Bootstrap solution

Author: tyranron

juniper/src/ast.rs

@@ -527,6 +527,12 @@ impl<'a, S: From<String>> From<&'a str> for InputValue<S> {
     }
 }
 
+impl<S: ScalarValue> From<&ArcStr> for InputValue<S> {
+    fn from(value: &ArcStr) -> Self {
+        Self::scalar(S::from_custom_string(value))
+    }
+}
+
 impl<'a, S: From<String>> From<Cow<'a, str>> for InputValue<S> {
     fn from(s: Cow<'a, str>) -> Self {
         Self::scalar(s.into_owned())

juniper/src/lib.rs

@@ -102,7 +102,9 @@ pub use crate::{
         },
     },
     validation::RuleError,
-    value::{DefaultScalarValue, Object, ParseScalarResult, ParseScalarValue, ScalarValue, Value},
+    value::{
+        AnyExt, DefaultScalarValue, Object, ParseScalarResult, ParseScalarValue, ScalarValue, Value,
+    },
 };
 
 /// An error that prevented query execution

juniper/src/types/scalars.rs

@@ -181,7 +181,7 @@ mod impl_arcstr_scalar {
     use super::ArcStr;
 
     pub(super) fn to_output<S: ScalarValue>(v: &ArcStr) -> Value<S> {
-        Value::scalar(v.to_string())
+        Value::scalar(S::from_custom_string(v))
     }
 
     pub(super) fn from_input<S: ScalarValue>(v: &InputValue<S>) -> Result<ArcStr, String> {
@@ -201,7 +201,7 @@ mod impl_compactstring_scalar {
     use super::CompactString;
 
     pub(super) fn to_output<S: ScalarValue>(v: &CompactString) -> Value<S> {
-        Value::scalar(v.to_string())
+        Value::scalar(S::from_custom_string(v))
     }
 
     pub(super) fn from_input<S: ScalarValue>(v: &InputValue<S>) -> Result<CompactString, String> {

juniper/src/value/mod.rs

@@ -3,6 +3,7 @@ mod scalar;
 
 use std::{any::TypeId, borrow::Cow, fmt, mem};
 
+use arcstr::ArcStr;
 use derive_more::with_trait::From;
 
 use crate::{
@@ -12,7 +13,7 @@ use crate::{
 
 pub use self::{
     object::Object,
-    scalar::{DefaultScalarValue, ParseScalarResult, ParseScalarValue, ScalarValue},
+    scalar::{DefaultScalarValue, ParseScalarResult, ParseScalarValue, ScalarValue, AnyExt},
 };
 
 /// Serializable value returned from query and field execution.
@@ -251,6 +252,12 @@ impl<'a, S: From<String>> From<&'a str> for Value<S> {
     }
 }
 
+impl<S: ScalarValue> From<&ArcStr> for Value<S> {
+    fn from(value: &ArcStr) -> Self {
+        Self::scalar(S::from_custom_string(value))
+    }
+}
+
 impl<'a, S: From<String>> From<Cow<'a, str>> for Value<S> {
     fn from(s: Cow<'a, str>) -> Self {
         Self::scalar(s.into_owned())

juniper/src/value/scalar.rs

@@ -1,4 +1,8 @@
-use std::{borrow::Cow, fmt};
+use std::{
+    any::{Any, TypeId},
+    borrow::Cow,
+    fmt, ptr,
+};
 
 use derive_more::with_trait::From;
 use serde::{Serialize, de::DeserializeOwned};
@@ -223,8 +227,44 @@ pub trait ScalarValue:
             unreachable!("`ScalarValue` must represent at least one of the GraphQL spec types")
         }
     }
+
+    /// Creates this [`ScalarValue`] from the provided custom string type.
+    ///
+    /// This method should be implemented if [`ScalarValue`] implementation uses some custom string 
+    /// type inside to enable efficient conversion from values of this type.
+    ///
+    /// Default implementation allocates by converting [`ToString`] and [`From`]`<`[`String`]`>`.
+    fn from_custom_string<Str: fmt::Display + Any + ?Sized>(s: &Str) -> Self {
+        s.to_string().into()
+    }
 }
 
+/// Extension of [`Any`] for using its methods directly on the value without `dyn`.
+pub trait AnyExt: Any {
+    /// Returns `true` if the this type is the same as `T`.
+    #[must_use]
+    fn is<T: Any + ?Sized>(&self) -> bool {
+        TypeId::of::<T>() == self.type_id()
+    }
+
+    /// Returns [`Some`] reference to this value if it's of type `T`, or [`None`] otherwise.
+    #[must_use]
+    fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        self.is::<T>()
+            .then(|| unsafe { &*(ptr::from_ref(self) as *const T) })
+    }
+
+    /// Returns [`Some`] mutable reference to this value if it's of type `T`, or [`None`] otherwise.
+    #[must_use]
+    fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
+        // `self.is::<T>()` produces a false positive here: borrowed data escapes outside of method
+        (TypeId::of::<Self>() == TypeId::of::<T>())
+            .then(|| unsafe { &mut *(ptr::from_mut(self) as *mut T) })
+    }
+}
+
+impl<T: Any + ?Sized> AnyExt for T {}
+
 /// The default [`ScalarValue`] representation in [`juniper`].
 ///
 /// These types closely follow the [GraphQL specification][0].