Emit a single error on if expr with expectation and no else clause

src/librustc_typeck/check/_match.rs

@@ -112,19 +112,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
 
             self.diverges.set(pats_diverge);
-            let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
-            all_arms_diverge &= self.diverges.get();
-
-            let span = expr.span;
-
+            let arm_ty = if source_if && if_no_else && i != 0 && self.if_fallback_coercion(
+                expr.span,
+                &arms[0].body,
+                &mut coercion,
+            ) {
+                    tcx.types.err
+            } else {
+                // Only call this if this is not an `if` expr with an expected type and no `else`
+                // clause to avoid duplicated type errors. (#60254)
+                let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
+                all_arms_diverge &= self.diverges.get();
+                arm_ty
+            };
             if source_if {
                 let then_expr = &arms[0].body;
                 match (i, if_no_else) {
-                    (0, _) => coercion.coerce(self, &self.misc(span), &arm.body, arm_ty),
-                    (_, true) => self.if_fallback_coercion(span, then_expr, &mut coercion),
+                    (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
+                    (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
                     (_, _) => {
                         let then_ty = prior_arm_ty.unwrap();
-                        let cause = self.if_cause(span, then_expr, &arm.body, then_ty, arm_ty);
+                        let cause = self.if_cause(expr.span, then_expr, &arm.body, then_ty, arm_ty);
                         coercion.coerce(self, &cause, &arm.body, arm_ty);
                     }
                 }
@@ -139,7 +147,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // The reason for the first arm to fail is not that the match arms diverge,
                     // but rather that there's a prior obligation that doesn't hold.
                     0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
-                    _ => (span, ObligationCauseCode::MatchExpressionArm {
+                    _ => (expr.span, ObligationCauseCode::MatchExpressionArm {
                         arm_span,
                         source: match_src,
                         prior_arms: other_arms.clone(),
@@ -185,11 +193,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
         then_expr: &'tcx hir::Expr,
         coercion: &mut CoerceMany<'tcx, '_, rustc::hir::Arm>,
-    ) {
+    ) -> bool {
         // If this `if` expr is the parent's function return expr,
         // the cause of the type coercion is the return type, point at it. (#25228)
         let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
         let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
+        let mut error = false;
         coercion.coerce_forced_unit(self, &cause, &mut |err| {
             if let Some((span, msg)) = &ret_reason {
                 err.span_label(*span, msg.as_str());
@@ -200,7 +209,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
             err.note("`if` expressions without `else` evaluate to `()`");
             err.help("consider adding an `else` block that evaluates to the expected type");
+            error = true;
         }, ret_reason.is_none());
+        error
     }
 
     fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {

src/test/ui/if/if-without-else-as-fn-expr.rs

@@ -3,15 +3,13 @@ fn foo(bar: usize) -> usize {
         return 3;
     }
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
 }
 
 fn foo2(bar: usize) -> usize {
     let x: usize = if bar % 5 == 0 {
         return 3;
     };
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
     x
 }
 
@@ -20,23 +18,20 @@ fn foo3(bar: usize) -> usize {
         3
     }
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
 }
 
 fn foo_let(bar: usize) -> usize {
     if let 0 = 1 {
         return 3;
     }
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
 }
 
 fn foo2_let(bar: usize) -> usize {
     let x: usize = if let 0 = 1 {
         return 3;
     };
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
     x
 }
 
@@ -45,7 +40,6 @@ fn foo3_let(bar: usize) -> usize {
         3
     }
     //~^^^ ERROR if may be missing an else clause
-    //~| ERROR mismatched types [E0308]
 }
 
 // FIXME(60254): deduplicate first error in favor of second.

src/test/ui/if/if-without-else-as-fn-expr.stderr

@@ -1,14 +1,3 @@
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:2:5
-   |
-LL | /     if bar % 5 == 0 {
-LL | |         return 3;
-LL | |     }
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
   --> $DIR/if-without-else-as-fn-expr.rs:2:5
    |
@@ -24,20 +13,8 @@ LL | |     }
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:10:20
-   |
-LL |       let x: usize = if bar % 5 == 0 {
-   |  ____________________^
-LL | |         return 3;
-LL | |     };
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
-  --> $DIR/if-without-else-as-fn-expr.rs:10:20
+  --> $DIR/if-without-else-as-fn-expr.rs:9:20
    |
 LL |       let x: usize = if bar % 5 == 0 {
    |  _________-__________^
@@ -52,19 +29,8 @@ LL | |     };
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:19:5
-   |
-LL | /     if bar % 5 == 0 {
-LL | |         3
-LL | |     }
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
-  --> $DIR/if-without-else-as-fn-expr.rs:19:5
+  --> $DIR/if-without-else-as-fn-expr.rs:17:5
    |
 LL |   fn foo3(bar: usize) -> usize {
    |                          ----- expected `usize` because of this return type
@@ -78,19 +44,8 @@ LL | |     }
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:27:5
-   |
-LL | /     if let 0 = 1 {
-LL | |         return 3;
-LL | |     }
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
-  --> $DIR/if-without-else-as-fn-expr.rs:27:5
+  --> $DIR/if-without-else-as-fn-expr.rs:24:5
    |
 LL |   fn foo_let(bar: usize) -> usize {
    |                             ----- expected `usize` because of this return type
@@ -104,20 +59,8 @@ LL | |     }
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:35:20
-   |
-LL |       let x: usize = if let 0 = 1 {
-   |  ____________________^
-LL | |         return 3;
-LL | |     };
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
-  --> $DIR/if-without-else-as-fn-expr.rs:35:20
+  --> $DIR/if-without-else-as-fn-expr.rs:31:20
    |
 LL |       let x: usize = if let 0 = 1 {
    |  _________-__________^
@@ -132,19 +75,8 @@ LL | |     };
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error[E0308]: mismatched types
-  --> $DIR/if-without-else-as-fn-expr.rs:44:5
-   |
-LL | /     if let 0 = 1 {
-LL | |         3
-LL | |     }
-   | |_____^ expected usize, found ()
-   |
-   = note: expected type `usize`
-              found type `()`
-
 error[E0317]: if may be missing an else clause
-  --> $DIR/if-without-else-as-fn-expr.rs:44:5
+  --> $DIR/if-without-else-as-fn-expr.rs:39:5
    |
 LL |   fn foo3_let(bar: usize) -> usize {
    |                              ----- expected `usize` because of this return type
@@ -158,7 +90,6 @@ LL | |     }
    = note: `if` expressions without `else` evaluate to `()`
    = help: consider adding an `else` block that evaluates to the expected type
 
-error: aborting due to 12 previous errors
+error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0308, E0317.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0317`.

src/test/ui/issues/issue-50577.rs

@@ -2,6 +2,5 @@ fn main() {
     enum Foo {
         Drop = assert_eq!(1, 1)
         //~^ ERROR if may be missing an else clause
-        //~| ERROR mismatched types [E0308]
     }
 }

src/test/ui/issues/issue-50577.stderr

@@ -1,13 +1,3 @@
-error[E0308]: mismatched types
-  --> $DIR/issue-50577.rs:3:16
-   |
-LL |         Drop = assert_eq!(1, 1)
-   |                ^^^^^^^^^^^^^^^^ expected isize, found ()
-   |
-   = note: expected type `isize`
-              found type `()`
-   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
-
 error[E0317]: if may be missing an else clause
   --> $DIR/issue-50577.rs:3:16
    |
@@ -23,7 +13,6 @@ LL |         Drop = assert_eq!(1, 1)
    = help: consider adding an `else` block that evaluates to the expected type
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0308, E0317.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0317`.