if-let.coverage

   LL|       |#![feature(coverage_attribute, let_chains)]
   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 if_let(input: Option<&str>) {
   LL|      3|    no_merge!();
   LL|       |
   LL|      3|    if let Some(x) = input {
                              ^2
  ------------------
  |  Branch (LL:12): [True: 2, False: 1]
  ------------------
   LL|      2|        say(x);
   LL|      2|    } else {
   LL|      1|        say("none");
   LL|      1|    }
   LL|      3|    say("done");
   LL|      3|}
   LL|       |
   LL|     15|fn if_let_chain(a: Option<&str>, b: Option<&str>) {
   LL|     15|    if let Some(x) = a
                              ^12
  ------------------
  |  Branch (LL:12): [True: 12, False: 3]
  ------------------
   LL|     12|        && let Some(y) = b
                                  ^8
  ------------------
  |  Branch (LL:16): [True: 8, False: 4]
  ------------------
   LL|      8|    {
   LL|      8|        say(x);
   LL|      8|        say(y);
   LL|      8|    } else {
   LL|      7|        say("not both");
   LL|      7|    }
   LL|     15|    say("done");
   LL|     15|}
   LL|       |
   LL|       |#[coverage(off)]
   LL|       |fn say(message: &str) {
   LL|       |    core::hint::black_box(message);
   LL|       |}
   LL|       |
   LL|       |#[coverage(off)]
   LL|       |fn main() {
   LL|       |    if_let(Some("x"));
   LL|       |    if_let(Some("x"));
   LL|       |    if_let(None);
   LL|       |
   LL|       |    for _ in 0..8 {
   LL|       |        if_let_chain(Some("a"), Some("b"));
   LL|       |    }
   LL|       |    for _ in 0..4 {
   LL|       |        if_let_chain(Some("a"), None);
   LL|       |    }
   LL|       |    for _ in 0..2 {
   LL|       |        if_let_chain(None, Some("b"));
   LL|       |    }
   LL|       |    if_let_chain(None, None);
   LL|       |}
   LL|       |
   LL|       |// FIXME(#124118) Actually instrument if-let and let-chains for branch coverage.