in which we plug the crack where ?-desugaring leaked into errors

src/librustc/infer/error_reporting/mod.rs

@@ -500,7 +500,17 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     } else {
                         err.span_label(arm_span, msg);
                     }
-                }
+                },
+                hir::MatchSource::TryDesugar => { // Issue #51632
+                    if let Ok(try_snippet) = self.tcx.sess.codemap().span_to_snippet(arm_span) {
+                        err.span_suggestion_with_applicability(
+                            arm_span,
+                            "try wrapping with a success variant",
+                            format!("Ok({})", try_snippet),
+                            Applicability::MachineApplicable
+                        );
+                    }
+                },
                 _ => {
                     let msg = "match arm with an incompatible type";
                     if self.tcx.sess.codemap().is_multiline(arm_span) {
@@ -1294,7 +1304,12 @@ impl<'tcx> ObligationCause<'tcx> {
         match self.code {
             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
             MatchExpressionArm { source, .. } => Error0308(match source {
-                hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have incompatible types",
+                hir::MatchSource::IfLetDesugar { .. } => {
+                    "`if let` arms have incompatible types"
+                },
+                hir::MatchSource::TryDesugar => {
+                    "try expression alternatives have incompatible types"
+                },
                 _ => "match arms have incompatible types",
             }),
             IfExpression => Error0308("if and else have incompatible types"),

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

@@ -0,0 +1,25 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+fn missing_discourses() -> Result<isize, ()> {
+    Ok(1)
+}
+
+fn forbidden_narratives() -> Result<isize, ()> {
+    Ok(missing_discourses()?)
+    //~^ ERROR try expression alternatives have incompatible types
+    //~| HELP try wrapping with a success variant
+}
+
+fn main() {}

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

@@ -0,0 +1,25 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+fn missing_discourses() -> Result<isize, ()> {
+    Ok(1)
+}
+
+fn forbidden_narratives() -> Result<isize, ()> {
+    missing_discourses()?
+    //~^ ERROR try expression alternatives have incompatible types
+    //~| HELP try wrapping with a success variant
+}
+
+fn main() {}

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

@@ -0,0 +1,15 @@
+error[E0308]: try expression alternatives have incompatible types
+  --> $DIR/issue-51632-try-desugar-incompatible-types.rs:20:5
+   |
+LL |     missing_discourses()?
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     expected enum `std::result::Result`, found isize
+   |     help: try wrapping with a success variant: `Ok(missing_discourses()?)`
+   |
+   = note: expected type `std::result::Result<isize, ()>`
+              found type `isize`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.