderr

@@ -1,8 +1,8 @@
-error: functions used as tests returning Result<_, _> must not use #[should_panic]
+error: functions using `#[should_panic]` must return `()`
   --> $DIR/termination-trait-in-test-should-panic.rs:22:1
    |
 LL | / fn not_a_num() -> Result<(), ParseIntError> {
-LL | |     //~^ ERROR functions used as tests returning Result<_, _> must not use #[should_panic]
+LL | |     //~^ ERROR functions using `#[should_panic]` must return `()`
 LL | |     let _: u32 = "abc".parse()?;
 LL | |     Ok(())
 LL | | }