Auto merge of #861 - photoszzt:derive_default, r=fitzgen

Can derive default analysis

r? @fitzgen

Author: bors-servo

src/codegen/mod.rs

@@ -1418,7 +1418,7 @@ impl CodeGenerator for CompInfo {
             derives.push("Debug");
         }
 
-        if item.can_derive_default(ctx, ()) {
+        if item.can_derive_default(ctx) {
             derives.push("Default");
         } else {
             needs_default_impl = ctx.options().derive_default;

src/ir/analysis/derive_default.rs

@@ -0,0 +1,360 @@
+//! Determining which types for which we can emit `#[derive(Default)]`.
+
+use super::{ConstrainResult, MonotoneFramework, HasVtable};
+use std::collections::HashSet;
+use std::collections::HashMap;
+use ir::context::{BindgenContext, ItemId};
+use ir::item::IsOpaque;
+use ir::traversal::EdgeKind;
+use ir::ty::RUST_DERIVE_IN_ARRAY_LIMIT;
+use ir::ty::TypeKind;
+use ir::comp::Field;
+use ir::comp::FieldMethods;
+use ir::derive::CanTriviallyDeriveDefault;
+use ir::comp::CompKind;
+use ir::traversal::Trace;
+use ir::item::ItemSet;
+
+/// An analysis that finds for each IR item whether default cannot be derived.
+///
+/// We use the monotone constraint function `cannot_derive_default`, defined as
+/// follows:
+///
+/// * If T is Opaque and layout of the type is known, get this layout as opaque
+///   type and check whether it can be derived using trivial checks.
+/// * If T is Array type, default cannot be derived if the length of the array is
+///   larger than the limit or the type of data the array contains cannot derive
+///   default.
+/// * If T is a type alias, a templated alias or an indirection to another type,
+///   default cannot be derived if the type T refers to cannot be derived default.
+/// * If T is a compound type, default cannot be derived if any of its base member
+///   or field cannot be derived default.
+#[derive(Debug, Clone)]
+pub struct CannotDeriveDefault<'ctx, 'gen>
+    where 'gen: 'ctx
+{
+    ctx: &'ctx BindgenContext<'gen>,
+
+    // The incremental result of this analysis's computation. Everything in this
+    // set cannot derive debug.
+    cannot_derive_default: HashSet<ItemId>,
+
+    // Dependencies saying that if a key ItemId has been inserted into the
+    // `cannot_derive_default` set, then each of the ids in Vec<ItemId> need to be
+    // considered again.
+    //
+    // This is a subset of the natural IR graph with reversed edges, where we
+    // only include the edges from the IR graph that can affect whether a type
+    // can derive debug or not.
+    dependencies: HashMap<ItemId, Vec<ItemId>>,
+}
+
+impl<'ctx, 'gen> CannotDeriveDefault<'ctx, 'gen> {
+    fn consider_edge(kind: EdgeKind) -> bool {
+        match kind {
+            // These are the only edges that can affect whether a type can derive
+            // debug or not.
+            EdgeKind::BaseMember |
+            EdgeKind::Field |
+            EdgeKind::TypeReference |
+            EdgeKind::VarType |
+            EdgeKind::TemplateArgument |
+            EdgeKind::TemplateDeclaration |
+            EdgeKind::TemplateParameterDefinition => true,
+
+            EdgeKind::Constructor |
+            EdgeKind::Destructor |
+            EdgeKind::FunctionReturn |
+            EdgeKind::FunctionParameter |
+            EdgeKind::InnerType |
+            EdgeKind::InnerVar |
+            EdgeKind::Method => false,
+            EdgeKind::Generic => false,
+        }
+    }
+
+    fn insert(&mut self, id: ItemId) -> ConstrainResult {
+        trace!("inserting {:?} into the cannot_derive_default set", id);
+
+        let was_not_already_in_set = self.cannot_derive_default.insert(id);
+        assert!(
+            was_not_already_in_set,
+            "We shouldn't try and insert {:?} twice because if it was \
+             already in the set, `constrain` should have exited early.",
+            id
+        );
+
+        ConstrainResult::Changed
+    }
+}
+
+impl<'ctx, 'gen> MonotoneFramework for CannotDeriveDefault<'ctx, 'gen> {
+    type Node = ItemId;
+    type Extra = &'ctx BindgenContext<'gen>;
+    type Output = HashSet<ItemId>;
+
+    fn new(ctx: &'ctx BindgenContext<'gen>) -> CannotDeriveDefault<'ctx, 'gen> {
+        let mut dependencies = HashMap::new();
+        let cannot_derive_default = HashSet::new();
+
+        let whitelisted_items: HashSet<_> = ctx.whitelisted_items()
+            .iter()
+            .cloned()
+            .collect();
+
+        let whitelisted_and_blacklisted_items: ItemSet = whitelisted_items.iter()
+            .cloned()
+            .flat_map(|i| {
+                let mut reachable = vec![i];
+                i.trace(ctx, &mut |s, _| {
+                    reachable.push(s);
+                }, &());
+                reachable
+            })
+            .collect();
+
+        for item in whitelisted_and_blacklisted_items {
+            dependencies.entry(item).or_insert(vec![]);
+
+            {
