CTFE: LongRunningWarn is getting implicitly deduplicated

[lcnr]

I tried the following code:

#![allow(long_running_const_eval)]
const FOO: () = loop {};

fn main() {
    FOO
}

output after running for a very long time:

warning: constant evaluation is taking a long time
 --> src/main.rs:2:17
  |
2 | const FOO: () = loop {};
  |                 ^^^^^^^ the const evaluator is currently interpreting this expression
  |
help: the constant being evaluated
 --> src/main.rs:2:1
  |
2 | const FOO: () = loop {};
  | ^^^^^^^^^^^^^

modify rustc as follows results in the following output:

diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index 2d8ca67c3a5..1bf7949725b 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -593,6 +593,9 @@ fn increment_const_eval_counter(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpR
             };
 
             ecx.machine.num_evaluated_steps = new_steps;
+            if new_steps.is_power_of_two() {
+                eprintln!("power of two : {new_steps}");
+            }
             // By default, we have a *deny* lint kicking in after some time
             // to ensure `loop {}` doesn't just go forever.
             // In case that lint got reduced, in particular for `--cap-lint` situations, we also

   Compiling test1 v0.1.0 (/home/lcnr/test1)
power of two : 1
power of two : 2
power of two : 4
power of two : 8
power of two : 16
power of two : 32
power of two : 64
power of two : 128
power of two : 256
power of two : 512
power of two : 1024
power of two : 2048
power of two : 4096
power of two : 8192
power of two : 16384
power of two : 32768
power of two : 65536
power of two : 131072
power of two : 262144
power of two : 524288
power of two : 1048576
power of two : 2097152
power of two : 4194304
warning: constant evaluation is taking a long time
 --> src/main.rs:2:17
  |
2 | const FOO: () = loop {};
  |                 ^^^^^^^ the const evaluator is currently interpreting this expression
  |
help: the constant being evaluated
 --> src/main.rs:2:1
  |
2 | const FOO: () = loop {};
  | ^^^^^^^^^^^^^

power of two : 8388608
power of two : 16777216
power of two : 33554432
power of two : 67108864
power of two : 134217728

changing the invocation to cargo rustc -- -Zdeduplicate-diagnostics=no actually prints the warning at each power of two of steps.

This warning should not be affected by deduplicate-diagnostics. It's whole point is to be repeated multiple times.

cc @oli-obk @RalfJung

[RalfJung]

It's whole point is to be repeated multiple times.

I honestly don't remember if we want it to be repeated or not.

Is there even a way for a single diagnostic to opt-out of deduplication? I guess we could put some number into the error message so that it's not the exact same diagnostic each time.

[lcnr]

Is there even a way for a single diagnostic to opt-out of deduplication? I guess we could put some number into the error message so that it's not the exact same diagnostic each time.

yes, we have full ownership of rustc, we can the deduplication code to stop duplicating this single warning while keeping everything else the same 😁

[veera-sivarajan]

@rustbot claim