coverage: Branch coverage test for let-else

tests/coverage/branch/let-else.cov-map

@@ -0,0 +1,18 @@
+Function name: let_else::let_else
+Raw bytes (38): 0x[01, 01, 02, 05, 09, 09, 02, 06, 01, 0c, 01, 01, 10, 02, 03, 0e, 00, 0f, 05, 00, 13, 00, 18, 09, 01, 09, 01, 0f, 02, 04, 05, 00, 0b, 07, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 1 operands: lhs = Counter(2), rhs = Expression(0, Sub)
+Number of file 0 mappings: 6
+- Code(Counter(0)) at (prev + 12, 1) to (start + 1, 16)
+- Code(Expression(0, Sub)) at (prev + 3, 14) to (start + 0, 15)
+    = (c1 - c2)
+- Code(Counter(1)) at (prev + 0, 19) to (start + 0, 24)
+- Code(Counter(2)) at (prev + 1, 9) to (start + 1, 15)
+- Code(Expression(0, Sub)) at (prev + 4, 5) to (start + 0, 11)
+    = (c1 - c2)
+- Code(Expression(1, Add)) at (prev + 1, 1) to (start + 0, 2)
+    = (c2 + (c1 - c2))
+

tests/coverage/branch/let-else.coverage

@@ -0,0 +1,37 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2021
+   LL|       |//@ compile-flags: -Zcoverage-options=branch
+   LL|       |//@ llvm-cov-flags: --show-branches=count
+   LL|       |
+   LL|       |macro_rules! no_merge {
+   LL|       |    () => {
+   LL|       |        for _ in 0..1 {}
+   LL|       |    };
+   LL|       |}
+   LL|       |
+   LL|      3|fn let_else(value: Option<&str>) {
+   LL|      3|    no_merge!();
+   LL|       |
+   LL|      3|    let Some(x) = value else {
+                           ^2
+   LL|      1|        say("none");
+   LL|      1|        return;
+   LL|       |    };
+   LL|       |
+   LL|      2|    say(x);
+   LL|      3|}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn say(message: &str) {
+   LL|       |    core::hint::black_box(message);
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    let_else(Some("x"));
+   LL|       |    let_else(Some("x"));
+   LL|       |    let_else(None);
+   LL|       |}
+   LL|       |
+   LL|       |// FIXME(#124118) Actually instrument let-else for branch coverage.
+

tests/coverage/branch/let-else.rs

@@ -0,0 +1,35 @@
+#![feature(coverage_attribute)]
+//@ edition: 2021
+//@ compile-flags: -Zcoverage-options=branch
+//@ llvm-cov-flags: --show-branches=count
+
+macro_rules! no_merge {
+    () => {
+        for _ in 0..1 {}
+    };
+}
+
+fn let_else(value: Option<&str>) {
+    no_merge!();
+
+    let Some(x) = value else {
+        say("none");
+        return;
+    };
+
+    say(x);
+}
+
+#[coverage(off)]
+fn say(message: &str) {
+    core::hint::black_box(message);
+}
+
+#[coverage(off)]
+fn main() {
+    let_else(Some("x"));
+    let_else(Some("x"));
+    let_else(None);
+}
+
+// FIXME(#124118) Actually instrument let-else for branch coverage.