Tweak output for note pointing at root cause of const error

```
error[E0080]: evaluation of constant value failed
  --> $DIR/issue-81899.rs:4:24
   |
LL | const _CONST: &[u8] = &f(&[], |_| {});
   |                        ^^^^^^^^^^^^^^ panic: explicit panic
   |
note: the evaluated program failed inside `f::<{closure@$DIR/issue-81899.rs:4:31: 4:34}>`
  --> $DIR/issue-81899.rs:11:5
   |
LL |     panic!()
   |     ^^^^^^^^ the evaluated program failed here
   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
```

Author: estebank

compiler/rustc_const_eval/messages.ftl

@@ -103,12 +103,17 @@ const_eval_frame_note = {$times ->
     *[other] [... {$times} additional calls {const_eval_frame_note_inner} ...]
 }
 
-const_eval_frame_note_inner = inside {$where_ ->
+const_eval_frame_note_inner = {$is_last ->
+    [true] {"the evaluated program failed "}
+    *[false] {""}
+    }inside {$where_ ->
     [closure] closure
     [instance] `{$instance}`
     *[other] {""}
 }
 
+const_eval_frame_note_last = the evaluated program failed here
+
 const_eval_in_bounds_test = out-of-bounds pointer use
 const_eval_incompatible_calling_conventions =
     calling a function with calling convention {$callee_conv} using calling convention {$caller_conv}

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -115,6 +115,9 @@ pub fn get_span_and_frames<'tcx>(
     if frames.len() > 0 {
         frames.remove(0);
     }
+    if let Some(last) = frames.last_mut() {
+        last.is_last = true;
+    }
 
     (span, frames)
 }

compiler/rustc_const_eval/src/errors.rs

@@ -6,6 +6,7 @@ use rustc_abi::WrappingRange;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Diag, DiagArgValue, DiagCtxtHandle, DiagMessage, Diagnostic, EmissionGuarantee, Level,
+    MultiSpan, SubdiagMessageOp, Subdiagnostic,
 };
 use rustc_hir::ConstContext;
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -17,6 +18,7 @@ use rustc_middle::mir::interpret::{
 use rustc_middle::ty::{self, Mutability, Ty};
 use rustc_span::{Span, Symbol};
 
+use crate::fluent_generated as fluent;
 use crate::interpret::InternKind;
 
 #[derive(Diagnostic)]
@@ -278,14 +280,32 @@ pub(crate) struct NonConstImplNote {
     pub span: Span,
 }
 
-#[derive(Subdiagnostic, Clone)]
-#[note(const_eval_frame_note)]
+#[derive(Clone)]
 pub struct FrameNote {
-    #[primary_span]
     pub span: Span,
     pub times: i32,
     pub where_: &'static str,
     pub instance: String,
+    pub is_last: bool,
+}
+
+impl Subdiagnostic for FrameNote {
+    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
+        self,
+        diag: &mut Diag<'_, G>,
+        f: &F,
+    ) {
+        diag.arg("times", self.times);
+        diag.arg("where_", self.where_);
+        diag.arg("instance", self.instance);
+        diag.arg("is_last", self.is_last);
+        let mut span: MultiSpan = self.span.into();
+        if self.is_last && !self.span.is_dummy() {
+            span.push_span_label(self.span, fluent::const_eval_frame_note_last);
+        }
+        let msg = f(diag, fluent::const_eval_frame_note.into());
+        diag.span_note(span, msg);
+    }
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_const_eval/src/interpret/stack.rs

@@ -209,6 +209,7 @@ impl<'tcx, Prov: Provenance> LocalState<'tcx, Prov> {
 pub struct FrameInfo<'tcx> {
     pub instance: ty::Instance<'tcx>,
     pub span: Span,
+    pub is_last: bool,
 }
 
