Suggest adding an appropriate missing pattern excluding comments

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -849,22 +849,22 @@ fn non_exhaustive_match<'p, 'tcx>(
             ));
         }
         [.., prev, last] if prev.span.eq_ctxt(last.span) => {
-            if let Ok(snippet) = sm.span_to_snippet(prev.span.between(last.span)) {
-                let comma = if matches!(last.body.kind, hir::ExprKind::Block(..))
-                    && last.span.eq_ctxt(last.body.span)
-                {
-                    ""
-                } else {
-                    ","
-                };
+            let comma = if matches!(last.body.kind, hir::ExprKind::Block(..))
+                && last.span.eq_ctxt(last.body.span)
+            {
+                ""
+            } else {
+                ","
+            };
+            let spacing = if sm.is_multiline(prev.span.between(last.span)) {
+                sm.indentation_before(last.span).map(|indent| format!("\n{indent}"))
+            } else {
+                Some(" ".to_string())
+            };
+            if let Some(spacing) = spacing {
                 suggestion = Some((
                     last.span.shrink_to_hi(),
-                    format!(
-                        "{}{}{} => todo!()",
-                        comma,
-                        snippet.strip_prefix(',').unwrap_or(&snippet),
-                        pattern
-                    ),
+                    format!("{}{}{} => todo!()", comma, spacing, pattern),
                 ));
             }
         }

src/test/ui/pattern/suggest-adding-appropriate-missing-pattern-excluding-comments.fixed

@@ -0,0 +1,10 @@
+// run-rustfix
+
+fn main() {
+    match Some(1) { //~ ERROR non-exhaustive patterns: `None` not covered
+        Some(1) => {}
+        // hello
+        Some(_) => {}
+        None => todo!()
+    }
+}

src/test/ui/pattern/suggest-adding-appropriate-missing-pattern-excluding-comments.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+fn main() {
+    match Some(1) { //~ ERROR non-exhaustive patterns: `None` not covered
+        Some(1) => {}
+        // hello
+        Some(_) => {}
+    }
+}

src/test/ui/pattern/suggest-adding-appropriate-missing-pattern-excluding-comments.stderr

@@ -0,0 +1,24 @@
+error[E0004]: non-exhaustive patterns: `None` not covered
+  --> $DIR/suggest-adding-appropriate-missing-pattern-excluding-comments.rs:4:11
+   |
+LL |     match Some(1) {
+   |           ^^^^^^^ pattern `None` not covered
+   |
+note: `Option<i32>` defined here
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+   |
+LL | pub enum Option<T> {
+   | ------------------
+...
+LL |     None,
+   |     ^^^^ not covered
+   = note: the matched value is of type `Option<i32>`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
+   |
+LL ~         Some(_) => {}
+LL +         None => todo!()
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0004`.