Rollup merge of #55970 - RalfJung:miri-backtrace, r=@oli-obk

Miri backtrace improvements

Nicer pretty-printing of the `RUST_CTFE_BACKTRACE`-backtraces:
```
  0: backtrace::backtrace::libunwind::trace::hc410fcb66fe85b11
           at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/libunwind.rs:53
     backtrace::backtrace::trace::h2106294a22648407
           at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/backtrace/mod.rs:42
  1: backtrace::capture::Backtrace::new_unresolved::h5d8d98b993d092ba
           at /home/r/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.9/src/capture.rs:88
  2: <rustc::mir::interpret::error::EvalError<'tcx> as core::convert::From<rustc::mir::interpret::error::EvalErrorKind<'tcx, u64>>>::from::h6355269b2a661412
           at librustc/mir/interpret/error.rs:236
  3: <T as core::convert::Into<U>>::into::h70fcb917509539bd
           at /home/r/src/rust/rustc.2/src/libcore/convert.rs:455
  4: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, miri::Evaluator<'tcx>> as miri::fn_call::EvalContextExt<'tcx, 'mir>>::emulate_foreign_item::h9cde0e3ce7455a4a
           at src/fn_call.rs:292
  5: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, miri::Evaluator<'tcx>> as miri::fn_call::EvalContextExt<'tcx, 'mir>>::find_fn::h83f89524b9d1a49a
           at src/fn_call.rs:74
  6: <miri::Evaluator<'tcx> as rustc_mir::interpret::machine::Machine<'a, 'mir, 'tcx>>::find_fn::hf9980473c4775f0c
           at src/lib.rs:345
     rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::eval_fn_call::h401dec4a687f96e9
           at /home/r/src/rust/rustc.2/src/librustc_mir/interpret/terminator.rs:285
```
Indentation is now consistent with `RUST_BACKTRACE`, and the frame number is not repeated when there are multiple symbols for a frame.

Also preserve the `ty::Instance` for the internal backtrace (showing which frames in the user code where on the interpreter stack when the error happened), used by miri to avoid printing spans for libstd internals:
```
error[E0080]: constant evaluation error: the evaluated program panicked
   --> /home/r/src/rust/rustc.2/src/libstd/panicking.rs:525:9
    |
525 |         __rust_start_panic(obj as usize)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked
    |
    = note: inside call to `std::panicking::rust_panic` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:496:5
    = note: inside call to `std::panicking::rust_panic_with_hook` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:390:5
    = note: inside call to `std::panicking::continue_panic_fmt` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:345:5
note: inside call to `std::rt::begin_panic_fmt` at <::std::macros::panic macros>:8:1
   --> tests/compile-fail/panic.rs:4:5
    |
4   |     assert_eq!(5, 6);
    |     ^^^^^^^^^^^^^^^^^
    = note: inside call to `main` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:74:34
    = note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/rt.rs:59:75
    = note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/sys_common/backtrace.rs:136:5
    = note: inside call to `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1/1:1913 ~ std[78f0]::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/src/rust/rustc.2/src/libstd/rt.rs:59:13
    = note: inside call to closure at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:310:40
    = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/src/rust/rustc.2/src/libstd/panicking.rs:306:5
    = note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/r/src/rust/rustc.2/src/libstd/panic.rs:398:9
    = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1912 ~ std[78f0]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:58:25
    = note: inside call to `std::rt::lang_start_internal` at /home/r/src/rust/rustc.2/src/libstd/rt.rs:74:5
    = note: inside call to `std::rt::lang_start::<()>`
```
Also notice that we show filenames and line numbers here now.

r? @oli-obk

1 files changed, 43 insertions, 12 deletions

diff --git a/src/librustc/mir/interpret/error.rs b/src/librustc/mir/interpret/error.rs
index bf678db51c9..f1c95e0f000 100644
--- a/src/librustc/mir/interpret/error.rs
+++ b/src/librustc/mir/interpret/error.rs
@@ -10,8 +10,9 @@
 
 use std::{fmt, env};
 
+use hir::map::definitions::DefPathData;
 use mir;
-use ty::{Ty, layout};
+use ty::{self, Ty, layout};
 use ty::layout::{Size, Align, LayoutError};
 use rustc_target::spec::abi::Abi;
 
