Use tracing crate for logging (#1192)

* egui_web: use tracing crate
* egui_glow: use tracing crate
* Log at the debug level
* egui_demo_app: enable tracing to log to stdout
* Use tracing in egui-winit
* Add opt-in tracing support to egui

egui/Cargo.toml

@@ -28,8 +28,12 @@ epaint = { version = "0.16.0", path = "../epaint", default-features = false }
 
 ahash = "0.7"
 nohash-hasher = "0.2"
+
+# Optional:
 ron = { version = "0.7", optional = true }
 serde = { version = "1", features = ["derive", "rc"], optional = true }
+# egui doesn't log much, but when it does, it uses `tracing`
+tracing = { version = "0.1", optional = true }
 
 [features]
 default = ["default_fonts", "single_threaded"]
@@ -37,6 +41,9 @@ default = ["default_fonts", "single_threaded"]
 # add compatibility with https://crates.io/crates/cint
 cint = ["epaint/cint"]
 
+# implement bytemuck on most types.
+convert_bytemuck = ["epaint/convert_bytemuck"]
+
 # If set, egui will use `include_bytes!` to bundle some fonts.
 # If you plan on specifying your own fonts you may disable this feature.
 default_fonts = ["epaint/default_fonts"]
@@ -55,9 +62,6 @@ persistence = ["serde", "epaint/serialize", "ron"]
 # implement serde on most types.
 serialize = ["serde", "epaint/serialize"]
 
-# implement bytemuck on most types.
-convert_bytemuck = ["epaint/convert_bytemuck"]
-
 # multi_threaded is only needed if you plan to use the same egui::Context
 # from multiple threads. It comes with a minor performance impact.
 single_threaded = ["epaint/single_threaded"]

egui/src/util/id_type_map.rs

@@ -275,11 +275,12 @@ impl Element {
 fn from_ron_str<T: serde::de::DeserializeOwned>(ron: &str) -> Option<T> {
     match ron::from_str::<T>(ron) {
         Ok(value) => Some(value),
-        Err(err) => {
-            eprintln!(
+        Err(_err) => {
+            #[cfg(feature = "tracing")]
+            tracing::warn!(
                 "egui: Failed to deserialize {} from memory: {}, ron error: {:?}",
                 std::any::type_name::<T>(),
-                err,
+                _err,
                 ron
             );
             None