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use crate::error::StrictCoherenceNeedsNegativeCoherence;
use crate::ty::fast_reject::SimplifiedType;
use crate::ty::visit::TypeVisitableExt;
use crate::ty::{self, TyCtxt};
use rustc_data_structures::fx::FxIndexMap;
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def_id::{DefId, DefIdMap};
use rustc_span::symbol::sym;
/// A per-trait graph of impls in specialization order. At the moment, this
/// graph forms a tree rooted with the trait itself, with all other nodes
/// representing impls, and parent-child relationships representing
/// specializations.
///
/// The graph provides two key services:
///
/// - Construction. This implicitly checks for overlapping impls (i.e., impls
/// that overlap but where neither specializes the other -- an artifact of the
/// simple "chain" rule.
///
/// - Parent extraction. In particular, the graph can give you the *immediate*
/// parents of a given specializing impl, which is needed for extracting
/// default items amongst other things. In the simple "chain" rule, every impl
/// has at most one parent.
#[derive(TyEncodable, TyDecodable, HashStable, Debug)]
pub struct Graph {
/// All impls have a parent; the "root" impls have as their parent the `def_id`
/// of the trait.
pub parent: DefIdMap<DefId>,
/// The "root" impls are found by looking up the trait's def_id.
pub children: DefIdMap<Children>,
/// Whether an error was emitted while constructing the graph.
pub has_errored: Option<ErrorGuaranteed>,
}
impl Graph {
pub fn new() -> Graph {
Graph { parent: Default::default(), children: Default::default(), has_errored: None }
}
/// The parent of a given impl, which is the `DefId` of the trait when the
/// impl is a "specialization root".
pub fn parent(&self, child: DefId) -> DefId {
*self.parent.get(&child).unwrap_or_else(|| panic!("Failed to get parent for {child:?}"))
}
}
/// What kind of overlap check are we doing -- this exists just for testing and feature-gating
/// purposes.
#[derive(Copy, Clone, PartialEq, Eq, Hash, HashStable, Debug, TyEncodable, TyDecodable)]
pub enum OverlapMode {
/// The 1.0 rules (either types fail to unify, or where clauses are not implemented for crate-local types)
Stable,
/// Feature-gated test: Stable, *or* there is an explicit negative impl that rules out one of the where-clauses.
WithNegative,
/// Just check for negative impls, not for "where clause not implemented": used for testing.
Strict,
}
impl OverlapMode {
pub fn get(tcx: TyCtxt<'_>, trait_id: DefId) -> OverlapMode {
let with_negative_coherence = tcx.features().with_negative_coherence;
let strict_coherence = tcx.has_attr(trait_id, sym::rustc_strict_coherence);
if with_negative_coherence {
if strict_coherence { OverlapMode::Strict } else { OverlapMode::WithNegative }
} else {
if strict_coherence {
let attr_span = trait_id
.as_local()
.into_iter()
.flat_map(|local_def_id| {
tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(local_def_id))
})
.find(|attr| attr.has_name(sym::rustc_strict_coherence))
.map(|attr| attr.span);
tcx.sess.emit_err(StrictCoherenceNeedsNegativeCoherence {
span: tcx.def_span(trait_id),
attr_span,
});
}
OverlapMode::Stable
}
}
pub fn use_negative_impl(&self) -> bool {
*self == OverlapMode::Strict || *self == OverlapMode::WithNegative
}
pub fn use_implicit_negative(&self) -> bool {
*self == OverlapMode::Stable || *self == OverlapMode::WithNegative
}
}
/// Children of a given impl, grouped into blanket/non-blanket varieties as is
/// done in `TraitDef`.
#[derive(Default, TyEncodable, TyDecodable, Debug, HashStable)]
pub struct Children {
// Impls of a trait (or specializations of a given impl). To allow for
// quicker lookup, the impls are indexed by a simplified version of their
// `Self` type: impls with a simplifiable `Self` are stored in
// `non_blanket_impls` keyed by it, while all other impls are stored in
// `blanket_impls`.
//
// A similar division is used within `TraitDef`, but the lists there collect
// together *all* the impls for a trait, and are populated prior to building
// the specialization graph.
/// Impls of the trait.
pub non_blanket_impls: FxIndexMap<SimplifiedType, Vec<DefId>>,
/// Blanket impls associated with the trait.
pub blanket_impls: Vec<DefId>,
}
/// A node in the specialization graph is either an impl or a trait
/// definition; either can serve as a source of item definitions.
