Auto merge of #53109 - nikomatsakis:nll-escaping-into-return-revert, r=nikomatsakis

revert #52991

Reverts https://github.com/rust-lang/rust/pull/52991 which is flawed. I have an idea how to fix it but might as well revert first since it is so wildly flawed. That's what I get for opening PRs while on PTO =)

r? @pnkfelix

src/test/ui/borrowck/issue-52713-bug.rs

@@ -0,0 +1,29 @@
+// Copyright 2014 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.
+
+// Regression test for a bug in #52713: this was an optimization for
+// computing liveness that wound up accidentally causing the program
+// below to be accepted.
+
+#![feature(nll)]
+
+fn foo<'a>(x: &'a mut u32) -> u32 {
+    let mut x = 22;
+    let y = &x;
+    if false {
+        return x;
+    }
+
+    x += 1; //~ ERROR
+    println!("{}", y);
+    return 0;
+}
+
+fn main() { }

src/test/ui/borrowck/issue-52713-bug.stderr

@@ -0,0 +1,14 @@
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/issue-52713-bug.rs:24:5
+   |
+LL |     let y = &x;
+   |             -- borrow of `x` occurs here
+...
+LL |     x += 1; //~ ERROR
+   |     ^^^^^^ assignment to borrowed `x` occurs here
+LL |     println!("{}", y);
+   |                    - borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr

@@ -1,24 +1,30 @@
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:21
    |
-LL |     let ref mut x = 1234543; //~ ERROR
-   |                     ^^^^^^^ temporary value does not live long enough
-LL |     x
-LL | }
-   | - temporary value only lives until here
-   |
-   = note: borrowed value must be valid for the static lifetime...
+LL |   fn gimme_static_mut_let() -> &'static mut u32 {
+   |  _______________________________________________-
+LL | |     let ref mut x = 1234543; //~ ERROR
+   | |                     ^^^^^^^ temporary value does not live long enough
+LL | |     x
+LL | | }
+   | | -
+   | | |
+   | |_temporary value only lives until here
+   |   borrow later used here
 
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:20:25
    |
-LL |     let (ref mut x, ) = (1234543, ); //~ ERROR
-   |                         ^^^^^^^^^^^ temporary value does not live long enough
-LL |     x
-LL | }
-   | - temporary value only lives until here
-   |
-   = note: borrowed value must be valid for the static lifetime...
+LL |   fn gimme_static_mut_let_nested() -> &'static mut u32 {
+   |  ______________________________________________________-
+LL | |     let (ref mut x, ) = (1234543, ); //~ ERROR
+   | |                         ^^^^^^^^^^^ temporary value does not live long enough
+LL | |     x
+LL | | }
+   | | -
+   | | |
+   | |_temporary value only lives until here
+   |   borrow later used here
 
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:25:11

src/test/ui/nll/get_default.nll.stderr

@@ -63,18 +63,9 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
-   |
-note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
-  --> $DIR/get_default.rs:41:1
-   |
-LL | / fn err(map: &mut Map) -> &String {
-LL | |     loop {
-LL | |         match map.get() {
-LL | |             Some(v) => {
-...  |
-LL | |     }
-LL | | }
-   | |_^
+...
+LL |                 return v;
+   |                        - borrow later used here
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17

src/test/ui/nll/get_default.stderr

@@ -63,18 +63,9 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^ mutable borrow occurs here
-   |
-note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
-  --> $DIR/get_default.rs:41:1
-   |
-LL | / fn err(map: &mut Map) -> &String {
-LL | |     loop {
-LL | |         match map.get() {
-LL | |             Some(v) => {
-...  |
-LL | |     }
-LL | | }
-   | |_^
+...
+LL |                 return v;
+   |                        - borrow later used here
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17

src/test/ui/nll/return-ref-mut-issue-46557.stderr

@@ -1,13 +1,16 @@
 error[E0597]: borrowed value does not live long enough
   --> $DIR/return-ref-mut-issue-46557.rs:17:21
    |
-LL |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
-   |                     ^^^^^^^ temporary value does not live long enough
-LL |     x
-LL | }
-   | - temporary value only lives until here
-   |
-   = note: borrowed value must be valid for the static lifetime...
+LL |   fn gimme_static_mut() -> &'static mut u32 {
+   |  ___________________________________________-
+LL | |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
+   | |                     ^^^^^^^ temporary value does not live long enough
+LL | |     x
+LL | | }
+   | | -
+   | | |
+   | |_temporary value only lives until here
+   |   borrow later used here
 
 error: aborting due to previous error