Update panic message to be clearer about env-vars

Esteban Kuber requested that the panic message make it clear
that `RUST_BACKTRACE=1` is an environment variable.  This change
makes that clear.  Wording provided in part by David Tolnay.

4 files changed, 7 insertions, 5 deletions

diff --git a/src/libstd/panicking.rs b/src/libstd/panicking.rs
index 6b47ba6d1cb..b6180fbeb50 100644
--- a/src/libstd/panicking.rs
+++ b/src/libstd/panicking.rs
@@ -209,7 +209,8 @@ fn default_hook(info: &PanicInfo) {
             if let Some(format) = log_backtrace {
                 let _ = backtrace::print(err, format);
             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
-                let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
+                let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` \
+                                       environment variable to display a backtrace.");
             }
         }
     };
diff --git a/src/test/run-make-fulldeps/libtest-json/output.json b/src/test/run-make-fulldeps/libtest-json/output.json
index 80e75c89bfc..2b831ea2bdc 100644
--- a/src/test/run-make-fulldeps/libtest-json/output.json
+++ b/src/test/run-make-fulldeps/libtest-json/output.json
@@ -2,7 +2,7 @@
 { "type": "test", "event": "started", "name": "a" }
 { "type": "test", "name": "a", "event": "ok" }
 { "type": "test", "event": "started", "name": "b" }
-{ "type": "test", "name": "b", "event": "failed", "stdout": "thread 'main' panicked at 'assertion failed: false', f.rs:18:5\nnote: Run with `RUST_BACKTRACE=1` for a backtrace.\n" }
+{ "type": "test", "name": "b", "event": "failed", "stdout": "thread 'main' panicked at 'assertion failed: false', f.rs:18:5\nnote: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.\n" }
 { "type": "test", "event": "started", "name": "c" }
 { "type": "test", "name": "c", "event": "ok" }
 { "type": "test", "event": "started", "name": "d" }
diff --git a/src/test/run-pass/multi-panic.rs b/src/test/run-pass/multi-panic.rs
index 2e6e10935fa..03e58fc2387 100644
--- a/src/test/run-pass/multi-panic.rs
+++ b/src/test/run-pass/multi-panic.rs
@@ -17,7 +17,8 @@ fn check_for_no_backtrace(test: std::process::Output) {
     let mut it = err.lines();
 
     assert_eq!(it.next().map(|l| l.starts_with("thread '<unnamed>' panicked at")), Some(true));
-    assert_eq!(it.next(), Some("note: Run with `RUST_BACKTRACE=1` for a backtrace."));
+    assert_eq!(it.next(), Some("note: Run with `RUST_BACKTRACE=1` \
+                                environment variable to display a backtrace."));
     assert_eq!(it.next().map(|l| l.starts_with("thread 'main' panicked at")), Some(true));
     assert_eq!(it.next(), None);
 }
diff --git a/src/test/rustdoc-ui/failed-doctest-output.stdout b/src/test/rustdoc-ui/failed-doctest-output.stdout
index cd19099647c..bd6fa1f84b4 100644
--- a/src/test/rustdoc-ui/failed-doctest-output.stdout
+++ b/src/test/rustdoc-ui/failed-doctest-output.stdout
@@ -13,13 +13,13 @@ error[E0425]: cannot find value `no` in this scope
   | ^^ not found in this scope
 
 thread '$DIR/failed-doctest-output.rs - OtherStruct (line 27)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:326:13
-note: Run with `RUST_BACKTRACE=1` for a backtrace.
+note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 ---- $DIR/failed-doctest-output.rs - SomeStruct (line 21) stdout ----
 thread '$DIR/failed-doctest-output.rs - SomeStruct (line 21)' panicked at 'test executable failed:
 
 thread 'main' panicked at 'oh no', $DIR/failed-doctest-output.rs:3:1
-note: Run with `RUST_BACKTRACE=1` for a backtrace.
+note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 ', src/librustdoc/test.rs:361:17