/// There is always exactly one trait definition node: the root.
#[derive(Debug, Copy, Clone)]
pub enum Node {
Impl(DefId),
Trait(DefId),
}
impl Node {
pub fn is_from_trait(&self) -> bool {
matches!(self, Node::Trait(..))
}
/// Tries to find the associated item that implements `trait_item_def_id`
/// defined in this node.
///
/// If this returns `None`, the item can potentially still be found in
/// parents of this node.
pub fn item<'tcx>(&self, tcx: TyCtxt<'tcx>, trait_item_def_id: DefId) -> Option<ty::AssocItem> {
match *self {
Node::Trait(_) => Some(tcx.associated_item(trait_item_def_id)),
Node::Impl(impl_def_id) => {
let id = tcx.impl_item_implementor_ids(impl_def_id).get(&trait_item_def_id)?;
Some(tcx.associated_item(*id))
}
}
}
pub fn def_id(&self) -> DefId {
match *self {
Node::Impl(did) => did,
Node::Trait(did) => did,
}
}
}
#[derive(Copy, Clone)]
pub struct Ancestors<'tcx> {
trait_def_id: DefId,
specialization_graph: &'tcx Graph,
current_source: Option<Node>,
}
impl Iterator for Ancestors<'_> {
type Item = Node;
fn next(&mut self) -> Option<Node> {
let cur = self.current_source.take();
if let Some(Node::Impl(cur_impl)) = cur {
let parent = self.specialization_graph.parent(cur_impl);
self.current_source = if parent == self.trait_def_id {
Some(Node::Trait(parent))
} else {
Some(Node::Impl(parent))
};
}
cur
}
}
/// Information about the most specialized definition of an associated item.
#[derive(Debug)]
pub struct LeafDef {
/// The associated item described by this `LeafDef`.
pub item: ty::AssocItem,
/// The node in the specialization graph containing the definition of `item`.
pub defining_node: Node,
/// The "top-most" (ie. least specialized) specialization graph node that finalized the
/// definition of `item`.
///
/// Example:
///
/// ```
/// #![feature(specialization)]
/// trait Tr {
/// fn assoc(&self);
/// }
///
/// impl<T> Tr for T {
/// default fn assoc(&self) {}
/// }
///
/// impl Tr for u8 {}
/// ```
///
/// If we start the leaf definition search at `impl Tr for u8`, that impl will be the
/// `finalizing_node`, while `defining_node` will be the generic impl.
///
/// If the leaf definition search is started at the generic impl, `finalizing_node` will be
/// `None`, since the most specialized impl we found still allows overriding the method
/// (doesn't finalize it).
pub finalizing_node: Option<Node>,
}
impl LeafDef {
/// Returns whether this definition is known to not be further specializable.
pub fn is_final(&self) -> bool {
self.finalizing_node.is_some()
}
}
impl<'tcx> Ancestors<'tcx> {
/// Finds the bottom-most (ie. most specialized) definition of an associated
/// item.
pub fn leaf_def(mut self, tcx: TyCtxt<'tcx>, trait_item_def_id: DefId) -> Option<LeafDef> {
let mut finalizing_node = None;
self.find_map(|node| {
if let Some(item) = node.item(tcx, trait_item_def_id) {
if finalizing_node.is_none() {
let is_specializable = item.defaultness(tcx).is_default()
|| tcx.defaultness(node.def_id()).is_default();
if !is_specializable {
finalizing_node = Some(node);
}
}
Some(LeafDef { item, defining_node: node, finalizing_node })
} else {
// Item not mentioned. This "finalizes" any defaulted item provided by an ancestor.
finalizing_node = Some(node);
None
}
})
}
}
/// Walk up the specialization ancestors of a given impl, starting with that
/// impl itself.
///
/// Returns `Err` if an error was reported while building the specialization
/// graph.
pub fn ancestors(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
start_from_impl: DefId,
) -> Result<Ancestors<'_>, ErrorGuaranteed> {
let specialization_graph = tcx.specialization_graph_of(trait_def_id);
if let Some(reported) = specialization_graph.has_errored {
Err(reported)
} else if let Err(reported) =
tcx.type_of(start_from_impl).instantiate_identity().error_reported()
{
Err(reported)
} else {
Ok(Ancestors {
trait_def_id,
specialization_graph,
current_source: Some(Node::Impl(start_from_impl)),
})
}
}