we can now print on entering/leaving the topmost frame, and make sure it stays that way

Author: RalfJung

src/librustc_mir/const_eval.rs

@@ -602,14 +602,15 @@ pub fn const_eval_raw_provider<'a, 'tcx>(
             other => return other,
         }
     }
-    // the first trace is for replicating an ice
-    // There's no tracking issue, but the next two lines concatenated link to the discussion on
-    // zulip. It's not really possible to test this, because it doesn't show up in diagnostics
-    // or MIR.
-    // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
-    // subject/anon_const_instance_printing/near/135980032
-    trace!("const eval: {}", key.value.instance);
-    trace!("const eval: {:?}", key);
+    if cfg!(debug_assertions) {
+        // Make sure we format the instance even if we do not print it.
+        // This serves as a regression test against an ICE on printing.
+        // The next two lines concatenated contain some discussion:
+        // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
+        // subject/anon_const_instance_printing/near/135980032
+        let instance = key.value.instance.to_string();
+        trace!("const eval: {:?} ({})", key, instance);
+    }
 
     let cid = key.value;
     let def_id = cid.instance.def.def_id();

src/librustc_mir/interpret/eval_context.rs

@@ -450,7 +450,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         return_place: Option<PlaceTy<'tcx, M::PointerTag>>,
         return_to_block: StackPopCleanup,
     ) -> EvalResult<'tcx> {
-        if self.stack.len() > 1 { // FIXME should be "> 0", printing topmost frame crashes rustc...
+        if self.stack.len() > 0 {
             info!("PAUSING({}) {}", self.cur_frame(), self.frame().instance);
         }
         ::log_settings::settings().indentation += 1;
@@ -525,9 +525,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
             self.frame_mut().locals = locals;
         }
 
-        if self.stack.len() > 1 { // FIXME no check should be needed, but some instances ICE
-            info!("ENTERING({}) {}", self.cur_frame(), self.frame().instance);
-        }
+        info!("ENTERING({}) {}", self.cur_frame(), self.frame().instance);
 
         if self.stack.len() > self.tcx.sess.const_eval_stack_frame_limit {
             err!(StackFrameLimitReached)
@@ -537,9 +535,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
     }
 
     pub(super) fn pop_stack_frame(&mut self) -> EvalResult<'tcx> {
-        if self.stack.len() > 1 { // FIXME no check should be needed, but some instances ICE
-            info!("LEAVING({}) {}", self.cur_frame(), self.frame().instance);
-        }
+        info!("LEAVING({}) {}", self.cur_frame(), self.frame().instance);
         ::log_settings::settings().indentation -= 1;
         let frame = self.stack.pop().expect(
             "tried to pop a stack frame, but there were none",
@@ -591,7 +587,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
             StackPopCleanup::None { .. } => {}
         }
 
-        if self.stack.len() > 1 { // FIXME should be "> 0", printing topmost frame crashes rustc...
+        if self.stack.len() > 0 {
             info!("CONTINUING({}) {}", self.cur_frame(), self.frame().instance);
         }