Auto merge of #46537 - pnkfelix:two-phase-borrows, r=arielb1

[MIR-borrowck] Two phase borrows

This adds limited support for two-phase borrows as described in
  http://smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/

The support is off by default; you opt into it via the flag `-Z two-phase-borrows`

I have written "*limited* support" above because there are simple variants of the simple `v.push(v.len())` example that one would think should work but currently do not, such as the one documented in the test compile-fail/borrowck/two-phase-reservation-sharing-interference-2.rs

(To be clear, that test is not describing something that is unsound. It is just providing an explicit example of a limitation in the implementation given in this PR. I have ideas on how to fix, but I want to land the work that is in this PR first, so that I can stop repeatedly rebasing this branch.)

src/test/compile-fail/borrowck/two-phase-activation-sharing-interference.rs

@@ -0,0 +1,62 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is an important corner case pointed out by Niko: one is
+// allowed to initiate a shared borrow during a reservation, but it
+// *must end* before the activation occurs.
+//
+// FIXME: for clarity, diagnostics for these cases might be better off
+// if they specifically said "cannot activate mutable borrow of `x`"
+
+#![allow(dead_code)]
+
+fn read(_: &i32) { }
+
+fn ok() {
+    let mut x = 3;
+    let y = &mut x;
+    { let z = &x; read(z); }
+    *y += 1;
+}
+
+fn not_ok() {
+    let mut x = 3;
+    let y = &mut x;
+    let z = &x;
+    *y += 1;
+    //[lxl]~^  ERROR cannot borrow `x` as mutable because it is also borrowed as immutable
+    //[nll]~^^ ERROR cannot borrow `x` as mutable because it is also borrowed as immutable
+    read(z);
+}
+
+fn should_be_ok_with_nll() {
+    let mut x = 3;
+    let y = &mut x;
+    let z = &x;
+    read(z);
+    *y += 1;
+    //[lxl]~^  ERROR cannot borrow `x` as mutable because it is also borrowed as immutable
+    // (okay with nll today)
+}
+
+fn should_also_eventually_be_ok_with_nll() {
+    let mut x = 3;
+    let y = &mut x;
+    let _z = &x;
+    *y += 1;
+    //[lxl]~^  ERROR cannot borrow `x` as mutable because it is also borrowed as immutable
+    //[nll]~^^ ERROR cannot borrow `x` as mutable because it is also borrowed as immutable
+}
+
+fn main() { }

src/test/compile-fail/borrowck/two-phase-allow-access-during-reservation.rs

@@ -0,0 +1,37 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is the second counter-example from Niko's blog post
+// smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/
+//
+// It is "artificial". It is meant to illustrate directly that we
+// should allow an aliasing access during reservation, but *not* while
+// the mutable borrow is active.
+
+fn main() {
+    /*0*/ let mut i = 0;
+
+    /*1*/ let p = &mut i; // (reservation of `i` starts here)
+
+    /*2*/ let j = i;      // OK: `i` is only reserved here
+
+    /*3*/ *p += 1;        // (mutable borrow of `i` starts here, since `p` is used)
+
+    /*4*/ let k = i;      //[lxl]~  ERROR cannot use `i` because it was mutably borrowed [E0503]
+                          //[nll]~^ ERROR cannot use `i` because it was mutably borrowed [E0503]
+
+    /*5*/ *p += 1;
+
+    let _ = (j, k, p);
+}

src/test/compile-fail/borrowck/two-phase-cannot-nest-mut-self-calls.rs

@@ -0,0 +1,34 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is the third counter-example from Niko's blog post
+// smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/
+//
+// It shows that not all nested method calls on `self` are magically
+// allowed by this change. In particular, a nested `&mut` borrow is
+// still disallowed.
+
+fn main() {
+
+
+    let mut vec = vec![0, 1];
+    vec.get({
+
+        vec.push(2);
+        //[lxl]~^ ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+        //[nll]~^^   ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+
+        0
+    });
+}

src/test/compile-fail/borrowck/two-phase-reservation-sharing-interference-2.rs

@@ -0,0 +1,37 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is similar to two-phase-reservation-sharing-interference.rs
+// in that it shows a reservation that overlaps with a shared borrow.
+//
+// Currently, this test fails with lexical lifetimes, but succeeds
+// with non-lexical lifetimes. (The reason is because the activation
+// of the mutable borrow ends up overlapping with a lexically-scoped
+// shared borrow; but a non-lexical shared borrow can end before the
+// activation occurs.)
+//
+// So this test is just making a note of the current behavior.
+
+#![feature(rustc_attrs)]
+
+#[rustc_error]
+fn main() { //[nll]~ ERROR compilation successful
+    let mut v = vec![0, 1, 2];
+    let shared = &v;
+
+    v.push(shared.len());
+    //[lxl]~^  ERROR cannot borrow `v` as mutable because it is also borrowed as immutable [E0502]
+
+    assert_eq!(v, [0, 1, 2, 3]);
+}

src/test/compile-fail/borrowck/two-phase-reservation-sharing-interference.rs

@@ -0,0 +1,46 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is a corner case that the current implementation is (probably)
+// treating more conservatively than is necessary. But it also does
+// not seem like a terribly important use case to cover.
+//
+// So this test is just making a note of the current behavior, with
+// the caveat that in the future, the rules may be loosened, at which
+// point this test might be thrown out.
+
+fn main() {
+    let mut vec = vec![0, 1];
+    let delay: &mut Vec<_>;
+    {
+        let shared = &vec;
+
+        // we reserve here, which could (on its own) be compatible
+        // with the shared borrow. But in the current implementation,
+        // its an error.
+        delay = &mut vec;
+        //[lxl]~^ ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+        //[nll]~^^   ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+
+        shared[0];
+    }
+
+    // the &mut-borrow only becomes active way down here.
+    //
+    // (At least in theory; part of the reason this test fails is that
+    // the constructed MIR throws in extra &mut reborrows which
+    // flummoxes our attmpt to delay the activation point here.)
+    delay.push(2);
+}
+

src/test/compile-fail/borrowck/two-phase-sneaky.rs

@@ -0,0 +1,30 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is the first counter-example from Niko's blog post
+// smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/
+// of a danger for code to crash if we just turned off the check for whether
+// a mutable-borrow aliases another borrow.
+
+fn main() {
+    let mut v: Vec<String> = vec![format!("Hello, ")];
+    v[0].push_str({
+
+        v.push(format!("foo"));
+        //[lxl]~^ ERROR cannot borrow `v` as mutable more than once at a time [E0499]
+        //[nll]~^^   ERROR cannot borrow `v` as mutable more than once at a time [E0499]
+
+        "World!"
+    });
+}

src/test/run-pass/borrowck/two-phase-baseline.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is the "goto example" for why we want two phase borrows.
+
+fn main() {
+    let mut v = vec![0, 1, 2];
+    v.push(v.len());
+    assert_eq!(v, [0, 1, 2, 3]);
+}

src/test/run-pass/borrowck/two-phase-control-flow-split-before-activation.rs

@@ -0,0 +1,27 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+fn main() {
+    let mut a = 0;
+    let mut b = 0;
+    let p = if maybe() {
+        &mut a
+    } else {
+        &mut b
+    };
+    use_(p);
+}
+
+fn maybe() -> bool { false }
+fn use_<T>(_: T) { }