Deduplicate lines in long const-eval stack trace

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -156,9 +156,37 @@ impl<'tcx> ConstEvalErr<'tcx> {
  }
  // Add spans for the stacktrace. Don't print a single-line backtrace though.
  if self.stacktrace.len() > 1 {
+ // Helper closure to print duplicated lines.
+ let mut flush_last_line = |last_frame, times| {
+ if let Some((line, span)) = last_frame {
+ err.span_label(span, &line);
+ // Don't print [... additional calls ...] if the number of lines is small
+ if times < 3 {
+ for _ in 0..times {
+ err.span_label(span, &line);
+ }
+ } else {
+ err.span_label(
+ span,
+ format!("[... {} additional calls {} ...]", times, &line),
+ );
+ }
+ }
+ };
+
+ let mut last_frame = None;
+ let mut times = 0;
  for frame_info in &self.stacktrace {
- err.span_label(frame_info.span, frame_info.to_string());
+ let frame = (frame_info.to_string(), frame_info.span);
+ if last_frame.as_ref() == Some(&frame) {
+ times += 1;
+ } else {
+ flush_last_line(last_frame, times);
+ last_frame = Some(frame);
+ times = 0;
+ }
  }
+ flush_last_line(last_frame, times);
  }
  // Let the caller finish the job.
  emit(err)

src/test/ui/consts/recursive.rs

@@ -0,0 +1,11 @@
+#![allow(unused)]
+
+const fn f<T>(x: T) { //~ WARN function cannot return without recursing
+ f(x);
+ //~^ ERROR any use of this value will cause an error
+ //~| WARN this was previously accepted by the compiler
+}
+
+const X: () = f(1);
+
+fn main() {}

src/test/ui/consts/recursive.stderr

@@ -0,0 +1,31 @@
+warning: function cannot return without recursing
+ --> $DIR/recursive.rs:3:1
+ |
+LL | const fn f<T>(x: T) {
+ | ^^^^^^^^^^^^^^^^^^^ cannot return without recursing
+LL | f(x);
+ | ---- recursive call site
+ |
+ = note: `#[warn(unconditional_recursion)]` on by default
+ = help: a `loop` may express intention better if this is on purpose
+
+error: any use of this value will cause an error
+ --> $DIR/recursive.rs:4:5
+ |
+LL | f(x);
+ | ^^^^
+ | |
+ | reached the configured maximum number of stack frames
+ | inside `f::<i32>` at $DIR/recursive.rs:4:5
+ | [... 126 additional calls inside `f::<i32>` at $DIR/recursive.rs:4:5 ...]
+ | inside `X` at $DIR/recursive.rs:9:15
+...
+LL | const X: () = f(1);
+ | -------------------
+ |
+ = note: `#[deny(const_err)]` on by default
+ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+ = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to previous error; 1 warning emitted
+