+                // We reverse our natural IR graph edges to find dependencies
+                // between nodes.
+                item.trace(ctx, &mut |sub_item: ItemId, edge_kind| {
+                    if ctx.whitelisted_items().contains(&sub_item) &&
+                        Self::consider_edge(edge_kind) {
+                            dependencies.entry(sub_item)
+                                .or_insert(vec![])
+                                .push(item);
+                        }
+                }, &());
+            }
+        }
+
+        CannotDeriveDefault {
+            ctx,
+            cannot_derive_default,
+            dependencies,
+        }
+    }
+
+    fn initial_worklist(&self) -> Vec<ItemId> {
+        self.ctx.whitelisted_items().iter().cloned().collect()
+    }
+
+    fn constrain(&mut self, id: ItemId) -> ConstrainResult {
+        trace!("constrain: {:?}", id);
+
+        if self.cannot_derive_default.contains(&id) {
+            trace!("    already know it cannot derive Default");
+            return ConstrainResult::Same;
+        }
+
+        let item = self.ctx.resolve_item(id);
+        let ty = match item.as_type() {
+            Some(ty) => ty,
+            None => {
+                trace!("    not a type; ignoring");
+                return ConstrainResult::Same;
+            }
+        };
+
+        if ty.is_opaque(self.ctx, item) {
+            let layout_can_derive = ty.layout(self.ctx).map_or(true, |l| {
+                l.opaque().can_trivially_derive_default()
+            });
+            return if layout_can_derive {
+                trace!("    we can trivially derive Default for the layout");
+                ConstrainResult::Same
+            } else {
+                trace!("    we cannot derive Default for the layout");
+                self.insert(id)
+            };
+        }
+
+        if ty.layout(self.ctx).map_or(false, |l| l.align > RUST_DERIVE_IN_ARRAY_LIMIT) {
+            // We have to be conservative: the struct *could* have enough
+            // padding that we emit an array that is longer than
+            // `RUST_DERIVE_IN_ARRAY_LIMIT`. If we moved padding calculations
+            // into the IR and computed them before this analysis, then we could
+            // be precise rather than conservative here.
+            return self.insert(id);
+        }
+
+        match *ty.kind() {
+            // Handle the simple cases. These can derive debug without further
+            // information.
+            TypeKind::Function(..) |
+            TypeKind::Int(..) |
+            TypeKind::Float(..) |
+            TypeKind::Complex(..) => {
+                trace!("    simple type that can always derive Default");
+                ConstrainResult::Same
+            }
+
+            TypeKind::Void |
+            TypeKind::Named |
+            TypeKind::Reference(..) |
+            TypeKind::NullPtr |
+            TypeKind::Pointer(..) |
+            TypeKind::BlockPointer |
+            TypeKind::ObjCId |
+            TypeKind::ObjCSel |
+            TypeKind::ObjCInterface(..) |
+            TypeKind::Enum(..) => {
+                trace!("    types that always cannot derive Default");
+                self.insert(id)
+            }
+
+            TypeKind::Array(t, len) => {
+                if self.cannot_derive_default.contains(&t) {
+                    trace!("    arrays of T for which we cannot derive Default \
+                            also cannot derive Default");
+                    return self.insert(id);
+                }
+
+                if len <= RUST_DERIVE_IN_ARRAY_LIMIT {
+                    trace!("    array is small enough to derive Default");
+                    ConstrainResult::Same
+                } else {
+                    trace!("    array is too large to derive Default");
+                    self.insert(id)
+                }
+            }
+
+            TypeKind::ResolvedTypeRef(t) |
+            TypeKind::TemplateAlias(t, _) |
+            TypeKind::Alias(t) => {
+                if self.cannot_derive_default.contains(&t) {
+                    trace!("    aliases and type refs to T which cannot derive \
+                            Default also cannot derive Default");
+                    self.insert(id)
+                } else {
+                    trace!("    aliases and type refs to T which can derive \
+                            Default can also derive Default");
+                    ConstrainResult::Same
+                }
+            }
+
+            TypeKind::Comp(ref info) => {
+                assert!(
+                    !info.has_non_type_template_params(),
+                    "The early ty.is_opaque check should have handled this case"
+                );
+
+                if info.kind() == CompKind::Union {
+                    if self.ctx.options().unstable_rust {
+                        trace!("    cannot derive Default for Rust unions");
+                        return self.insert(id);
