Allow trait bounds to be manually specified

derive/Cargo.toml

@@ -21,7 +21,7 @@ rust-version = "1.63.0"
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.56", features = ['derive'] }
+syn = { version = "1.0.56", features = ['derive', 'parsing'] }
 
 [lib]
 proc_macro = true

derive/src/container_attributes.rs

@@ -0,0 +1,72 @@
+use crate::ARBITRARY_ATTRIBUTE_NAME;
+use syn::{
+    parse::Error, punctuated::Punctuated, DeriveInput, Lit, Meta, MetaNameValue, NestedMeta, Token,
+    TypeParam,
+};
+
+pub struct ContainerAttributes {
+    /// Specify type bounds to be applied to the derived `Arbitrary` implementation instead of the
+    /// default inferred bounds.
+    ///
+    /// ```
+    /// #[arbitrary(bound = "T: Default, U: Debug")]
+    /// ```
+    ///
+    /// Multiple attributes will be combined as long as they don't conflict, e.g.
+    ///
+    /// ```
+    /// #[arbitrary(bound = "T: Default")]
+    /// #[arbitrary(bound = "U: Default")]
+    /// ```
+    pub bounds: Option<Vec<Punctuated<TypeParam, Token![,]>>>,
+}
+
+impl ContainerAttributes {
+    pub fn from_derive_input(derive_input: &DeriveInput) -> Result<Self, Error> {
+        let mut bounds = None;
+
+        for attr in &derive_input.attrs {
+            if !attr.path.is_ident(ARBITRARY_ATTRIBUTE_NAME) {
+                continue;
+            }
+
+            let meta_list = match attr.parse_meta()? {
+                Meta::List(l) => l,
+                _ => {
+                    return Err(Error::new_spanned(
+                        attr,
+                        format!(
+                            "invalid `{}` attribute. expected list",
+                            ARBITRARY_ATTRIBUTE_NAME
+                        ),
+                    ))
+                }
+            };
+
+            for nested_meta in meta_list.nested.iter() {
+                match nested_meta {
+                    NestedMeta::Meta(Meta::NameValue(MetaNameValue {
+                        path,
+                        lit: Lit::Str(bound_str_lit),
+                        ..
+                    })) if path.is_ident("bound") => {
+                        bounds
+                            .get_or_insert_with(Vec::new)
+                            .push(bound_str_lit.parse_with(Punctuated::parse_terminated)?);
+                    }
+                    _ => {
+                        return Err(Error::new_spanned(
+                            attr,
+                            format!(
+                                "invalid `{}` attribute. expected `bound = \"..\"`",
+                                ARBITRARY_ATTRIBUTE_NAME,
+                            ),
+                        ))
+                    }
+                }
+            }
+        }
+
+        Ok(Self { bounds })
+    }
+}

derive/src/field_attributes.rs

@@ -1,10 +1,8 @@
+use crate::ARBITRARY_ATTRIBUTE_NAME;
 use proc_macro2::{Group, Span, TokenStream, TokenTree};
 use quote::quote;
 use syn::{spanned::Spanned, *};
 
-/// Used to filter out necessary field attribute and within error messages.
-static ARBITRARY_ATTRIBUTE_NAME: &str = "arbitrary";
-
 /// Determines how a value for a field should be constructed.
 #[cfg_attr(test, derive(Debug))]
 pub enum FieldConstructor {

derive/src/lib.rs

@@ -4,9 +4,12 @@ use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::*;
 
+mod container_attributes;
 mod field_attributes;
+use container_attributes::ContainerAttributes;
 use field_attributes::{determine_field_constructor, FieldConstructor};
 
+static ARBITRARY_ATTRIBUTE_NAME: &str = "arbitrary";
 static ARBITRARY_LIFETIME_NAME: &str = "'arbitrary";
 
 #[proc_macro_derive(Arbitrary, attributes(arbitrary))]
@@ -18,6 +21,8 @@ pub fn derive_arbitrary(tokens: proc_macro::TokenStream) -> proc_macro::TokenStr
 }
 
 fn expand_derive_arbitrary(input: syn::DeriveInput) -> Result<TokenStream> {
+    let container_attrs = ContainerAttributes::from_derive_input(&input)?;
+
     let (lifetime_without_bounds, lifetime_with_bounds) =
         build_arbitrary_lifetime(input.generics.clone());
 
