From 1e653aa96b392a04ed489eadc4cd040bc2859b4b Mon Sep 17 00:00:00 2001
From: Oliver Schneider <git-spam-no-reply9815368754983@oli-obk.de>
Date: Tue, 6 Feb 2018 15:35:43 +0100
Subject: [PATCH] Report a best guess span if no stack is available anymore

---
 src/librustc_mir/interpret/const_eval.rs   |  7 ++++---
 src/librustc_mir/interpret/eval_context.rs | 20 +++++++++++++++-----
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/librustc_mir/interpret/const_eval.rs b/src/librustc_mir/interpret/const_eval.rs
index ebb62070140..b44e91d7b74 100644
--- a/src/librustc_mir/interpret/const_eval.rs
+++ b/src/librustc_mir/interpret/const_eval.rs
@@ -24,7 +24,7 @@ pub fn mk_borrowck_eval_cx<'a, 'mir, 'tcx>(
 ) -> EvalResult<'tcx, EvalContext<'a, 'mir, 'tcx, CompileTimeEvaluator>> {
     debug!("mk_borrowck_eval_cx: {:?}", instance);
     let param_env = tcx.param_env(instance.def_id());
-    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, ());
+    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, (), span);
     // insert a stack frame so any queries have the correct substs
     ecx.push_stack_frame(
         instance,
@@ -42,7 +42,8 @@ pub fn mk_eval_cx<'a, 'tcx>(
     param_env: ty::ParamEnv<'tcx>,
 ) -> EvalResult<'tcx, EvalContext<'a, 'tcx, 'tcx, CompileTimeEvaluator>> {
     debug!("mk_eval_cx: {:?}, {:?}", instance, param_env);
-    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, ());
+    let span = tcx.def_span(instance.def_id());
+    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, (), span);
     let mir = ecx.load_mir(instance.def)?;
     // insert a stack frame so any queries have the correct substs
     ecx.push_stack_frame(
@@ -93,10 +94,10 @@ fn eval_body_and_ecx<'a, 'mir, 'tcx>(
     param_env: ty::ParamEnv<'tcx>,
 ) -> (EvalResult<'tcx, (Value, Pointer, Ty<'tcx>)>, EvalContext<'a, 'mir, 'tcx, CompileTimeEvaluator>) {
     debug!("eval_body: {:?}, {:?}", cid, param_env);
-    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, ());
     // we start out with the best span we have
     // and try improving it down the road when more information is available
     let mut span = tcx.def_span(cid.instance.def_id());
+    let mut ecx = EvalContext::new(tcx, param_env, CompileTimeEvaluator, (), mir.map(|mir| mir.span).unwrap_or(span));
     let res = (|| {
         let mut mir = match mir {
             Some(mir) => mir,
diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 2dc23e759c8..23ab6be1b46 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -45,6 +45,11 @@ pub struct EvalContext<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'mir, 'tcx>> {
     /// This prevents infinite loops and huge computations from freezing up const eval.
     /// Remove once halting problem is solved.
     pub(crate) steps_remaining: usize,
+
+    /// The span that is used if no more stack frames are available
+    ///
+    /// This happens after successful evaluation when the result is inspected
+    root_span: codemap::Span,
 }
 
 /// A stack frame.
@@ -186,6 +191,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
         param_env: ty::ParamEnv<'tcx>,
         machine: M,
         memory_data: M::MemoryData,
+        root_span: codemap::Span,
     ) -> Self {
         EvalContext {
             machine,
@@ -195,6 +201,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
             stack: Vec::new(),
             stack_limit: tcx.sess.const_eval_stack_frame_limit.get(),
             steps_remaining: tcx.sess.const_eval_step_limit.get(),
+            root_span,
         }
     }
 
@@ -1594,12 +1601,15 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
             };
             frames.push(FrameInfo { span, location });
         }
-        let frame = self.frame();
-        let bb = &frame.mir.basic_blocks()[frame.block];
-        let span = if let Some(stmt) = bb.statements.get(frame.stmt) {
-            stmt.source_info.span
+        let span = if let Some(frame) = self.stack().last() {
+            let bb = &frame.mir.basic_blocks()[frame.block];
+            if let Some(stmt) = bb.statements.get(frame.stmt) {
+                stmt.source_info.span
+            } else {
+                bb.terminator().source_info.span
+            }
         } else {
-            bb.terminator().source_info.span
+            self.root_span
         };
         trace!("generate stacktrace: {:#?}, {:?}", frames, explicit_span);
         (frames, span)