From 9946ff227d3b1ec7ee5de9ef9b2f81ee4547df34 Mon Sep 17 00:00:00 2001
From: Fabian Wolff <fabian.wolff@alumni.ethz.ch>
Date: Sun, 11 Jul 2021 17:04:11 +0200
Subject: [PATCH] Do not suggest adding a semicolon after `?`

---
 compiler/rustc_typeck/src/check/coercion.rs   | 10 ++++--
 .../try-operator-dont-suggest-semicolon.rs    | 27 +++++++++++++++
 ...try-operator-dont-suggest-semicolon.stderr | 33 +++++++++++++++++++
 3 files changed, 67 insertions(+), 3 deletions(-)
 create mode 100644 src/test/ui/suggestions/try-operator-dont-suggest-semicolon.rs
 create mode 100644 src/test/ui/suggestions/try-operator-dont-suggest-semicolon.stderr

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index afa4d0f1c4d..9e89804b747 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -1456,11 +1456,15 @@ fn report_return_mismatched_types<'a>(
                 expected.is_unit(),
                 pointing_at_return_type,
             ) {
-                // If the block is from an external macro, then do not suggest
-                // adding a semicolon, because there's nowhere to put it.
-                // See issue #81943.
+                // If the block is from an external macro or try (`?`) desugaring, then
+                // do not suggest adding a semicolon, because there's nowhere to put it.
+                // See issues #81943 and #87051.
                 if cond_expr.span.desugaring_kind().is_none()
                     && !in_external_macro(fcx.tcx.sess, cond_expr.span)
+                    && !matches!(
+                        cond_expr.kind,
+                        hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
+                    )
                 {
                     err.span_label(cond_expr.span, "expected this to be `()`");
                     if expr.can_have_side_effects() {
diff --git a/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.rs b/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.rs
new file mode 100644
index 00000000000..f882a159f98
--- /dev/null
+++ b/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.rs
@@ -0,0 +1,27 @@
+// Regression test for #87051, where a double semicolon was erroneously
+// suggested after a `?` operator.
+
+fn main() -> Result<(), ()> {
+    a(|| {
+        b()
+        //~^ ERROR: mismatched types [E0308]
+        //~| NOTE: expected `()`, found `i32`
+        //~| HELP: consider using a semicolon here
+    })?;
+
+    // Here, we do want to suggest a semicolon:
+    let x = Ok(42);
+    if true {
+    //~^ NOTE: expected this to be `()`
+        x?
+        //~^ ERROR: mismatched types [E0308]
+        //~| NOTE: expected `()`, found integer
+        //~| HELP: consider using a semicolon here
+    }
+    //~^ HELP: consider using a semicolon here
+
+    Ok(())
+}
+
+fn a<F>(f: F) -> Result<(), ()> where F: FnMut() { Ok(()) }
+fn b() -> i32 { 42 }
diff --git a/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.stderr b/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.stderr
new file mode 100644
index 00000000000..4f7e18742e2
--- /dev/null
+++ b/src/test/ui/suggestions/try-operator-dont-suggest-semicolon.stderr
@@ -0,0 +1,33 @@
+error[E0308]: mismatched types
+  --> $DIR/try-operator-dont-suggest-semicolon.rs:6:9
+   |
+LL |         b()
+   |         ^^^- help: consider using a semicolon here: `;`
+   |         |
+   |         expected `()`, found `i32`
+
+error[E0308]: mismatched types
+  --> $DIR/try-operator-dont-suggest-semicolon.rs:16:9
+   |
+LL | /     if true {
+LL | |
+LL | |         x?
+   | |         ^^ expected `()`, found integer
+LL | |
+LL | |
+LL | |
+LL | |     }
+   | |_____- expected this to be `()`
+   |
+help: consider using a semicolon here
+   |
+LL |         x?;
+   |           ^
+help: consider using a semicolon here
+   |
+LL |     };
+   |      ^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.