Auto merge of #56649 - davidtwco:issue-46589, r=pnkfelix

MIR borrowck doesn't accept the example of iterating and updating a mutable reference

Fixes #46589.

r? @pnkfelix or @nikomatsakis

4 files changed, 57 insertions, 31 deletions

diff --git a/src/test/ui/nll/issue-46589.rs b/src/test/ui/nll/issue-46589.rs
new file mode 100644
index 00000000000..82e73651f43
--- /dev/null
+++ b/src/test/ui/nll/issue-46589.rs
@@ -0,0 +1,37 @@
+// Copyright 2016 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.
+
+#![feature(nll)]
+
+struct Foo;
+
+impl Foo {
+    fn get_self(&mut self) -> Option<&mut Self> {
+        Some(self)
+    }
+
+    fn new_self(&mut self) -> &mut Self {
+        self
+    }
+
+    fn trigger_bug(&mut self) {
+        let other = &mut (&mut *self);
+
+        *other = match (*other).get_self() {
+            Some(s) => s,
+            None => (*other).new_self()
+            //~^ ERROR cannot borrow `**other` as mutable more than once at a time [E0499]
+        };
+
+        let c = other;
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/nll/issue-46589.stderr b/src/test/ui/nll/issue-46589.stderr
new file mode 100644
index 00000000000..6df2983f465
--- /dev/null
+++ b/src/test/ui/nll/issue-46589.stderr
@@ -0,0 +1,15 @@
+error[E0499]: cannot borrow `**other` as mutable more than once at a time
+  --> $DIR/issue-46589.rs:29:21
+   |
+LL |         *other = match (*other).get_self() {
+   |                        -------- first mutable borrow occurs here
+LL |             Some(s) => s,
+LL |             None => (*other).new_self()
+   |                     ^^^^^^^^
+   |                     |
+   |                     second mutable borrow occurs here
+   |                     first borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0499`.
diff --git a/src/test/ui/nll/loan_ends_mid_block_pair.rs b/src/test/ui/nll/loan_ends_mid_block_pair.rs
index 97126e98cbf..320d80438b0 100644
--- a/src/test/ui/nll/loan_ends_mid_block_pair.rs
+++ b/src/test/ui/nll/loan_ends_mid_block_pair.rs
@@ -27,10 +27,8 @@ fn nll_fail() {
     //~| ERROR (Mir) [E0506]
     data.0 = 'f';
     //~^ ERROR (Ast) [E0506]
-    //~| ERROR (Mir) [E0506]
     data.0 = 'g';
     //~^ ERROR (Ast) [E0506]
-    //~| ERROR (Mir) [E0506]
     capitalize(c);
 }
 
diff --git a/src/test/ui/nll/loan_ends_mid_block_pair.stderr b/src/test/ui/nll/loan_ends_mid_block_pair.stderr
index 9afae71edbe..3ba3fa15a53 100644
--- a/src/test/ui/nll/loan_ends_mid_block_pair.stderr
+++ b/src/test/ui/nll/loan_ends_mid_block_pair.stderr
@@ -17,7 +17,7 @@ LL |     data.0 = 'f';
    |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
 
 error[E0506]: cannot assign to `data.0` because it is borrowed (Ast)
-  --> $DIR/loan_ends_mid_block_pair.rs:31:5
+  --> $DIR/loan_ends_mid_block_pair.rs:30:5
    |
 LL |     let c = &mut data.0;
    |                  ------ borrow of `data.0` occurs here
@@ -26,7 +26,7 @@ LL |     data.0 = 'g';
    |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
 
 error[E0506]: cannot assign to `data.0` because it is borrowed (Ast)
-  --> $DIR/loan_ends_mid_block_pair.rs:41:5
+  --> $DIR/loan_ends_mid_block_pair.rs:39:5
    |
 LL |     let c = &mut data.0;
    |                  ------ borrow of `data.0` occurs here
@@ -35,7 +35,7 @@ LL |     data.0 = 'e';
    |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
 
 error[E0506]: cannot assign to `data.0` because it is borrowed (Ast)
-  --> $DIR/loan_ends_mid_block_pair.rs:43:5
+  --> $DIR/loan_ends_mid_block_pair.rs:41:5
    |
 LL |     let c = &mut data.0;
    |                  ------ borrow of `data.0` occurs here
@@ -44,7 +44,7 @@ LL |     data.0 = 'f';
    |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
 
 error[E0506]: cannot assign to `data.0` because it is borrowed (Ast)
-  --> $DIR/loan_ends_mid_block_pair.rs:45:5
+  --> $DIR/loan_ends_mid_block_pair.rs:43:5
    |
 LL |     let c = &mut data.0;
    |                  ------ borrow of `data.0` occurs here
@@ -64,30 +64,6 @@ LL |     data.0 = 'e';
 LL |     capitalize(c);
    |                - borrow later used here
 
-error[E0506]: cannot assign to `data.0` because it is borrowed (Mir)
-  --> $DIR/loan_ends_mid_block_pair.rs:28:5
-   |
-LL |     let c = &mut data.0;
-   |             ----------- borrow of `data.0` occurs here
-...
-LL |     data.0 = 'f';
-   |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
-...
-LL |     capitalize(c);
-   |                - borrow later used here
-
-error[E0506]: cannot assign to `data.0` because it is borrowed (Mir)
-  --> $DIR/loan_ends_mid_block_pair.rs:31:5
-   |
-LL |     let c = &mut data.0;
-   |             ----------- borrow of `data.0` occurs here
-...
-LL |     data.0 = 'g';
-   |     ^^^^^^^^^^^^ assignment to borrowed `data.0` occurs here
-...
-LL |     capitalize(c);
-   |                - borrow later used here
-
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0506`.