coverage: Branch coverage tests for match arms

tests/coverage/branch/match-trivial.rs

@@ -0,0 +1,48 @@
+#![feature(coverage_attribute)]
+//@ edition: 2021
+//@ compile-flags: -Zcoverage-options=branch
+//@ llvm-cov-flags: --show-branches=count
+
+// When instrumenting match expressions for branch coverage, make sure we don't
+// cause an ICE or produce weird coverage output for matches with <2 arms.
+
+// Helper macro to prevent start-of-function spans from being merged into
+// spans on the lines we care about.
+macro_rules! no_merge {
+    () => {
+        for _ in 0..1 {}
+    };
+}
+
+enum Uninhabited {}
+enum Trivial {
+    Value,
+}
+
+fn _uninhabited(x: Uninhabited) {
+    no_merge!();
+
+    match x {}
+
+    consume("done");
+}
+
+fn trivial(x: Trivial) {
+    no_merge!();
+
+    match x {
+        Trivial::Value => consume("trivial"),
+    }
+
+    consume("done");
+}
+
+#[coverage(off)]
+fn consume<T>(x: T) {
+    core::hint::black_box(x);
+}
+
+#[coverage(off)]
+fn main() {
+    trivial(Trivial::Value);
+}