@@ -30,8 +35,13 @@ fn expand_derive_arbitrary(input: syn::DeriveInput) -> Result<TokenStream> {
         gen_arbitrary_method(&input, lifetime_without_bounds.clone(), &recursive_count)?;
     let size_hint_method = gen_size_hint_method(&input)?;
     let name = input.ident;
-    // Add a bound `T: Arbitrary` to every type parameter T.
-    let generics = add_trait_bounds(input.generics, lifetime_without_bounds.clone());
+
+    // Apply user-supplied bounds or automatic `T: ArbitraryBounds`.
+    let generics = apply_trait_bounds(
+        input.generics,
+        lifetime_without_bounds.clone(),
+        &container_attrs,
+    )?;
 
     // Build ImplGeneric with a lifetime (https://github.com/dtolnay/syn/issues/90)
     let mut generics_with_lifetime = generics.clone();
@@ -77,6 +87,51 @@ fn build_arbitrary_lifetime(generics: Generics) -> (LifetimeDef, LifetimeDef) {
     (lifetime_without_bounds, lifetime_with_bounds)
 }
 
+fn apply_trait_bounds(
+    mut generics: Generics,
+    lifetime: LifetimeDef,
+    container_attrs: &ContainerAttributes,
+) -> Result<Generics> {
+    // If user-supplied bounds exist, apply them to their matching type parameters.
+    if let Some(config_bounds) = &container_attrs.bounds {
+        let mut config_bounds_applied = 0;
+        for param in generics.params.iter_mut() {
+            if let GenericParam::Type(type_param) = param {
+                if let Some(replacement) = config_bounds
+                    .iter()
+                    .flatten()
+                    .find(|p| p.ident == type_param.ident)
+                {
+                    *type_param = replacement.clone();
+                    config_bounds_applied += 1;
+                } else {
+                    // If no user-supplied bounds exist for this type, delete the original bounds.
+                    // This mimics serde.
+                    type_param.bounds = Default::default();
+                    type_param.default = None;
+                }
+            }
+        }
+        let config_bounds_supplied = config_bounds
+            .iter()
+            .map(|bounds| bounds.len())
+            .sum::<usize>();
+        if config_bounds_applied != config_bounds_supplied {
+            return Err(Error::new(
+                Span::call_site(),
+                format!(
+                    "invalid `{}` attribute. too many bounds, only {} out of {} are applicable",
+                    ARBITRARY_ATTRIBUTE_NAME, config_bounds_applied, config_bounds_supplied,
+                ),
+            ));
+        }
+        Ok(generics)
+    } else {
+        // Otherwise, inject a `T: Arbitrary` bound for every parameter.
+        Ok(add_trait_bounds(generics, lifetime))
+    }
+}
+
 // Add a bound `T: Arbitrary` to every type parameter T.
 fn add_trait_bounds(mut generics: Generics, lifetime: LifetimeDef) -> Generics {
     for param in generics.params.iter_mut() {

src/lib.rs

@@ -1338,5 +1338,56 @@ mod test {
 ///     x: i32,
 /// }
 /// ```
+///
+/// Multiple conflicting bounds at the container-level:
+/// ```compile_fail
+/// #[derive(::arbitrary::Arbitrary)]
+/// #[arbitrary(bound = "T: Default")]
+/// #[arbitrary(bound = "T: Default")]
+/// struct Point<T: Default> {
+///     #[arbitrary(default)]
+///     x: T,
+/// }
+/// ```
+///
+/// Multiple conflicting bounds in a single bound attribute:
+/// ```compile_fail
+/// #[derive(::arbitrary::Arbitrary)]
+/// #[arbitrary(bound = "T: Default, T: Default")]
+/// struct Point<T: Default> {
+///     #[arbitrary(default)]
+///     x: T,
+/// }
+/// ```
+///
+/// Multiple conflicting bounds in multiple bound attributes:
+/// ```compile_fail
+/// #[derive(::arbitrary::Arbitrary)]
+/// #[arbitrary(bound = "T: Default", bound = "T: Default")]
+/// struct Point<T: Default> {
+///     #[arbitrary(default)]
+///     x: T,
+/// }
+/// ```
+///
+/// Too many bounds supplied:
+/// ```compile_fail
+/// #[derive(::arbitrary::Arbitrary)]
+/// #[arbitrary(bound = "T: Default")]
+/// struct Point {
+///     x: i32,
+/// }
+/// ```
+///
+/// Too many bounds supplied across multiple attributes:
+/// ```compile_fail
+/// #[derive(::arbitrary::Arbitrary)]
+/// #[arbitrary(bound = "T: Default")]
+/// #[arbitrary(bound = "U: Default")]
+/// struct Point<T: Default> {
+///     #[arbitrary(default)]
+///     x: T,
+/// }
+/// ```
 #[cfg(all(doctest, feature = "derive"))]
 pub struct CompileFailTests;

