Auto merge of #1977 - saethlin:short-backtraces, r=RalfJung

Prune backtraces similar to RUST_BACKTRACE=1 logic

This removes the majority of output from `cargo miri run` and `cargo miri test` in common usage.

~~I've copied the logic almost directly from `std`:
https://github.com/rust-lang/rust/blob/3b186511f62b0ce20e72ede0e8e13f8787155f02/library/std/src/sys_common/backtrace.rs#L76-L77~~

~~It might be nice to have the "some details were omitted" note and a fallback to a setting where we print everything just in case this logic goes sideways, but~~
~~1. I'm not sure where to put the note~~
~~2. `MIRI_BACKTRACE`, `RUST_BACKTRACE`, and `RUSTC_CTFE_BACKTRACE` already do something else. Should we repurpose or add on to the semantics of `MIRI_BACKTRACE`?~~

---
Based on this tiny silly crate:
```rust
fn main() {
    some_function();
}

fn some_function() {
    unsafe {
        let _x: &u8 = core::mem::transmute(1usize);
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        unsafe {
            let _x: &'static u8 = core::mem::transmute(1usize);
        }
    }
}
```

`cargo miri run`:
Before:
```
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `/home/ben/.cargo/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/scratch`
error: Undefined Behavior: type validation failed: encountered a dangling reference (address 0x1 is unallocated)
 --> src/main.rs:7:23
  |
7 |         let _x: &u8 = core::mem::transmute(1usize);
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a dangling reference (address 0x1 is unallocated)
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

  = note: inside `some_function` at src/main.rs:7:23
note: inside `main` at src/main.rs:2:5
 --> src/main.rs:2:5
  |
2 |     some_function();
  |     ^^^^^^^^^^^^^^^
  = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
  = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:122:18
  = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:145:18
  = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
  = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:492:40
  = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:456:19
  = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:137:14
  = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:128:48
  = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:492:40
  = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:456:19
  = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:137:14
  = note: inside `std::rt::lang_start_internal` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:128:20
  = note: inside `std::rt::lang_start::<()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:144:17

error: aborting due to previous error

```
After:
```    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `/home/ben/.cargo/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/scratch`
error: Undefined Behavior: type validation failed: encountered a dangling reference (address 0x1 is unallocated)
 --> src/main.rs:7:23
  |
7 |         let _x: &u8 = core::mem::transmute(1usize);
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a dangling reference (address 0x1 is unallocated)
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

  = note: inside `some_function` at src/main.rs:7:23
note: inside `main` at src/main.rs:2:5
 --> src/main.rs:2:5
  |
2 |     some_function();
  |     ^^^^^^^^^^^^^^^

note: Some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace.

error: aborting due to previous error
```

`cargo miri test`
Before:
```
    Finished test [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests (target/miri/x86_64-unknown-linux-gnu/debug/deps/scratch-9d7717efc37bb64c)

running 1 test
test tests::it_works ... error: Undefined Behavior: type validation failed: encountered a dangling reference (address 0x1 is unallocated)
  --> src/main.rs:16:35
   |
16 |             let _x: &'static u8 = core::mem::transmute(1usize);
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a dangling reference (address 0x1 is unallocated)
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

   = note: inside `tests::it_works` at src/main.rs:16:35
note: inside closure at src/main.rs:14:5
  --> src/main.rs:14:5
   |
13 |       #[test]
   |       ------- in this procedural macro expansion
14 | /     fn it_works() {
15 | |         unsafe {
16 | |             let _x: &'static u8 = core::mem::transmute(1usize);
17 | |         }
18 | |     }
   | |_____^
   = note: inside `<[closure@src/main.rs:14:5: 18:6] as std::ops::FnOnce<()>>::call_once - shim` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   = note: inside `tests::test::__rust_begin_short_backtrace::<fn()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:575:5
   = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:566:30
   = note: inside `<[closure@tests::test::run_test::{closure#1}] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   = note: inside `<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1854:9
   = note: inside `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/panic/unwind_safe.rs:271:9
   = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:492:40
   = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:456:19
   = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:137:14
   = note: inside `tests::test::run_test_in_process` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:598:18
   = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:492:39
   = note: inside `tests::test::run_test::run_test_inner` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:530:13
   = note: inside `tests::test::run_test` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:562:28
   = note: inside `tests::test::run_tests::<[closure@tests::test::run_tests_console::{closure#2}]>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:305:17
   = note: inside `tests::test::run_tests_console` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/console.rs:290:5
   = note: inside `tests::test::test_main` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:116:15
   = note: inside `tests::test::test_main_static` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/test/src/lib.rs:135:5
   = note: inside `main`
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:122:18
   = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:145:18
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
   = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:492:40
   = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:456:19
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:137:14
   = note: inside closure at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:128:48
   = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:492:40
   = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:456:19
   = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panic.rs:137:14
   = note: inside `std::rt::lang_start_internal` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:128:20
   = note: inside `std::rt::lang_start::<()>` at /home/ben/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/rt.rs:144:17
   = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: test failed, to rerun pass '--bin scratch'
```

