add error message for the other case too

src/libcore/ops/try.rs

@@ -27,7 +27,12 @@
                   message="the `?` operator can only be used in a \
                            function that returns `Result` \
                            (or another type that implements `{Try}`)",
-                  label="cannot use the `?` operator in a function that returns `{Self}`")))]
+                  label="cannot use the `?` operator in a function that returns `{Self}`"),
+               on(all(from_method="into_result", from_desugaring="?"),
+                  message="the `?` operator can only be applied to values \
+                           that implement `{Try}`",
+                  label="the `?` operator cannot be applied to type `{Self}`")
+))]
 pub trait Try {
     /// The type of this value when viewed as successful.
     #[unstable(feature = "try_trait", issue = "42327")]

src/test/ui/suggestions/try-operator-on-main.rs

@@ -16,7 +16,7 @@ fn main() {
     // error for a `Try` type on a non-`Try` fn
     std::fs::File::open("foo")?;
 
-    // a non-`Try` type on a `Try` fn
+    // a non-`Try` type on a non-`Try` fn
     ()?;
 
     // an unrelated use of `Try`

src/test/ui/suggestions/try-operator-on-main.stderr

@@ -10,15 +10,16 @@ error[E0277]: the `?` operator can only be used in a function that returns `Resu
    = help: the trait `std::ops::Try` is not implemented for `()`
    = note: required by `std::ops::Try::from_error`
 
-error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
   --> $DIR/try-operator-on-main.rs:20:5
    |
 20 |     ()?;
    |     ---
    |     |
-   |     the trait `std::ops::Try` is not implemented for `()`
+   |     the `?` operator cannot be applied to type `()`
    |     in this macro invocation
    |
+   = help: the trait `std::ops::Try` is not implemented for `()`
    = note: required by `std::ops::Try::into_result`
 
 error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
@@ -29,15 +30,16 @@ error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
    |
    = note: required by `try_trait_generic`
 
-error[E0277]: the trait bound `(): std::ops::Try` is not satisfied
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
   --> $DIR/try-operator-on-main.rs:30:5
    |
 30 |     ()?;
    |     ---
    |     |
-   |     the trait `std::ops::Try` is not implemented for `()`
+   |     the `?` operator cannot be applied to type `()`
    |     in this macro invocation
    |
+   = help: the trait `std::ops::Try` is not implemented for `()`
    = note: required by `std::ops::Try::into_result`
 
 error: aborting due to 4 previous errors