 // FIXME: only used by miri, should be removed once translatable.
@@ -231,13 +232,25 @@ impl<'tcx> FrameInfo<'tcx> {
     pub fn as_note(&self, tcx: TyCtxt<'tcx>) -> errors::FrameNote {
         let span = self.span;
         if tcx.def_key(self.instance.def_id()).disambiguated_data.data == DefPathData::Closure {
-            errors::FrameNote { where_: "closure", span, instance: String::new(), times: 0 }
+            errors::FrameNote {
+                where_: "closure",
+                span,
+                instance: String::new(),
+                times: 0,
+                is_last: self.is_last,
+            }
         } else {
             let instance = format!("{}", self.instance);
             // Note: this triggers a `must_produce_diag` state, which means that if we ever get
             // here we must emit a diagnostic. We should never display a `FrameInfo` unless we
             // actually want to emit a warning or error to the user.
-            errors::FrameNote { where_: "instance", span, instance, times: 0 }
+            errors::FrameNote {
+                where_: "instance",
+                span,
+                instance,
+                times: 0,
+                is_last: self.is_last,
+            }
         }
     }
 }
@@ -322,15 +335,15 @@ impl<'tcx, Prov: Provenance, Extra> Frame<'tcx, Prov, Extra> {
                     let mir::SourceInfo { mut span, scope } = *frame.body.source_info(loc);
                     let mut scope_data = &frame.body.source_scopes[scope];
                     while let Some((instance, call_span)) = scope_data.inlined {
-                        frames.push(FrameInfo { span, instance });
+                        frames.push(FrameInfo { span, instance, is_last: false });
                         span = call_span;
                         scope_data = &frame.body.source_scopes[scope_data.parent_scope.unwrap()];
                     }
                     span
                 }
                 Right(span) => span,
             };
-            frames.push(FrameInfo { span, instance: frame.instance });
+            frames.push(FrameInfo { span, instance: frame.instance, is_last: false });
         }
         trace!("generate stacktrace: {:#?}", frames);
         frames

tests/ui/borrowck/issue-81899.stderr

@@ -4,11 +4,11 @@ error[E0080]: evaluation of constant value failed
 LL | const _CONST: &[u8] = &f(&[], |_| {});
    |                        ^^^^^^^^^^^^^^ panic: explicit panic
    |
-note: inside `f::<{closure@$DIR/issue-81899.rs:4:31: 4:34}>`
+note: the evaluated program failed inside `f::<{closure@$DIR/issue-81899.rs:4:31: 4:34}>`
   --> $DIR/issue-81899.rs:11:5
    |
 LL |     panic!()
-   |     ^^^^^^^^
+   |     ^^^^^^^^ the evaluated program failed here
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered

tests/ui/borrowck/issue-88434-minimal-example.stderr

@@ -4,11 +4,11 @@ error[E0080]: evaluation of constant value failed
 LL | const _CONST: &() = &f(&|_| {});
    |                      ^^^^^^^^^^ panic: explicit panic
    |
-note: inside `f::<{closure@$DIR/issue-88434-minimal-example.rs:3:25: 3:28}>`
+note: the evaluated program failed inside `f::<{closure@$DIR/issue-88434-minimal-example.rs:3:25: 3:28}>`
   --> $DIR/issue-88434-minimal-example.rs:10:5
    |
 LL |     panic!()
-   |     ^^^^^^^^
+   |     ^^^^^^^^ the evaluated program failed here
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered

tests/ui/borrowck/issue-88434-removal-index-should-be-less.stderr

@@ -4,11 +4,11 @@ error[E0080]: evaluation of constant value failed
 LL | const _CONST: &[u8] = &f(&[], |_| {});
    |                        ^^^^^^^^^^^^^^ panic: explicit panic
    |
-note: inside `f::<{closure@$DIR/issue-88434-removal-index-should-be-less.rs:3:31: 3:34}>`
+note: the evaluated program failed inside `f::<{closure@$DIR/issue-88434-removal-index-should-be-less.rs:3:31: 3:34}>`
   --> $DIR/issue-88434-removal-index-should-be-less.rs:10:5
    |
 LL |     panic!()
-   |     ^^^^^^^^
+   |     ^^^^^^^^ the evaluated program failed here
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered

