Add useful Debug impl to PropertyBag and ConfigBag (#2612)

## Motivation and Context
- it's hard to debug what properties are set, especially for layered
configuration

## Description
Add Debug Info for PropertyBag and Config bag:
```
PropertyBag { contents: ["aws_smithy_http::property_bag::test::test_extensions::MyType"] }
```

```

ConfigBag {
    layers: [
        Layer {
            name: "c",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop3>",
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop4>",
            ],
        },
        Layer {
            name: "b",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop3>",
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop2>",
            ],
        },
        Layer {
            name: "a",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop1>",
            ],
        },
        Layer {
            name: "base",
            properties: [],
        },
    ],
}
```

There is still some work to do, but this is a start

## Testing
- [x] UT

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: John DiSanti <jdisanti@amazon.com>

CHANGELOG.next.toml

@@ -10,3 +10,10 @@
 # references = ["smithy-rs#920"]
 # meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client | server | all"}
 # author = "rcoh"
+
+[[smithy-rs]]
+message = "The `Debug` implementation for `PropertyBag` now prints a list of the types it contains. This significantly improves debuggability."
+author = "rcoh"
+references = ["smithy-rs#2612"]
+meta = { "breaking" = false, "tada" = false, "bug" = false }
+

rust-runtime/aws-smithy-http/src/property_bag.rs

@@ -13,12 +13,38 @@
 use std::any::{Any, TypeId};
 use std::collections::HashMap;
 use std::fmt;
-use std::fmt::Debug;
+use std::fmt::{Debug, Formatter};
 use std::hash::{BuildHasherDefault, Hasher};
 use std::ops::{Deref, DerefMut};
 use std::sync::{Arc, Mutex};
 
-type AnyMap = HashMap<TypeId, Box<dyn Any + Send + Sync>, BuildHasherDefault<IdHasher>>;
+type AnyMap = HashMap<TypeId, NamedType, BuildHasherDefault<IdHasher>>;
+
+struct NamedType {
+    name: &'static str,
+    value: Box<dyn Any + Send + Sync>,
+}
+
+impl NamedType {
+    fn as_mut<T: 'static>(&mut self) -> Option<&mut T> {
+        self.value.downcast_mut()
+    }
+
+    fn as_ref<T: 'static>(&self) -> Option<&T> {
+        self.value.downcast_ref()
+    }
+
+    fn assume<T: 'static>(self) -> Option<T> {
+        self.value.downcast().map(|t| *t).ok()
+    }
+
+    fn new<T: Any + Send + Sync>(value: T) -> Self {
+        Self {
+            name: std::any::type_name::<T>(),
+            value: Box::new(value),
+        }
+    }
+}
 
 // With TypeIds as keys, there's no need to hash them. They are already hashes
 // themselves, coming from the compiler. The IdHasher just holds the u64 of
@@ -82,13 +108,8 @@ impl PropertyBag {
     /// ```
     pub fn insert<T: Send + Sync + 'static>(&mut self, val: T) -> Option<T> {
         self.map
-            .insert(TypeId::of::<T>(), Box::new(val))
-            .and_then(|boxed| {
-                (boxed as Box<dyn Any + 'static>)
-                    .downcast()
-                    .ok()
-                    .map(|boxed| *boxed)
-            })
+            .insert(TypeId::of::<T>(), NamedType::new(val))
+            .and_then(|val| val.assume())
     }
 
     /// Get a reference to a type previously inserted on this `PropertyBag`.
@@ -106,7 +127,16 @@ impl PropertyBag {
     pub fn get<T: Send + Sync + 'static>(&self) -> Option<&T> {
         self.map
             .get(&TypeId::of::<T>())
-            .and_then(|boxed| (&**boxed as &(dyn Any + 'static)).downcast_ref())
+            .and_then(|val| val.as_ref())
+    }
+
+    /// Returns an iterator of the types contained in this PropertyBag
+    ///
+    /// # Stability
+    /// This method is unstable and may be removed or changed in a future release. The exact
+    /// format of the returned types may also change.
+    pub fn contents(&self) -> impl Iterator<Item = &'static str> + '_ {
+        self.map.values().map(|tpe| tpe.name)
     }
 
     /// Get a mutable reference to a type previously inserted on this `PropertyBag`.
@@ -124,7 +154,7 @@ impl PropertyBag {
     pub fn get_mut<T: Send + Sync + 'static>(&mut self) -> Option<&mut T> {
         self.map
             .get_mut(&TypeId::of::<T>())
-            .and_then(|boxed| (&mut **boxed as &mut (dyn Any + 'static)).downcast_mut())
+            .map(|val| val.as_mut().expect("type mismatch!"))
     }
 
     /// Remove a type from this `PropertyBag`.
@@ -141,8 +171,8 @@ impl PropertyBag {
     /// assert!(props.get::<i32>().is_none());
     /// ```
     pub fn remove<T: Send + Sync + 'static>(&mut self) -> Option<T> {
-        self.map.remove(&TypeId::of::<T>()).and_then(|boxed| {
-            (boxed as Box<dyn Any + 'static>)
+        self.map.remove(&TypeId::of::<T>()).and_then(|tpe| {
+            (tpe.value as Box<dyn Any + 'static>)
                 .downcast()
                 .ok()
                 .map(|boxed| *boxed)
@@ -168,7 +198,16 @@ impl PropertyBag {
 
 impl fmt::Debug for PropertyBag {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("PropertyBag").finish()
+        let mut fmt = f.debug_struct("PropertyBag");
+
+        struct Contents<'a>(&'a PropertyBag);
+        impl<'a> Debug for Contents<'a> {
+            fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+                f.debug_list().entries(self.0.contents()).finish()
+            }
+        }
+        fmt.field("contents", &Contents(self));
+        fmt.finish()
     }
 }
 
@@ -225,22 +264,30 @@ impl From<PropertyBag> for SharedPropertyBag {
 }
 
 #[cfg(test)]
-#[test]
-fn test_extensions() {
-    #[derive(Debug, PartialEq)]
-    struct MyType(i32);
+mod test {
+    use crate::property_bag::PropertyBag;
 
-    let mut extensions = PropertyBag::new();
+    #[test]
+    fn test_extensions() {
+        #[derive(Debug, PartialEq)]
+        struct MyType(i32);
 
-    extensions.insert(5i32);
-    extensions.insert(MyType(10));
+        let mut property_bag = PropertyBag::new();
 
-    assert_eq!(extensions.get(), Some(&5i32));
-    assert_eq!(extensions.get_mut(), Some(&mut 5i32));
+        property_bag.insert(5i32);
+        property_bag.insert(MyType(10));
 
-    assert_eq!(extensions.remove::<i32>(), Some(5i32));
-    assert!(extensions.get::<i32>().is_none());
+        assert_eq!(property_bag.get(), Some(&5i32));
+        assert_eq!(property_bag.get_mut(), Some(&mut 5i32));
 
-    assert_eq!(extensions.get::<bool>(), None);
-    assert_eq!(extensions.get(), Some(&MyType(10)));
+        assert_eq!(property_bag.remove::<i32>(), Some(5i32));
+        assert!(property_bag.get::<i32>().is_none());
+
+        assert_eq!(property_bag.get::<bool>(), None);
+        assert_eq!(property_bag.get(), Some(&MyType(10)));
+        assert_eq!(
+            format!("{:?}", property_bag),
+            r#"PropertyBag { contents: ["aws_smithy_http::property_bag::test::test_extensions::MyType"] }"#
+        );
+    }
 }

rust-runtime/aws-smithy-runtime-api/src/config_bag.rs

@@ -10,20 +10,40 @@
 //! 1. A new layer of configuration may be applied onto an existing configuration structure without modifying it or taking ownership.
 //! 2. No lifetime shenanigans to deal with
 use aws_smithy_http::property_bag::PropertyBag;
-use std::fmt::Debug;
+use std::fmt::{Debug, Formatter};
 use std::ops::Deref;
 use std::sync::Arc;
 
 /// Layered Configuration Structure
 ///
 /// [`ConfigBag`] is the "unlocked" form of the bag. Only the top layer of the bag may be unlocked.
 #[must_use]
-#[derive(Debug)]
 pub struct ConfigBag {
     head: Layer,
     tail: Option<FrozenConfigBag>,
 }
 
+impl Debug for ConfigBag {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        struct Layers<'a>(&'a ConfigBag);
+        impl Debug for Layers<'_> {
+            fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+                let mut list = f.debug_list();
+                list.entry(&self.0.head);
+                let mut us = self.0.tail.as_ref();
+                while let Some(bag) = us {
+                    list.entry(&bag.head);
+                    us = bag.tail.as_ref()
+                }
+                list.finish()
+            }
+        }
+        f.debug_struct("ConfigBag")
+            .field("layers", &Layers(self))
+            .finish()
+    }
+}
+
 /// Layered Configuration Structure
 ///
 /// [`FrozenConfigBag`] is the "locked" form of the bag.
@@ -55,12 +75,26 @@ enum Value<T> {
     ExplicitlyUnset,
 }
 
-#[derive(Debug)]
 struct Layer {
     name: &'static str,
     props: PropertyBag,
 }
 
+impl Debug for Layer {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        struct Contents<'a>(&'a Layer);
+        impl Debug for Contents<'_> {
+            fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+                f.debug_list().entries(self.0.props.contents()).finish()
+            }
+        }
+        f.debug_struct("Layer")
+            .field("name", &self.name)
+            .field("properties", &Contents(self))
+            .finish()
+    }
+}
+
 fn no_op(_: &mut ConfigBag) {}
 
 impl FrozenConfigBag {
@@ -258,6 +292,7 @@ mod test {
         assert!(final_bag.get::<Prop2>().is_some());
         // we unset prop3
         assert!(final_bag.get::<Prop3>().is_none());
+        println!("{:#?}", final_bag);
     }
 
     #[test]