Suggest removing ? to resolve type errors.

This commit adds a suggestion to remove the `?` from expressions if
removing the `?` would resolve a type error.

Author: davidtwco

src/librustc/infer/error_reporting/mod.rs

@@ -604,13 +604,39 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 source,
                 ref prior_arms,
                 last_ty,
+                discrim_hir_id,
                 ..
             } => match source {
                 hir::MatchSource::IfLetDesugar { .. } => {
                     let msg = "`if let` arms have incompatible types";
                     err.span_label(cause.span, msg);
                 }
-                hir::MatchSource::TryDesugar => {}
+                hir::MatchSource::TryDesugar => {
+                    if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
+                        let discrim_expr = self.tcx.hir().expect_expr_by_hir_id(discrim_hir_id);
+                        let discrim_ty = if let hir::ExprKind::Call(_, args) = &discrim_expr.node {
+                            let arg_expr = args.first().expect("try desugaring call w/out arg");
+                            self.in_progress_tables.and_then(|tables| {
+                                tables.borrow().expr_ty_opt(arg_expr)
+                            })
+                        } else {
+                            bug!("try desugaring w/out call expr as discriminant");
+                        };
+
+                        match discrim_ty {
+                            Some(ty) if expected == ty => {
+                                let source_map = self.tcx.sess.source_map();
+                                err.span_suggestion(
+                                    source_map.end_point(cause.span),
+                                    "try removing this `?`",
+                                    "".to_string(),
+                                    Applicability::MachineApplicable,
+                                );
+                            },
+                            _ => {},
+                        }
+                    }
+                }
                 _ => {
                     let msg = "`match` arms have incompatible types";
                     err.span_label(cause.span, msg);

src/librustc/traits/mod.rs

@@ -227,6 +227,7 @@ pub enum ObligationCauseCode<'tcx> {
         source: hir::MatchSource,
         prior_arms: Vec<Span>,
         last_ty: Ty<'tcx>,
+        discrim_hir_id: hir::HirId,
     },
 
     /// Computing common supertype in the pattern guard for the arms of a match expression

src/librustc/traits/structural_impls.rs

@@ -519,13 +519,15 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 source,
                 ref prior_arms,
                 last_ty,
+                discrim_hir_id,
             } => {
                 tcx.lift(&last_ty).map(|last_ty| {
                     super::MatchExpressionArm {
                         arm_span,
                         source,
                         prior_arms: prior_arms.clone(),
                         last_ty,
+                        discrim_hir_id,
                     }
                 })
             }

src/librustc_typeck/check/_match.rs

@@ -732,6 +732,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
                         source: match_src,
                         prior_arms: other_arms.clone(),
                         last_ty: prior_arm_ty.unwrap(),
+                        discrim_hir_id: discrim.hir_id,
                     })
                 };
                 coercion.coerce(self, &cause, &arm.body, arm_ty);

src/test/ui/issue-59756.fixed

@@ -0,0 +1,17 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+struct A;
+struct B;
+
+fn foo() -> Result<A, B> {
+    Ok(A)
+}
+
+fn bar() -> Result<A, B> {
+    foo()
+    //~^ ERROR try expression alternatives have incompatible types [E0308]
+}
+
+fn main() {}

src/test/ui/issue-59756.rs

@@ -1,3 +1,5 @@
+// run-rustfix
+
 #![allow(warnings)]
 
 struct A;

src/test/ui/issue-59756.stderr

@@ -1,8 +1,11 @@
 error[E0308]: try expression alternatives have incompatible types
-  --> $DIR/issue-59756.rs:11:5
+  --> $DIR/issue-59756.rs:13:5
    |
 LL |     foo()?
-   |     ^^^^^^ expected enum `std::result::Result`, found struct `A`
+   |     ^^^^^-
+   |     |    |
+   |     |    help: try removing this `?`
+   |     expected enum `std::result::Result`, found struct `A`
    |
    = note: expected type `std::result::Result<A, B>`
               found type `A`

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

@@ -2,7 +2,10 @@ error[E0308]: try expression alternatives have incompatible types
   --> $DIR/issue-51632-try-desugar-incompatible-types.rs:8:5
    |
 LL |     missing_discourses()?
-   |     ^^^^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found isize
+   |     ^^^^^^^^^^^^^^^^^^^^-
+   |     |                   |
+   |     |                   help: try removing this `?`
+   |     expected enum `std::result::Result`, found isize
    |
    = note: expected type `std::result::Result<isize, ()>`
               found type `isize`