tests/ui/coherence/const-errs-dont-conflict-103369.stderr

@@ -4,11 +4,11 @@ error[E0080]: evaluation of constant value failed
 LL | impl ConstGenericTrait<{my_fn(1)}> for () {}
    |                         ^^^^^^^^ panic: Some error occurred
    |
-note: inside `my_fn`
+note: the evaluated program failed inside `my_fn`
   --> $DIR/const-errs-dont-conflict-103369.rs:10:5
    |
 LL |     panic!("Some error occurred");
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program failed here
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0080]: evaluation of constant value failed
@@ -17,11 +17,11 @@ error[E0080]: evaluation of constant value failed
 LL | impl ConstGenericTrait<{my_fn(2)}> for () {}
    |                         ^^^^^^^^ panic: Some error occurred
    |
-note: inside `my_fn`
+note: the evaluated program failed inside `my_fn`
   --> $DIR/const-errs-dont-conflict-103369.rs:10:5
    |
 LL |     panic!("Some error occurred");
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program failed here
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

tests/ui/const-generics/issues/issue-100313.stderr

@@ -4,11 +4,11 @@ error[E0080]: evaluation of constant value failed
 LL |     x.set_false();
    |     ^^^^^^^^^^^^^ writing to ALLOC0 which is read-only
    |
-note: inside `T::<&true>::set_false`
+note: the evaluated program failed inside `T::<&true>::set_false`
   --> $DIR/issue-100313.rs:9:13
    |
 LL |             *(B as *const bool as *mut bool) = false;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program failed here
 
 error: aborting due to 1 previous error
 

tests/ui/const-ptr/forbidden_slices.stderr

@@ -107,8 +107,10 @@ LL | pub static R1: &[()] = unsafe { from_ptr_range(ptr::null()..ptr::null()) };
    |
 note: inside `from_ptr_range::<'_, ()>`
   --> $SRC_DIR/core/src/slice/raw.rs:LL:COL
-note: inside `std::ptr::const_ptr::<impl *const ()>::sub_ptr`
+note: the evaluated program failed inside `std::ptr::const_ptr::<impl *const ()>::sub_ptr`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+   = note: the evaluated program failed here
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0080]: could not evaluate static initializer
@@ -117,8 +119,10 @@ error[E0080]: could not evaluate static initializer
 LL |     from_ptr_range(ptr..ptr.add(2)) // errors inside libcore
    |                         ^^^^^^^^^^ out-of-bounds pointer arithmetic: expected a pointer to 8 bytes of memory, but got ALLOC10 which is only 4 bytes from the end of the allocation
    |
-note: inside `std::ptr::const_ptr::<impl *const u32>::add`
+note: the evaluated program failed inside `std::ptr::const_ptr::<impl *const u32>::add`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+   = note: the evaluated program failed here
 
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/forbidden_slices.rs:57:1
@@ -172,8 +176,10 @@ error[E0080]: could not evaluate static initializer
 LL |     from_ptr_range(ptr..ptr.add(1))
    |                         ^^^^^^^^^^ out-of-bounds pointer arithmetic: expected a pointer to 8 bytes of memory, but got ALLOC11+0x1 which is only 7 bytes from the end of the allocation
    |
-note: inside `std::ptr::const_ptr::<impl *const u64>::add`
+note: the evaluated program failed inside `std::ptr::const_ptr::<impl *const u64>::add`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+   = note: the evaluated program failed here
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/forbidden_slices.rs:85:34
@@ -183,8 +189,10 @@ LL | pub static R9: &[u32] = unsafe { from_ptr_range(&D0..(&D0 as *const u32).ad
    |
 note: inside `from_ptr_range::<'_, u32>`
   --> $SRC_DIR/core/src/slice/raw.rs:LL:COL
-note: inside `std::ptr::const_ptr::<impl *const u32>::sub_ptr`
+note: the evaluated program failed inside `std::ptr::const_ptr::<impl *const u32>::sub_ptr`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+   = note: the evaluated program failed here
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/forbidden_slices.rs:87:35
@@ -194,8 +202,10 @@ LL | pub static R10: &[u32] = unsafe { from_ptr_range(&D0..&D0) };
    |
 note: inside `from_ptr_range::<'_, u32>`
   --> $SRC_DIR/core/src/slice/raw.rs:LL:COL
-note: inside `std::ptr::const_ptr::<impl *const u32>::sub_ptr`
+note: the evaluated program failed inside `std::ptr::const_ptr::<impl *const u32>::sub_ptr`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+   = note: the evaluated program failed here
 
 error: aborting due to 18 previous errors