@@ -19,11 +20,10 @@ use super::{Pointer, InboundsCheck, ScalarMaybeUndef};
 
 use backtrace::Backtrace;
 
-use ty;
 use ty::query::TyCtxtAt;
 use errors::DiagnosticBuilder;
 
-use syntax_pos::Span;
+use syntax_pos::{Pos, Span};
 use syntax::ast;
 use syntax::symbol::Symbol;
 
@@ -52,16 +52,35 @@ pub type ConstEvalResult<'tcx> = Result<&'tcx ty::Const<'tcx>, ErrorHandled>;
 pub struct ConstEvalErr<'tcx> {
     pub span: Span,
     pub error: ::mir::interpret::EvalErrorKind<'tcx, u64>,
-    pub stacktrace: Vec<FrameInfo>,
+    pub stacktrace: Vec<FrameInfo<'tcx>>,
 }
 
 #[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
-pub struct FrameInfo {
-    pub span: Span,
-    pub location: String,
+pub struct FrameInfo<'tcx> {
+    pub call_site: Span, // this span is in the caller!
+    pub instance: ty::Instance<'tcx>,
     pub lint_root: Option<ast::NodeId>,
 }
 
+impl<'tcx> fmt::Display for FrameInfo<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        ty::tls::with(|tcx| {
+            if tcx.def_key(self.instance.def_id()).disambiguated_data.data
+                == DefPathData::ClosureExpr
+            {
+                write!(f, "inside call to closure")?;
+            } else {
+                write!(f, "inside call to `{}`", self.instance)?;
+            }
+            if !self.call_site.is_dummy() {
+                let lo = tcx.sess.source_map().lookup_char_pos_adj(self.call_site.lo());
+                write!(f, " at {}:{}:{}", lo.filename, lo.line, lo.col.to_usize() + 1)?;
+            }
+            Ok(())
+        })
+    }
+}
+
 impl<'a, 'gcx, 'tcx> ConstEvalErr<'tcx> {
     pub fn struct_error(&self,
         tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
@@ -135,8 +154,13 @@ impl<'a, 'gcx, 'tcx> ConstEvalErr<'tcx> {
             struct_error(tcx, message)
         };
         err.span_label(self.span, self.error.to_string());
-        for FrameInfo { span, location, .. } in &self.stacktrace {
-            err.span_label(*span, format!("inside call to `{}`", location));
+        // Skip the last, which is just the environment of the constant.  The stacktrace
+        // is sometimes empty because we create "fake" eval contexts in CTFE to do work
+        // on constant values.
+        if self.stacktrace.len() > 0 {
+            for frame_info in &self.stacktrace[..self.stacktrace.len()-1] {
+                err.span_label(frame_info.call_site, frame_info.to_string());
+            }
         }
         Ok(err)
     }
@@ -172,16 +196,23 @@ fn print_backtrace(backtrace: &mut Backtrace) -> String {
     write!(trace_text, "backtrace frames: {}\n", backtrace.frames().len()).unwrap();
     'frames: for (i, frame) in backtrace.frames().iter().enumerate() {
         if frame.symbols().is_empty() {
-            write!(trace_text, "{}: no symbols\n", i).unwrap();
+            write!(trace_text, "  {}: no symbols\n", i).unwrap();
         }
+        let mut first = true;
         for symbol in frame.symbols() {
-            write!(trace_text, "{}: ", i).unwrap();
+            if first {
+                write!(trace_text, "  {}: ", i).unwrap();
+                first = false;
+            } else {
+                let len = i.to_string().len();
+                write!(trace_text, "  {}  ", " ".repeat(len)).unwrap();
+            }
             if let Some(name) = symbol.name() {
                 write!(trace_text, "{}\n", name).unwrap();
             } else {
                 write!(trace_text, "<unknown>\n").unwrap();
             }
-            write!(trace_text, "\tat ").unwrap();
+            write!(trace_text, "           at ").unwrap();
             if let Some(file_path) = symbol.filename() {
                 write!(trace_text, "{}", file_path.display()).unwrap();
             } else {