coverage: Migrate tests/run-coverage into tests/coverage

tests/coverage/unreachable.rs

@@ -0,0 +1,37 @@
+#![feature(core_intrinsics)]
+#![feature(coverage_attribute)]
+// compile-flags: --edition=2021
+
+// <https://github.com/rust-lang/rust/issues/116171>
+// If we instrument a function for coverage, but all of its counter-increment
+// statements are removed by MIR optimizations, LLVM will think it isn't
+// instrumented and it will disappear from coverage maps and coverage reports.
+// Most MIR opts won't cause this because they tend not to remove statements
+// from bb0, but `UnreachablePropagation` can do so if it sees that bb0 ends
+// with `TerminatorKind::Unreachable`.
+
+use std::hint::{black_box, unreachable_unchecked};
+
+static UNREACHABLE_CLOSURE: fn() = || unsafe { unreachable_unchecked() };
+
+fn unreachable_function() {
+    unsafe { unreachable_unchecked() }
+}
+
+// Use an intrinsic to more reliably trigger unreachable-propagation.
+fn unreachable_intrinsic() {
+    unsafe { std::intrinsics::unreachable() }
+}
+
+#[coverage(off)]
+fn main() {
+    if black_box(false) {
+        UNREACHABLE_CLOSURE();
+    }
+    if black_box(false) {
+        unreachable_function();
+    }
+    if black_box(false) {
+        unreachable_intrinsic();
+    }
+}