Make diagnostics clearer for ? operator

src/librustc_infer/infer/error_reporting/mod.rs

@@ -1604,6 +1604,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         trace: TypeTrace<'tcx>,
         terr: &TypeError<'tcx>,
     ) -> DiagnosticBuilder<'tcx> {
+        use crate::traits::ObligationCauseCode::MatchExpressionArm;
         debug!("report_and_explain_type_error(trace={:?}, terr={:?})", trace, terr);
 
         let span = trace.cause.span(self.tcx);
@@ -1620,7 +1621,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
             }
             FailureCode::Error0308(failure_str) => {
-                struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
+                let mut diag = struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str);
+                if let MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =
+                    trace.cause.code
+                {
+                    if let hir::MatchSource::TryDesugar = source {
+                        if let Some((expected_ty, found_ty)) = self.values_str(&trace.values) {
+                            diag.note(&format!(
+                                "`?` operator cannot convert from `{}` to `{}`",
+                                found_ty.content(),
+                                expected_ty.content()
+                            ));
+                            diag.help(
+                                "`?` operator unwraps `Result` or `Option` \
+                                and propagates its `Err` or `None`",
+                            );
+                        }
+                    }
+                }
+                diag
             }
             FailureCode::Error0644(failure_str) => {
                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
@@ -2088,9 +2107,7 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
                     hir::MatchSource::IfLetDesugar { .. } => {
                         "`if let` arms have incompatible types"
                     }
-                    hir::MatchSource::TryDesugar => {
-                        "try expression alternatives have incompatible types"
-                    }
+                    hir::MatchSource::TryDesugar => "`?` operator has incompatible types",
                     _ => "`match` arms have incompatible types",
                 })
             }

src/test/ui/issues/issue-51632-try-desugar-incompatible-types.rs

@@ -6,7 +6,7 @@ fn missing_discourses() -> Result<isize, ()> {
 
 fn forbidden_narratives() -> Result<isize, ()> {
     missing_discourses()?
-    //~^ ERROR try expression alternatives have incompatible types
+    //~^ ERROR: `?` operator has incompatible types
 }
 
 fn main() {}

src/test/ui/issues/issue-51632-try-desugar-incompatible-types.stderr

@@ -1,9 +1,11 @@
-error[E0308]: try expression alternatives have incompatible types
+error[E0308]: `?` operator has incompatible types
   --> $DIR/issue-51632-try-desugar-incompatible-types.rs:8:5
    |
 LL |     missing_discourses()?
    |     ^^^^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found `isize`
    |
+   = note: `?` operator cannot convert from `isize` to `std::result::Result<isize, ()>`
+   = help: `?` operator unwraps `Result` or `Option` and propagates its `Err` or `None`
    = note: expected enum `std::result::Result<isize, ()>`
               found type `isize`
 help: try removing this `?`

src/test/ui/issues/issue-59756.rs

@@ -15,7 +15,7 @@ fn foo() -> Result<A, B> {
 
 fn bar() -> Result<A, B> {
     foo()?
-    //~^ ERROR try expression alternatives have incompatible types [E0308]
+    //~^ ERROR `?` operator has incompatible types [E0308]
 }
 
 fn main() {}

src/test/ui/issues/issue-71309.rs

@@ -0,0 +1,7 @@
+fn foo(x: Result<i32, ()>) -> Result<(), ()> {
+    let y: u32 = x?;
+    //~^ ERROR: `?` operator has incompatible types
+    Ok(())
+}
+
+fn main() {}

src/test/ui/issues/issue-71309.stderr

@@ -0,0 +1,16 @@
+error[E0308]: `?` operator has incompatible types
+  --> $DIR/issue-71309.rs:2:18
+   |
+LL |     let y: u32 = x?;
+   |                  ^^ expected `u32`, found `i32`
+   |
+   = note: `?` operator cannot convert from `i32` to `u32`
+   = help: `?` operator unwraps `Result` or `Option` and propagates its `Err` or `None`
+help: you can convert an `i32` to `u32` and panic if the converted value wouldn't fit
+   |
+LL |     let y: u32 = x?.try_into().unwrap();
+   |                  ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.