Improve readability of CLI trace output

core/engine/src/vm/mod.rs

@@ -21,9 +21,6 @@ use std::{future::Future, ops::ControlFlow, path::Path, pin::Pin, task};
 #[cfg(feature = "trace")]
 use crate::sys::time::Instant;
 
-#[cfg(feature = "trace")]
-use std::fmt::Write as _;
-
 #[allow(unused_imports)]
 pub(crate) use opcode::{Instruction, InstructionIterator, Opcode};
 
@@ -277,26 +274,100 @@ impl Stack {
     }
 
     #[cfg(feature = "trace")]
-    /// Display the stack trace of the current frame.
+    const MAX_VALUE_LEN: usize = 18;
+    #[cfg(feature = "trace")]
+    const MAX_STACK_WIDTH: usize = 68;
+
+    #[cfg(feature = "trace")]
+    fn raw_value(value: &JsValue) -> String {
+        match value {
+            v if v.is_callable() => "func".to_string(),
+            v if v.is_object() => "obj".to_string(),
+            v if v.is_undefined() => "und".to_string(),
+            v if v.is_null() => "null".to_string(),
+            v => v.display().to_string(),
+        }
+    }
+
+    #[cfg(feature = "trace")]
+    fn truncate_display(val: &str) -> String {
+        if val.len() <= Self::MAX_VALUE_LEN {
+            return val.to_string();
+        }
+        let mut end = Self::MAX_VALUE_LEN - 2;
+        while !val.is_char_boundary(end) && end > 0 {
+            end -= 1;
+        }
+        format!("{}..", &val[..end])
+    }
+
+    #[cfg(feature = "trace")]
     fn display_trace(&self, frame: &CallFrame, frame_count: usize) -> String {
-        let mut string = String::from("[ ");
-        for (i, (j, value)) in self.stack.iter().enumerate().rev().enumerate() {
-            match value {
-                value if value.is_callable() => string.push_str("[function]"),
-                value if value.is_object() => string.push_str("[object]"),
-                value => string.push_str(&value.display().to_string()),
-            }
+        let total = self.stack.len();
+        if total == 0 {
+            return "[ <empty> ]".to_string();
+        }
+
+        let mut groups: Vec<(String, usize, Option<usize>)> = Vec::new();
+        let mut force_truncate = false;
 
-            if frame.frame_pointer() == j {
-                let _ = write!(string, " |{frame_count}|");
-            } else if i + 1 != self.stack.len() {
-                string.push(',');
+        // Lazily group values to avoid eagerly evaluating `raw_value` for the entire stack.
+        for (idx, v) in self.stack.iter().enumerate().rev() {
+            let is_frame = frame.frame_pointer() == idx;
+            let raw = Self::raw_value(v);
+
+            if !is_frame
+                && let Some(last) = groups.last_mut()
+                && last.0 == raw
+                && last.2.is_none()
+            {
+                last.1 += 1;
+            } else {
+                let marker = if is_frame { Some(frame_count) } else { None };
+                groups.push((raw, 1, marker));
+
+                // If groups is large enough to mathematically guarantee overflowing the display width,
+                // we can stop evaluating to save instruction budget / time.
+                if groups.len() > Self::MAX_STACK_WIDTH / 2 {
+                    force_truncate = true;
+                    break;
+                }
             }
+        }
+
+        let mut string = String::from("[ ");
+        let mut truncated = force_truncate;
+        let suffix = format!(".. ({total} total) ]");
+
+        for (i, (val, count, marker)) in groups.iter().enumerate() {
+            let display_val = Self::truncate_display(val);
+            let part = if *count > 1 {
+                format!("{display_val} (x{count})")
+            } else {
+                display_val
+            };
+
+            let separator = if let Some(fc) = marker {
+                format!(" |{fc}|")
+            } else if i + 1 < groups.len() {
+                ",".to_string()
+            } else {
+                String::new()
+            };
 
-            string.push(' ');
+            let addition = format!("{part}{separator} ");
+            if string.len() + addition.len() + suffix.len() > Self::MAX_STACK_WIDTH {
+                truncated = true;
+                break;
+            }
+            string.push_str(&addition);
         }
 
-        string.push(']');
+        if truncated {
+            string.push_str(&suffix);
+        } else {
+            string.push(']');
+        }
         string
     }
 }