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

Most of the time, it's not a problem that the types of the arm bodies in
a desugared-from-`?` match are different (that is, specifically: in `x?`
where x is a `Result<A, B>`, the `Ok` arm body is an `A`, whereas the
`Err` arm diverges to return a `Result<A, B>`), because they're being
assigned to different places. But in tail position, the types do need to
match, and our error message was explicitly referring to "match arms",
which is confusing when there's no `match` in the sweetly sugared
source.

It is not without some misgivings that we pollute the clarity-of-purpose
of `note_error_origin` with the suggestion to wrap with `Ok` (the other
branches are pointing out the odd-arm-out in the HIR that is the origin
of the error; the new branch that issues the `Ok` suggestion is serving
a different purpose), but it's the natural place to do it given that
we're already matching on `ObligationCauseCode::MatchExpressionArm {
arm_span, source }` there.

Resolves rust-lang#51632.

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`.