Rollup merge of rust-lang#78133 - JohnTitor:mir-tests, r=lcnr

Add some MIR-related regression tests

Closes rust-lang#68841
Closes rust-lang#75053
Closes rust-lang#76375
Closes rust-lang#77911

I think they're fixed by rust-lang#77306.

src/test/ui/mir/auxiliary/issue_76375_aux.rs

@@ -0,0 +1,14 @@
+// edition:2018
+// compile-flags: -Z mir-opt-level=2 -Z unsound-mir-opts
+
+#[inline(always)]
+pub fn f(s: bool) -> String {
+    let a = "Hello world!".to_string();
+    let b = a;
+    let c = b;
+    if s {
+        c
+    } else {
+        String::new()
+    }
+}

src/test/ui/mir/issue-68841.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Z mir-opt-level=2
+// edition:2018
+// build-pass
+
+#![feature(async_closure)]
+
+use std::future::Future;
+
+fn async_closure() -> impl Future<Output = u8> {
+    (async move || -> u8 { 42 })()
+}
+
+fn main() {
+    let _fut = async_closure();
+}

src/test/ui/mir/issue-75053.rs

@@ -0,0 +1,48 @@
+// compile-flags: -Z mir-opt-level=2
+// build-pass
+
+#![feature(type_alias_impl_trait)]
+
+use std::marker::PhantomData;
+
+trait MyIndex<T> {
+    type O;
+    fn my_index(self) -> Self::O;
+}
+trait MyFrom<T>: Sized {
+    type Error;
+    fn my_from(value: T) -> Result<Self, Self::Error>;
+}
+
+trait F {}
+impl F for () {}
+type DummyT<T> = impl F;
+fn _dummy_t<T>() -> DummyT<T> {}
+
+struct Phantom1<T>(PhantomData<T>);
+struct Phantom2<T>(PhantomData<T>);
+struct Scope<T>(Phantom2<DummyT<T>>);
+
+impl<T> Scope<T> {
+    fn new() -> Self {
+        unimplemented!()
+    }
+}
+
+impl<T> MyFrom<Phantom2<T>> for Phantom1<T> {
+    type Error = ();
+    fn my_from(_: Phantom2<T>) -> Result<Self, Self::Error> {
+        unimplemented!()
+    }
+}
+
+impl<T: MyFrom<Phantom2<DummyT<U>>>, U> MyIndex<Phantom1<T>> for Scope<U> {
+    type O = T;
+    fn my_index(self) -> Self::O {
+        MyFrom::my_from(self.0).ok().unwrap()
+    }
+}
+
+fn main() {
+    let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
+}

src/test/ui/mir/issue-76375.rs

@@ -0,0 +1,15 @@
+// edition:2018
+// build-pass
+// compile-flags: -Z mir-opt-level=2 -L.
+// aux-build:issue_76375_aux.rs
+
+#![crate_type = "lib"]
+
+extern crate issue_76375_aux;
+
+pub async fn g() {
+    issue_76375_aux::f(true);
+    h().await;
+}
+
+pub async fn h() {}

src/test/ui/mir/issue-77911.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Z mir-opt-level=2
+// ignore-cloudabi no std::fs
+// build-pass
+
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+fn file_lines() -> impl Iterator<Item = String> {
+    BufReader::new(File::open("").unwrap())
+        .lines()
+        .map(Result::unwrap)
+}
+
+fn main() {
+    for _ in file_lines() {}
+}