Auto merge of #1304 - RalfJung:backtrace-sep, r=RalfJung

add empty line before backtrace, to separate it from help text

I think this improves how things look, and makes it easier to see the help text (it currently kind of drowns next to the backtrace).

Before:
```
error: unsupported operation: Miri does not support threading
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   |
68 |         let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri does not support threading
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
   = note: inside `std::sys::unix::thread::Thread::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   = note: inside `std::thread::Builder::spawn_unchecked::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:492:26
   = note: inside `std::thread::Builder::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:386:18
   = note: inside `std::thread::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:619:5
note: inside `main` at tests/compile-fail/thread-spawn.rs:6:5
  --> tests/compile-fail/thread-spawn.rs:6:5
   |
6  |     thread::spawn(|| {});
   |     ^^^^^^^^^^^^^^^^^^^^
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:73
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6030 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:130:5
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:13
   = note: inside `std::panicking::try::do_call::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:331:40
   = note: inside `std::panicking::try::<i32, [closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:274:15
   = note: inside `std::panic::catch_unwind::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
   = note: inside `std::rt::lang_start_internal` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:51:25
   = note: inside `std::rt::lang_start::<()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:5
```
After:
```
error: unsupported operation: Miri does not support threading
  --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   |
68 |         let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri does not support threading
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support

   = note: inside `std::sys::unix::thread::Thread::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/thread.rs:68:19
   = note: inside `std::thread::Builder::spawn_unchecked::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:492:26
   = note: inside `std::thread::Builder::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:386:18
   = note: inside `std::thread::spawn::<[closure@tests/compile-fail/thread-spawn.rs:6:19: 6:24], ()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/thread/mod.rs:619:5
note: inside `main` at tests/compile-fail/thread-spawn.rs:6:5
  --> tests/compile-fail/thread-spawn.rs:6:5
   |
6  |     thread::spawn(|| {});
   |     ^^^^^^^^^^^^^^^^^^^^
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:73
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6030 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:130:5
   = note: inside closure at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:52:13
   = note: inside `std::panicking::try::do_call::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:331:40
   = note: inside `std::panicking::try::<i32, [closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panicking.rs:274:15
   = note: inside `std::panic::catch_unwind::<[closure@DefId(1:6029 ~ std[2f86]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/panic.rs:394:14
   = note: inside `std::rt::lang_start_internal` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:51:25
   = note: inside `std::rt::lang_start::<()>` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:5
```

What do you think?

Author: bors

src/diagnostics.rs

@@ -106,7 +106,7 @@ pub fn report_error<'tcx, 'mir>(
 
     e.print_backtrace();
     let msg = e.to_string();
-    report_msg(ecx, &format!("{}: {}", title, msg), msg, &helps, true)
+    report_msg(ecx, &format!("{}: {}", title, msg), msg, helps, true)
 }
 
 /// Report an error or note (depending on the `error` argument) at the current frame's current statement.
@@ -115,7 +115,7 @@ fn report_msg<'tcx, 'mir>(
     ecx: &InterpCx<'mir, 'tcx, Evaluator<'tcx>>,
     title: &str,
     span_msg: String,
-    helps: &[String],
+    mut helps: Vec<String>,
     error: bool,
 ) -> Option<i64> {
     let span = if let Some(frame) = ecx.stack().last() {
@@ -129,8 +129,12 @@ fn report_msg<'tcx, 'mir>(
         ecx.tcx.sess.diagnostic().span_note_diag(span, title)
     };
     err.span_label(span, span_msg);
-    for help in helps {
-        err.help(help);
+    if !helps.is_empty() {
+        // Add visual separator before backtrace.
+        helps.last_mut().unwrap().push_str("\n");
+        for help in helps {
+            err.help(&help);
+        }
     }
     // Add backtrace
     let frames = ecx.generate_stacktrace();
@@ -182,7 +186,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     CreatedAlloc(AllocId(id)) =>
                         format!("created allocation with id {}", id),
                 };
-                report_msg(this, "tracking was triggered", msg, &[], false);
+                report_msg(this, "tracking was triggered", msg, vec![], false);
             }
         });
     }

src/eval.rs

@@ -186,8 +186,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
 /// Returns `Some(return_code)` if program executed completed.
 /// Returns `None` if an evaluation error occured.
 pub fn eval_main<'tcx>(tcx: TyCtxt<'tcx>, main_id: DefId, config: MiriConfig) -> Option<i64> {
-    // FIXME: on Windows, locks and TLS dtor management allocate and leave that memory in `static`s.
-    // So we need https://github.com/rust-lang/miri/issues/940 to fix the leaks there.
+    // FIXME: on Windows, we ignore leaks (https://github.com/rust-lang/miri/issues/1302).
     let ignore_leaks = config.ignore_leaks || tcx.sess.target.target.target_os == "windows";
 
     let (mut ecx, ret_place) = match create_ecx(tcx, main_id, config) {