Auto merge of #28833 - jryans:borrowck-linear-errors, r=pnkfelix

Change error reporting of conflicting loans to stop earlier after printing
an error for a given borrow, instead of proceeding to error on possibly every
issued loan.  This keeps us down to O(n) errors (for n problem lines), instead
of O(n^2) errors in some cases.

Fixes #27485.

src/test/compile-fail/borrowck-mut-borrow-linear-errors.rs

@@ -0,0 +1,28 @@
+// Copyright 2015 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.
+
+// Test to ensure we only report an error for the first issued loan that
+// conflicts with a new loan, as opposed to every issued loan.  This keeps us
+// down to O(n) errors (for n problem lines), instead of O(n^2) errors.
+
+fn main() {
+    let mut x = 1;
+    let mut addr;
+    loop {
+        match 1 {
+            1 => { addr = &mut x; }
+            //~^ ERROR cannot borrow `x` as mutable more than once at a time
+            2 => { addr = &mut x; }
+            //~^ ERROR cannot borrow `x` as mutable more than once at a time
+            _ => { addr = &mut x; }
+            //~^ ERROR cannot borrow `x` as mutable more than once at a time
+        }
+    }
+}