+                    }
+
+                    if ty.layout(self.ctx)
+                        .map_or(true,
+                                |l| l.opaque().can_trivially_derive_default()) {
+                        trace!("    union layout can trivially derive Default");
+                        return ConstrainResult::Same;
+                    } else {
+                        trace!("    union layout cannot derive Default");
+                        return self.insert(id);
+                    }
+                }
+
+                if item.has_vtable(self.ctx) {
+                    trace!("    comp with vtable cannot derive Default");
+                    return self.insert(id);
+                }
+
+                let bases_cannot_derive = info.base_members()
+                    .iter()
+                    .any(|base| self.cannot_derive_default.contains(&base.ty));
+                if bases_cannot_derive {
+                    trace!("    base members cannot derive Default, so we can't \
+                            either");
+                    return self.insert(id);
+                }
+
+                let fields_cannot_derive = info.fields()
+                    .iter()
+                    .any(|f| {
+                        match *f {
+                            Field::DataMember(ref data) => {
+                                self.cannot_derive_default.contains(&data.ty())
+                            }
+                            Field::Bitfields(ref bfu) => {
+                                bfu.bitfields()
+                                    .iter().any(|b| {
+                                        self.cannot_derive_default.contains(&b.ty())
+                                    })
+                            }
+                        }
+                    });
+                if fields_cannot_derive {
+                    trace!("    fields cannot derive Default, so we can't either");
+                    return self.insert(id);
+                }
+
+                trace!("    comp can derive Default");
+                ConstrainResult::Same
+            }
+
+            TypeKind::TemplateInstantiation(ref template) => {
+                if self.ctx.whitelisted_items().contains(&template.template_definition()) {
+                    let args_cannot_derive = template.template_arguments()
+                        .iter()
+                        .any(|arg| self.cannot_derive_default.contains(&arg));
+                    if args_cannot_derive {
+                        trace!("    template args cannot derive Default, so \
+                                insantiation can't either");
+                        return self.insert(id);
+                    }
+
+                    assert!(
+                        !template.template_definition().is_opaque(self.ctx, &()),
+                        "The early ty.is_opaque check should have handled this case"
+                    );
+                    let def_cannot_derive = self.cannot_derive_default
+                        .contains(&template.template_definition());
+                    if def_cannot_derive {
+                        trace!("    template definition cannot derive Default, so \
+                                insantiation can't either");
+                        return self.insert(id);
+                    }
+
+                    trace!("    template instantiation can derive Default");
+                    ConstrainResult::Same
+                } else {
+                    trace!("    blacklisted template instantiation cannot derive default");
+                    return self.insert(id);
+                }
+            }
+
+            TypeKind::Opaque => {
+                unreachable!(
+                    "The early ty.is_opaque check should have handled this case"
+                )
+            }
+
+            TypeKind::UnresolvedTypeRef(..) => {
+                unreachable!(
+                    "Type with unresolved type ref can't reach derive default"
+                )
+            }
+        }
+    }
+
+    fn each_depending_on<F>(&self, id: ItemId, mut f: F)
+        where F: FnMut(ItemId),
+    {
+        if let Some(edges) = self.dependencies.get(&id) {
+            for item in edges {
+                trace!("enqueue {:?} into worklist", item);
+                f(*item);
+            }
+        }
+    }
+}
+
+impl<'ctx, 'gen> From<CannotDeriveDefault<'ctx, 'gen>> for HashSet<ItemId> {
+    fn from(analysis: CannotDeriveDefault<'ctx, 'gen>) -> Self {
+        analysis.cannot_derive_default
+    }
+}

src/ir/analysis/mod.rs

@@ -45,6 +45,8 @@ pub use self::derive_debug::CannotDeriveDebug;
 mod has_vtable;
 pub use self::has_vtable::HasVtableAnalysis;
 pub use self::has_vtable::HasVtable;
+mod derive_default;
+pub use self::derive_default::CannotDeriveDefault;
 
 use ir::context::{BindgenContext, ItemId};
 use ir::traversal::{EdgeKind, Trace};