tests/bound.rs

@@ -0,0 +1,142 @@
+#![cfg(feature = "derive")]
+
+use arbitrary::{Arbitrary, Unstructured};
+
+fn arbitrary_from<'a, T: Arbitrary<'a>>(input: &'a [u8]) -> T {
+    let mut buf = Unstructured::new(input);
+    T::arbitrary(&mut buf).expect("can create arbitrary instance OK")
+}
+
+/// This wrapper trait *implies* `Arbitrary`, but the compiler isn't smart enough to work that out
+/// so when using this wrapper we *must* opt-out of the auto-generated `T: Arbitrary` bounds.
+pub trait WrapperTrait: for<'a> Arbitrary<'a> {}
+
+impl WrapperTrait for u32 {}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: WrapperTrait")]
+struct GenericSingleBound<T: WrapperTrait> {
+    t: T,
+}
+
+#[test]
+fn single_bound() {
+    let v: GenericSingleBound<u32> = arbitrary_from(&[0, 0, 0, 0]);
+    assert_eq!(v.t, 0);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: WrapperTrait, U: WrapperTrait")]
+struct GenericMultipleBoundsSingleAttribute<T: WrapperTrait, U: WrapperTrait> {
+    t: T,
+    u: U,
+}
+
+#[test]
+fn multiple_bounds_single_attribute() {
+    let v: GenericMultipleBoundsSingleAttribute<u32, u32> =
+        arbitrary_from(&[1, 0, 0, 0, 2, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+    assert_eq!(v.u, 2);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: WrapperTrait")]
+#[arbitrary(bound = "U: Default")]
+struct GenericMultipleArbitraryAttributes<T: WrapperTrait, U: Default> {
+    t: T,
+    #[arbitrary(default)]
+    u: U,
+}
+
+#[test]
+fn multiple_arbitrary_attributes() {
+    let v: GenericMultipleArbitraryAttributes<u32, u32> = arbitrary_from(&[1, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+    assert_eq!(v.u, 0);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: WrapperTrait", bound = "U: Default")]
+struct GenericMultipleBoundAttributes<T: WrapperTrait, U: Default> {
+    t: T,
+    #[arbitrary(default)]
+    u: U,
+}
+
+#[test]
+fn multiple_bound_attributes() {
+    let v: GenericMultipleBoundAttributes<u32, u32> = arbitrary_from(&[1, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+    assert_eq!(v.u, 0);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: WrapperTrait", bound = "U: Default")]
+#[arbitrary(bound = "V: WrapperTrait, W: Default")]
+struct GenericMultipleArbitraryAndBoundAttributes<
+    T: WrapperTrait,
+    U: Default,
+    V: WrapperTrait,
+    W: Default,
+> {
+    t: T,
+    #[arbitrary(default)]
+    u: U,
+    v: V,
+    #[arbitrary(default)]
+    w: W,
+}
+
+#[test]
+fn multiple_arbitrary_and_bound_attributes() {
+    let v: GenericMultipleArbitraryAndBoundAttributes<u32, u32, u32, u32> =
+        arbitrary_from(&[1, 0, 0, 0, 2, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+    assert_eq!(v.u, 0);
+    assert_eq!(v.v, 2);
+    assert_eq!(v.w, 0);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "T: Default")]
+struct GenericDefault<T: Default> {
+    #[arbitrary(default)]
+    x: T,
+}
+
+#[test]
+fn default_bound() {
+    // We can write a generic func without any `Arbitrary` bound.
+    fn generic_default<T: Default>() -> GenericDefault<T> {
+        arbitrary_from(&[])
+    }
+
+    assert_eq!(generic_default::<u64>().x, 0);
+    assert_eq!(generic_default::<String>().x, String::new());
+    assert_eq!(generic_default::<Vec<u8>>().x, Vec::new());
+}
+
+#[derive(Arbitrary)]
+#[arbitrary()]
+struct EmptyArbitraryAttribute {
+    t: u32,
+}
+
+#[test]
+fn empty_arbitrary_attribute() {
+    let v: EmptyArbitraryAttribute = arbitrary_from(&[1, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+}
+
+#[derive(Arbitrary)]
+#[arbitrary(bound = "")]
+struct EmptyBoundAttribute {
+    t: u32,
+}
+
+#[test]
+fn empty_bound_attribute() {
+    let v: EmptyBoundAttribute = arbitrary_from(&[1, 0, 0, 0]);
+    assert_eq!(v.t, 1);
+}