Give custom error for E0277 on ? error case

Author: estebank

src/librustc/traits/error_reporting.rs

@@ -638,6 +638,18 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                         let OnUnimplementedNote { message, label, note }
                             = self.on_unimplemented_note(trait_ref, obligation);
                         let have_alt_message = message.is_some() || label.is_some();
+                        let is_try = self.tcx.sess.source_map().span_to_snippet(span)
+                            .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!(
+                                "`?` couldn't convert the error to `{}`",
+                                trait_ref.self_ty(),
+                            ))
+                        } else {
+                            message
+                        };
 
                         let mut err = struct_span_err!(
                             self.tcx.sess,

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

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `(): std::convert::From<{integer}>` is not satisfied
+error[E0277]: `?` couldn't convert the error to `()`
   --> $DIR/issue-32709.rs:4:11
    |
 LL |     Err(5)?;

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

@@ -4,7 +4,7 @@
 
 pub fn main() {
     let res: Result<u32, i32> = try {
-        Err("")?; //~ ERROR the trait bound `i32: std::convert::From<&str>` is not satisfied
+        Err("")?; //~ ERROR `?` couldn't convert the error
         5
     };
 

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

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `i32: std::convert::From<&str>` is not satisfied
+error[E0277]: `?` couldn't convert the error to `i32`
   --> $DIR/try-block-bad-type.rs:7:16
    |
 LL |         Err("")?;

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

@@ -4,12 +4,12 @@ fn main() {}
 
 fn foo() -> Result<u32, ()> {
     let x: Option<u32> = None;
-    x?; //~ the trait bound
+    x?; //~ ERROR `?` couldn't convert the error
     Ok(22)
 }
 
 fn bar() -> u32 {
     let x: Option<u32> = None;
-    x?; //~ the `?` operator
+    x?; //~ ERROR the `?` operator
     22
 }

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

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `(): std::convert::From<std::option::NoneError>` is not satisfied
+error[E0277]: `?` couldn't convert the error to `()`
   --> $DIR/try-on-option.rs:7:6
    |
 LL |     x?;