After:
```
    Finished test [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests (target/miri/x86_64-unknown-linux-gnu/debug/deps/scratch-9d7717efc37bb64c)

running 1 test
test tests::it_works ... error: Undefined Behavior: type validation failed: encountered a dangling reference (address 0x1 is unallocated)
  --> src/main.rs:16:35
   |
16 |             let _x: &'static u8 = core::mem::transmute(1usize);
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a dangling reference (address 0x1 is unallocated)
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

   = note: inside `tests::it_works` at src/main.rs:16:35
note: inside closure at src/main.rs:14:5
  --> src/main.rs:14:5
   |
13 |       #[test]
   |       ------- in this procedural macro expansion
14 | /     fn it_works() {
15 | |         unsafe {
16 | |             let _x: &'static u8 = core::mem::transmute(1usize);
17 | |         }
18 | |     }
   | |_____^
   = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

note: Some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace.

error: aborting due to previous error

error: test failed, to rerun pass '--bin scratch'
```

Author: bors

src/bin/miri.rs

@@ -29,6 +29,8 @@ use rustc_middle::{
 };
 use rustc_session::{config::ErrorOutputType, search_paths::PathKind, CtfeBacktrace};
 
+use miri::BacktraceStyle;
+
 struct MiriCompilerCalls {
     miri_config: miri::MiriConfig,
 }
@@ -462,6 +464,14 @@ fn main() {
                     let measureme_out = arg.strip_prefix("-Zmiri-measureme=").unwrap();
                     miri_config.measureme_out = Some(measureme_out.to_string());
                 }
+                arg if arg.starts_with("-Zmiri-backtrace=") => {
+                    miri_config.backtrace_style = match arg.strip_prefix("-Zmiri-backtrace=") {
+                        Some("0") => BacktraceStyle::Off,
+                        Some("1") => BacktraceStyle::Short,
+                        Some("full") => BacktraceStyle::Full,
+                        _ => panic!("-Zmiri-backtrace may only be 0, 1, or full"),
+                    };
+                }
                 _ => {
                     // Forward to rustc.
                     rustc_args.push(arg);

src/diagnostics.rs

@@ -157,6 +157,46 @@ pub fn report_error<'tcx, 'mir>(
         }
     };
 
