Auto merge of #1076 - RalfJung:coercion-test, r=RalfJung

test closure-to-fn-ptr coercions a bit more

Also add some commented-out failing tests, Cc rust-lang/rust#66738 #1075

Author: bors

tests/run-pass/coerce_non_capture_closure_to_fn_ptr.rs

@@ -0,0 +1,36 @@
+// allow(const_err) to work around a bug in warnings
+#[allow(const_err)]
+static FOO: fn() = || { assert_ne!(42, 43) };
+#[allow(const_err)]
+static BAR: fn(i32, i32) = |a, b| { assert_ne!(a, b) };
+
+// use to first make the closure FnOnce() before making it fn()
+fn magic0<R, F: FnOnce() -> R>(f: F) -> F { f }
+fn magic1<T, R, F: FnOnce(T) -> R>(f: F) -> F { f }
+
+fn main() {
+    FOO();
+    BAR(44, 45);
+    let bar: unsafe fn(i32, i32) = BAR;
+    unsafe { bar(46, 47) };
+    let boo: &dyn Fn(i32, i32) = &BAR;
+    boo(48, 49);
+
+    let f: fn() = ||{};
+    f();
+    let f = magic0(||{}) as fn();
+    f();
+
+    let g: fn(i32) = |i| assert_eq!(i, 2);
+    g(2);
+    let g = magic1(|i| assert_eq!(i, 2)) as fn(i32);
+    g(2);
+
+    // FIXME: This fails with "invalid use of NULL pointer" <https://github.com/rust-lang/miri/issues/1075>
+    //let h: fn() -> ! = || std::process::exit(0);
+    //h();
+    // FIXME: This does not even compile?!? <https://github.com/rust-lang/rust/issues/66738>
+    //let h = magic0(|| std::process::exit(0)) as fn() -> !;
+    //h();
+    // Once these tests pass, they should be in separate files as they terminate the process.
+}