serde/serde_codegen/src/bound.rs

use std::collections::HashSet;

use aster::AstBuilder;

use syntax::ast;
use syntax::ext::base::ExtCtxt;
use syntax::ptr::P;
use syntax::visit;

pub fn with_bound(
    cx: &ExtCtxt,
    builder: &AstBuilder,
    item: &ast::Item,
    generics: &ast::Generics,
    filter: &Fn(&ast::StructField) -> bool,
    bound: &[&'static str],
) -> ast::Generics {
    let path = builder.path().global().ids(bound).build();

    builder.from_generics(generics.clone())
        .with_predicates(
            all_variants(cx, item).iter()
                .flat_map(|variant_data| all_struct_fields(variant_data))
                .filter(|field| filter(field))
                .map(|field| &field.ty)
                // TODO this filter can be removed later, see comment on function
                .filter(|ty| contains_generic(ty, generics))
                .map(|ty| strip_reference(ty))
                .map(|ty| builder.where_predicate()
                    // the type that is being bounded e.g. T
                    .bound().build(ty.clone())
                    // the bound e.g. Serialize
                    .bound().trait_(path.clone()).build()
                    .build()))
        .build()
}

fn all_variants<'a>(cx: &ExtCtxt, item: &'a ast::Item) -> Vec<&'a ast::VariantData> {
    match item.node {
        ast::ItemKind::Struct(ref variant_data, _) => {
            vec![variant_data]
        }
        ast::ItemKind::Enum(ref enum_def, _) => {
            enum_def.variants.iter()
                .map(|variant| &variant.node.data)
                .collect()
        }
        _ => {
            cx.span_bug(item.span, "expected Item to be Struct or Enum");
        }
    }
}

fn all_struct_fields(variant_data: &ast::VariantData) -> &[ast::StructField] {
    match *variant_data {
        ast::VariantData::Struct(ref fields, _) |
        ast::VariantData::Tuple(ref fields, _) => {
            fields
        }
        ast::VariantData::Unit(_) => {
            &[]
        }
    }
}

// Rust <1.7 enforces that `where` clauses involve generic type parameters. The
// corresponding compiler error is E0193. It is no longer enforced in Rust >=1.7
// so this filtering can be removed in the future when we stop supporting <1.7.
//
// E0193 means we must not generate a `where` clause like `i32: Serialize`
// because even though i32 implements Serialize, i32 is not a generic type
// parameter. Clauses like `T: Serialize` and `Option<T>: Serialize` are okay.
// This function decides whether a given type references any of the generic type
// parameters in the input `Generics`.
fn contains_generic(ty: &ast::Ty, generics: &ast::Generics) -> bool {
    struct FindGeneric<'a> {
        generic_names: &'a HashSet<ast::Name>,
        found_generic: bool,
    }
    impl<'a, 'v> visit::Visitor<'v> for FindGeneric<'a> {
        fn visit_path(&mut self, path: &'v ast::Path, _id: ast::NodeId) {
            if !path.global
                    && path.segments.len() == 1
                    && self.generic_names.contains(&path.segments[0].identifier.name) {
                self.found_generic = true;
            } else {
                visit::walk_path(self, path);
            }
        }
    }

    let generic_names: HashSet<_> = generics.ty_params.iter()
        .map(|ty_param| ty_param.ident.name)
        .collect();

    let mut visitor = FindGeneric {
        generic_names: &generic_names,
        found_generic: false,
    };
    visit::walk_ty(&mut visitor, ty);
    visitor.found_generic
}

// This is required to handle types that use both a reference and a value of
// the same type, as in:
//
//    enum Test<'a, T> where T: 'a {
//        Lifetime(&'a T),
//        NoLifetime(T),
//    }
//
// Preserving references, we would generate an impl like:
//
//    impl<'a, T> Serialize for Test<'a, T>
//        where &'a T: Serialize,
//              T: Serialize { ... }
//
// And taking a reference to one of the elements would fail with:
//
//    error: cannot infer an appropriate lifetime for pattern due
//    to conflicting requirements [E0495]
//        Test::NoLifetime(ref v) => { ... }
//                         ^~~~~
//
// Instead, we strip references before adding `T: Serialize` bounds in order to
// generate:
//
//    impl<'a, T> Serialize for Test<'a, T>
//        where T: Serialize { ... }
fn strip_reference(ty: &P<ast::Ty>) -> &P<ast::Ty> {
    match ty.node {
        ast::TyKind::Rptr(_, ref mut_ty) => &mut_ty.ty,
        _ => ty
    }
}