Explain that ? converts the error type using From

Author: estebank

src/librustc/traits/error_reporting.rs

@@ -643,13 +643,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             .map(|s| &s == "?")
                             .unwrap_or(false);
                         let is_from = format!("{}", trait_ref).starts_with("std::convert::From<");
-                        let message = if is_try && is_from {
-                            Some(format!(
+                        let (message, note) = if is_try && is_from {
+                            (Some(format!(
                                 "`?` couldn't convert the error to `{}`",
                                 trait_ref.self_ty(),
+                            )), Some(
+                                "the question mark operation (`?`) implicitly performs a \
+                                 conversion on the error value using the `From` trait".to_owned()
                             ))
                         } else {
-                            message
+                            (message, note)
                         };
 
                         let mut err = struct_span_err!(

src/test/ui/issues/issue-32709.stderr

@@ -4,6 +4,7 @@ error[E0277]: `?` couldn't convert the error to `()`
 LL |     Err(5)?;
    |           ^ the trait `std::convert::From<{integer}>` is not implemented for `()`
    |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = note: required by `std::convert::From::from`
 
 error: aborting due to previous error

src/test/ui/try-block/try-block-bad-type.stderr

@@ -4,6 +4,7 @@ error[E0277]: `?` couldn't convert the error to `i32`
 LL |         Err("")?;
    |                ^ the trait `std::convert::From<&str>` is not implemented for `i32`
    |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = help: the following implementations were found:
              <i32 as std::convert::From<bool>>
              <i32 as std::convert::From<i16>>

src/test/ui/try-on-option.stderr

@@ -4,6 +4,7 @@ error[E0277]: `?` couldn't convert the error to `()`
 LL |     x?;
    |      ^ the trait `std::convert::From<std::option::NoneError>` is not implemented for `()`
    |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = note: required by `std::convert::From::from`
 
 error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)