Improve backtraces when hovering widgets with modifiers pressed (#4696)

A useful debug-feature in egui is pressing down all modifiers keys and
hovering any widget to see its backtrace (only in `dev` builds).
Unfortunately this is incompatible with `panic="abort"`, something I
just now discovered.

So I removed `panic="abort"` from `Cargo.toml` for `dev` builds. If the
backtrace returns empty-handed, I also suggests this as a fix to the
user. Finally, I cleaned up the backtraces a bit, making them slightly
shorter and more readable.

Cargo.toml

@@ -43,7 +43,10 @@ panic = "abort" # This leads to better optimizations and smaller binaries (and i
 # split-debuginfo = "unpacked" # faster debug builds on mac
 # opt-level = 1                # Make debug builds run faster
 
-panic = "abort" # This leads to better optimizations and smaller binaries (and is the default in Wasm anyways).
+# panic = "abort" leads to better optimizations and smaller binaries (and is the default in Wasm anyways),
+# but it also means backtraces don't work with the `backtrace` library (https://github.com/rust-lang/backtrace-rs/issues/397).
+# egui has a feature where if you hold down all modifiers keys on your keyboard and hover any UI widget,
+# you will see the backtrace to that widget, and we don't want to break that feature in dev builds.
 
 [profile.dev.package."*"]
 # Optimize all dependencies even in debug builds (does not affect workspace packages):

crates/egui/src/callstack.rs

@@ -10,6 +10,7 @@ struct Frame {
 ///
 /// In particular: slips everything before `egui::Context::run`,
 /// and skipping all frames in the `egui::` namespace.
+#[inline(never)]
 pub fn capture() -> String {
     let mut frames = vec![];
     let mut depth = 0;
@@ -28,7 +29,7 @@ pub fn capture() -> String {
 
             let name = symbol
                 .name()
-                .map(|name| name.to_string())
+                .map(|name| clean_symbol_name(name.to_string()))
                 .unwrap_or_default();
 
             frames.push(Frame {
@@ -44,12 +45,13 @@ pub fn capture() -> String {
     });
 
     if frames.is_empty() {
-        return Default::default();
+        return
+            "Failed to capture a backtrace. A common cause of this is compiling with panic=\"abort\" (https://github.com/rust-lang/backtrace-rs/issues/397)".to_owned();
     }
 
     // Inclusive:
     let mut min_depth = 0;
-    let mut max_depth = frames.len() - 1;
+    let mut max_depth = usize::MAX;
 
     for frame in &frames {
         if frame.name.starts_with("egui::callstack::capture") {
@@ -60,14 +62,23 @@ pub fn capture() -> String {
         }
     }
 
+    /// Is this the name of some sort of useful entry point?
+    fn is_start_name(name: &str) -> bool {
+        name == "main"
+            || name == "_main"
+            || name.starts_with("eframe::run_native")
+            || name.starts_with("egui::context::Context::run")
+    }
+
+    let mut has_kept_any_start_names = false;
+
+    frames.reverse(); // main on top, i.e. chronological order. Same as Python.
+
     // Remove frames that are uninteresting:
     frames.retain(|frame| {
-        // Keep some special frames to give the user a sense of chronology:
-        if frame.name == "main"
-            || frame.name == "_main"
-            || frame.name.starts_with("egui::context::Context::run")
-            || frame.name.starts_with("eframe::run_native")
-        {
+        // Keep the first "start" frame we can detect (e.g. `main`) to give the user a sense of chronology:
+        if is_start_name(&frame.name) && !has_kept_any_start_names {
+            has_kept_any_start_names = true;
             return true;
         }
 
@@ -96,8 +107,6 @@ pub fn capture() -> String {
         true
     });
 
-    frames.reverse(); // main on top, i.e. chronological order. Same as Python.
-
     let mut deepest_depth = 0;
     let mut widest_file_line = 0;
     for frame in &frames {
@@ -135,6 +144,28 @@ pub fn capture() -> String {
     formatted
 }
 
+fn clean_symbol_name(mut s: String) -> String {
+    // We get a hex suffix (at least on macOS) which is quite unhelpful,
+    // e.g. `my_crate::my_function::h3bedd97b1e03baa5`.
+    // Let's strip that.
+    if let Some(h) = s.rfind("::h") {
+        let hex = &s[h + 3..];
+        if hex.len() == 16 && hex.chars().all(|c| c.is_ascii_hexdigit()) {
+            s.truncate(h);
+        }
+    }
+
+    s
+}
+
+#[test]
+fn test_clean_symbol_name() {
+    assert_eq!(
+        clean_symbol_name("my_crate::my_function::h3bedd97b1e03baa5".to_owned()),
+        "my_crate::my_function"
+    );
+}
+
 /// Shorten a path to a Rust source file from a callstack.
 ///
 /// Example input: