chore(config): Store default value in a box (#16487)

* Drop unused functions

* Add dummy phantom data to hold T

* Drop the `Clone` derive on `struct Metadata`

* Make Metadata store a Boxed default value

* generate a qualified expression path for Default::default

---------

Co-authored-by: Toby Lawrence <toby@nuclearfurnace.com>

lib/vector-config-macros/src/ast/container.rs

@@ -236,7 +236,7 @@ impl<'a> Container<'a> {
 
                 let original = input;
                 let name = serde.attrs.name().deserialize_name();
-                let default_value = get_serde_default_value(serde.attrs.default());
+                let default_value = get_serde_default_value(&serde.ident, serde.attrs.default());
 
                 let container = Container {
                     original,

lib/vector-config-macros/src/ast/field.rs

@@ -33,7 +33,7 @@ impl<'a> Field<'a> {
         let original = serde.original;
 
         let name = serde.attrs.name().deserialize_name();
-        let default_value = get_serde_default_value(serde.attrs.default());
+        let default_value = get_serde_default_value(&serde.ty, serde.attrs.default());
 
         Attributes::from_attributes(&original.attrs)
             .and_then(|attrs| {

lib/vector-config-macros/src/ast/util.rs

@@ -1,4 +1,5 @@
 use darling::error::Accumulator;
+use quote::{quote, ToTokens};
 use serde_derive_internals::{attr as serde_attr, Ctxt};
 use syn::{spanned::Spanned, Attribute, ExprPath, Lit, Meta, MetaNameValue, NestedMeta};
 
@@ -137,11 +138,6 @@ fn none_if_empty(s: String) -> Option<String> {
     }
 }
 
-pub fn get_default_exprpath() -> ExprPath {
-    syn::parse_str("::std::default::Default::default")
-        .expect("expression path for default should never be invalid")
-}
-
 pub fn err_field_missing_description<T: Spanned>(field: &T) -> darling::Error {
     darling::Error::custom(ERR_FIELD_MISSING_DESCRIPTION).with_span(field)
 }
@@ -150,10 +146,19 @@ pub fn err_field_implicit_transparent<T: Spanned>(field: &T) -> darling::Error {
     darling::Error::custom(ERR_FIELD_IMPLICIT_TRANSPARENT).with_span(field)
 }
 
-pub fn get_serde_default_value(default: &serde_attr::Default) -> Option<ExprPath> {
+pub fn get_serde_default_value<S: ToTokens>(
+    source: &S,
+    default: &serde_attr::Default,
+) -> Option<ExprPath> {
     match default {
         serde_attr::Default::None => None,
-        serde_attr::Default::Default => Some(get_default_exprpath()),
+        serde_attr::Default::Default => {
+            let qualified_path = syn::parse2(quote! {
+                <#source as ::std::default::Default>::default
+            })
+            .expect("should not fail to parse qualified default path");
+            Some(qualified_path)
+        }
         serde_attr::Default::Path(path) => Some(path.clone()),
     }
 }

lib/vector-config/src/metadata.rs

@@ -2,17 +2,19 @@ use std::fmt;
 
 use vector_config_common::{attributes::CustomAttribute, validation};
 
+use crate::ToValue;
+
 /// The metadata associated with a given type or field.
-#[derive(Clone)]
 pub struct Metadata<T> {
     title: Option<&'static str>,
     description: Option<&'static str>,
-    default_value: Option<T>,
+    default_value: Option<Box<dyn ToValue>>,
     custom_attributes: Vec<CustomAttribute>,
     deprecated: bool,
     deprecated_message: Option<&'static str>,
     transparent: bool,
     validations: Vec<validation::Validation>,
+    _dummy: std::marker::PhantomData<T>,
 }
 
 impl<T> Metadata<T> {
@@ -54,23 +56,12 @@ impl<T> Metadata<T> {
         self.description = None;
     }
 
-    pub fn default_value(&self) -> Option<&T> {
-        self.default_value.as_ref()
-    }
-
-    pub fn with_default_value(default: T) -> Self {
-        Self {
-            default_value: Some(default),
-            ..Default::default()
-        }
-    }
-
-    pub fn set_default_value(&mut self, default_value: T) {
-        self.default_value = Some(default_value);
+    pub fn default_value(&self) -> Option<&dyn ToValue> {
+        self.default_value.as_deref()
     }
 
-    pub fn consume_default_value(&mut self) -> Option<T> {
-        self.default_value.take()
+    pub fn set_default_value(&mut self, default_value: impl ToValue + 'static) {
+        self.default_value = Some(Box::new(default_value));
     }
 
     pub fn deprecated(&self) -> bool {
@@ -133,6 +124,7 @@ impl<T> Metadata<T> {
             deprecated_message: other.deprecated_message.or(self.deprecated_message),
             transparent: other.transparent,
             validations: self.validations,
+            _dummy: Default::default(),
         }
     }
 
@@ -149,6 +141,7 @@ impl<T> Metadata<T> {
             deprecated_message: self.deprecated_message,
             transparent: self.transparent,
             validations: self.validations.clone(),
+            _dummy: Default::default(),
         }
     }
 }
@@ -164,6 +157,7 @@ impl<T> Default for Metadata<T> {
             deprecated_message: None,
             transparent: false,
             validations: Vec::new(),
+            _dummy: Default::default(),
         }
     }
 }

src/config/mod.rs

@@ -336,7 +336,7 @@ impl Display for Resource {
 #[configurable_component]
 #[derive(Clone, Debug)]
 #[serde(deny_unknown_fields)]
-pub struct TestDefinition<T = OutputId> {
+pub struct TestDefinition<T: 'static = OutputId> {
     /// The name of the unit test.
     pub name: String,
 

src/sinks/util/buffer/compression.rs

@@ -263,7 +263,7 @@ impl Configurable for Compression {
             gzip_string_subschema,
             zlib_string_subschema,
         ]);
-        apply_metadata(&mut all_string_oneof_subschema, string_metadata.clone());
+        apply_metadata(&mut all_string_oneof_subschema, string_metadata);
 
         // Next we'll create a full schema for the given algorithms.
         //