Auto merge of #42541 - gilescope:patch-1, r=alexcrichton

assert_eq failure message easier to read

By having the left and right strings aligned with one another it helps spot the difference between the two far quicker than if they are on the same line.

E.g.
Before:

```
thread 'tests::test_safe_filename' panicked at 'assertion failed: `(left == right)` left:  `"-aandb--S123.html"` right: `"-aandb-S123.html"`',
```

After:

```
thread 'tests::test_safe_filename' panicked at 'assertion failed: `(left == right)`
left:  `"-aandb--S123.html"`
right: `"-aandb-S123.html"`',
```

When the strings are both on the same line it take a lot longer to spot the difference. It is a small change but the small time savings add up with repetition. This would help Rust be an excellent language to write tests in out of the box.

Closes #41615

Author: bors

src/libcore/macros.rs

@@ -116,8 +116,9 @@ macro_rules! assert_eq {
         match (&$left, &$right) {
             (left_val, right_val) => {
                 if !(*left_val == *right_val) {
-                    panic!("assertion failed: `(left == right)` \
-                           (left: `{:?}`, right: `{:?}`)", left_val, right_val)
+                    panic!(r#"assertion failed: `(left == right)`
+  left: `{:?}`,
+ right: `{:?}`"#, left_val, right_val)
                 }
             }
         }
@@ -126,8 +127,9 @@ macro_rules! assert_eq {
         match (&($left), &($right)) {
             (left_val, right_val) => {
                 if !(*left_val == *right_val) {
-                    panic!("assertion failed: `(left == right)` \
-                           (left: `{:?}`, right: `{:?}`): {}", left_val, right_val,
+                    panic!(r#"assertion failed: `(left == right)`
+  left: `{:?}`,
+ right: `{:?}`: {}"#, left_val, right_val,
                            format_args!($($arg)+))
                 }
             }
@@ -162,8 +164,9 @@ macro_rules! assert_ne {
         match (&$left, &$right) {
             (left_val, right_val) => {
                 if *left_val == *right_val {
-                    panic!("assertion failed: `(left != right)` \
-                           (left: `{:?}`, right: `{:?}`)", left_val, right_val)
+                    panic!(r#"assertion failed: `(left != right)`
+  left: `{:?}`,
+ right: `{:?}`"#, left_val, right_val)
                 }
             }
         }
@@ -172,8 +175,9 @@ macro_rules! assert_ne {
         match (&($left), &($right)) {
             (left_val, right_val) => {
                 if *left_val == *right_val {
-                    panic!("assertion failed: `(left != right)` \
-                           (left: `{:?}`, right: `{:?}`): {}", left_val, right_val,
+                    panic!(r#"assertion failed: `(left != right)`
+  left: `{:?}`,
+ right: `{:?}`: {}"#, left_val, right_val,
                            format_args!($($arg)+))
                 }
             }

src/test/run-fail/assert-eq-macro-panic.rs

@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:assertion failed: `(left == right)` (left: `14`, right: `15`)
+// error-pattern:assertion failed: `(left == right)`
+// error-pattern: left: `14`
+// error-pattern:right: `15`
 
 fn main() {
     assert_eq!(14, 15);

src/test/run-fail/assert-ne-macro-panic.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// error-pattern:assertion failed: `(left != right)`
+// error-pattern: left: `14`
+// error-pattern:right: `14`
+
+fn main() {
+    assert_ne!(14, 14);
+}