Rollup merge of rust-lang#143401 - Enselic:no-stack-backtrace-print-in-display, r=davidtwco

tests: Don't check for self-printed output in std-backtrace.rs test

The `Display` implementation for `Backtrace` used to print

    stack backtrace:

but that print was since removed. See rust-lang/backtrace-rs#286 and rust-lang#69042. To make the existing test pass, the print was added to the test instead. But it doesn't make sense to check for something that the test itself does since that will not detect any regressions in the implementation of `Backtrace`.

What the test _should_ check is that "stack backtrace:" is _not_ printed in `Display` of `Backtrace`. So do that instead.

This is one small steps towards resolving rust-lang#71706. The next steps after this step involves extending and hardening that test further.

Author: matthiaskrgr

tests/ui/backtrace/std-backtrace.rs

@@ -13,9 +13,9 @@ use std::str;
 fn main() {
     let args: Vec<String> = env::args().collect();
     if args.len() >= 2 && args[1] == "force" {
-        println!("stack backtrace:\n{}", std::backtrace::Backtrace::force_capture());
+        println!("{}", std::backtrace::Backtrace::force_capture());
     } else if args.len() >= 2 {
-        println!("stack backtrace:\n{}", std::backtrace::Backtrace::capture());
+        println!("{}", std::backtrace::Backtrace::capture());
     } else {
         runtest(&args[0]);
         println!("test ok");
@@ -28,7 +28,6 @@ fn runtest(me: &str) {
 
     let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "1").output().unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
     assert!(String::from_utf8_lossy(&p.stdout).contains("backtrace::main"));
 
     let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "0").output().unwrap();
@@ -46,7 +45,6 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 
     let p = Command::new(me)
         .arg("a")
@@ -64,9 +62,7 @@ fn runtest(me: &str) {
         .output()
         .unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 
     let p = Command::new(me).arg("force").output().unwrap();
     assert!(p.status.success());
-    assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
 }