+    let mut stacktrace = ecx.generate_stacktrace();
+    let has_local_frame = stacktrace.iter().any(|frame| frame.instance.def_id().is_local());
+    match ecx.machine.backtrace_style {
+        BacktraceStyle::Off => {
+            // Retain one frame so that we can print a span for the error itself
+            stacktrace.truncate(1);
+        }
+        BacktraceStyle::Short => {
+            // Only prune frames if there is at least one local frame. This check ensures that if
+            // we get a backtrace that never makes it to the user code because it has detected a
+            // bug in the Rust runtime, we don't prune away every frame.
+            if has_local_frame {
+                // This is part of the logic that `std` uses to select the relevant part of a
+                // backtrace. But here, we only look for __rust_begin_short_backtrace, not
+                // __rust_end_short_backtrace because the end symbol comes from a call to the default
+                // panic handler.
+                stacktrace = stacktrace
+                    .into_iter()
+                    .take_while(|frame| {
+                        let def_id = frame.instance.def_id();
+                        let path = ecx.tcx.tcx.def_path_str(def_id);
+                        !path.contains("__rust_begin_short_backtrace")
+                    })
+                    .collect::<Vec<_>>();
+
+                // After we prune frames from the bottom, there are a few left that are part of the
+                // Rust runtime. So we remove frames until we get to a local symbol, which should be
+                // main or a test.
+                // This len check ensures that we don't somehow remove every frame, as doing so breaks
+                // the primary error message.
+                while stacktrace.len() > 1
+                    && stacktrace.last().map_or(false, |e| !e.instance.def_id().is_local())
+                {
+                    stacktrace.pop();
+                }
+            }
+        }
+        BacktraceStyle::Full => {}
+    }
+
     e.print_backtrace();
     let msg = e.to_string();
     report_msg(
@@ -165,9 +205,17 @@ pub fn report_error<'tcx, 'mir>(
         &if let Some(title) = title { format!("{}: {}", title, msg) } else { msg.clone() },
         msg,
         helps,
-        &ecx.generate_stacktrace(),
+        &stacktrace,
     );
 
+    // Include a note like `std` does for short backtraces, but since we are opt-out not opt-in, we
+    // do not include a note when backtraces are off.
+    if ecx.machine.backtrace_style == BacktraceStyle::Short && has_local_frame {
+        ecx.tcx.sess.diagnostic().note_without_error(
+            "some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace",
+        );
+    }
+
     // Debug-dump all locals.
     for (i, frame) in ecx.active_thread_stack().iter().enumerate() {
         trace!("-------------------");

src/eval.rs

@@ -57,6 +57,16 @@ pub enum IsolatedOp {
     Allow,
 }
 
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum BacktraceStyle {
+    /// Prints a terser backtrace which ideally only contains relevant information.
+    Short,
+    /// Prints a backtrace with all possible information.
+    Full,
+    /// Prints only the frame that the error occurs in.
+    Off,
+}
+
 /// Configuration needed to spawn a Miri instance.
 #[derive(Clone)]
 pub struct MiriConfig {
@@ -98,6 +108,7 @@ pub struct MiriConfig {
     pub measureme_out: Option<String>,
     /// Panic when unsupported functionality is encountered
     pub panic_on_unsupported: bool,
+    pub backtrace_style: BacktraceStyle,
 }
 
 impl Default for MiriConfig {
@@ -121,6 +132,7 @@ impl Default for MiriConfig {
             cmpxchg_weak_failure_rate: 0.8,
             measureme_out: None,
             panic_on_unsupported: false,
+            backtrace_style: BacktraceStyle::Short,
         }
     }
 }

src/lib.rs

@@ -60,7 +60,7 @@ pub use crate::diagnostics::{
     NonHaltingDiagnostic, TerminationInfo,
 };
 pub use crate::eval::{
-    create_ecx, eval_entry, AlignmentCheck, IsolatedOp, MiriConfig, RejectOpWith,
+    create_ecx, eval_entry, AlignmentCheck, BacktraceStyle, IsolatedOp, MiriConfig, RejectOpWith,
 };
 pub use crate::helpers::EvalContextExt as HelpersEvalContextExt;
 pub use crate::machine::{

src/machine.rs

@@ -343,6 +343,9 @@ pub struct Evaluator<'mir, 'tcx> {
     /// functionality is encountered. If `false`, an error is propagated in the Miri application context
     /// instead (default behavior)
     pub(crate) panic_on_unsupported: bool,
+
+    /// Equivalent setting as RUST_BACKTRACE on encountering an error.
+    pub(crate) backtrace_style: BacktraceStyle,
 }
 
 impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
@@ -374,6 +377,7 @@ impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
             string_cache: Default::default(),
             exported_symbols_cache: FxHashMap::default(),
             panic_on_unsupported: config.panic_on_unsupported,
+            backtrace_style: config.backtrace_style,
         }
     }