dep_node.rs

//! Nodes in the dependency graph.
//!
//! A node in the [dependency graph] is represented by a [`DepNode`].
//! A `DepNode` consists of a [`DepKind`] (which
//! specifies the kind of thing it represents, like a piece of HIR, MIR, etc.)
//! and a [`Fingerprint`], a 128-bit hash value, the exact meaning of which
//! depends on the node's `DepKind`. Together, the kind and the fingerprint
//! fully identify a dependency node, even across multiple compilation sessions.
//! In other words, the value of the fingerprint does not depend on anything
//! that is specific to a given compilation session, like an unpredictable
//! interning key (e.g., `NodeId`, `DefId`, `Symbol`) or the numeric value of a
//! pointer. The concept behind this could be compared to how git commit hashes
//! uniquely identify a given commit. The fingerprinting approach has
//! a few advantages:
//!
//! * A `DepNode` can simply be serialized to disk and loaded in another session
//!   without the need to do any "rebasing" (like we have to do for Spans and
//!   NodeIds) or "retracing" (like we had to do for `DefId` in earlier
//!   implementations of the dependency graph).
//! * A `Fingerprint` is just a bunch of bits, which allows `DepNode` to
//!   implement `Copy`, `Sync`, `Send`, `Freeze`, etc.
//! * Since we just have a bit pattern, `DepNode` can be mapped from disk into
//!   memory without any post-processing (e.g., "abomination-style" pointer
//!   reconstruction).
//! * Because a `DepNode` is self-contained, we can instantiate `DepNodes` that
//!   refer to things that do not exist anymore. In previous implementations
//!   `DepNode` contained a `DefId`. A `DepNode` referring to something that
//!   had been removed between the previous and the current compilation session
//!   could not be instantiated because the current compilation session
//!   contained no `DefId` for thing that had been removed.
//!
//! `DepNode` definition happens in the `define_dep_nodes!()` macro. This macro
//! defines the `DepKind` enum. Each `DepKind` has its own parameters that are
//! needed at runtime in order to construct a valid `DepNode` fingerprint.
//! However, only `CompileCodegenUnit` and `CompileMonoItem` are constructed
//! explicitly (with `make_compile_codegen_unit` cq `make_compile_mono_item`).
//!
//! Because the macro sees what parameters a given `DepKind` requires, it can
//! "infer" some properties for each kind of `DepNode`:
//!
//! * Whether a `DepNode` of a given kind has any parameters at all. Some
//!   `DepNode`s could represent global concepts with only one value.
//! * Whether it is possible, in principle, to reconstruct a query key from a
//!   given `DepNode`. Many `DepKind`s only require a single `DefId` parameter,
//!   in which case it is possible to map the node's fingerprint back to the
//!   `DefId` it was computed from. In other cases, too much information gets
//!   lost during fingerprint computation.
//!
//! `make_compile_codegen_unit` and `make_compile_mono_items`, together with
//! `DepNode::new()`, ensures that only valid `DepNode` instances can be
//! constructed. For example, the API does not allow for constructing
//! parameterless `DepNode`s with anything other than a zeroed out fingerprint.
//! More generally speaking, it relieves the user of the `DepNode` API of
//! having to know how to compute the expected fingerprint for a given set of
//! node parameters.
//!
//! [dependency graph]: https://rustc-dev-guide.rust-lang.org/query.html

use crate::mir::mono::MonoItem;
use crate::ty::TyCtxt;

use rustc_data_structures::fingerprint::Fingerprint;
use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LocalModDefId, ModDefId, LOCAL_CRATE};
use rustc_hir::definitions::DefPathHash;
use rustc_hir::{HirId, ItemLocalId, OwnerId};
use rustc_query_system::dep_graph::FingerprintStyle;
use rustc_span::symbol::Symbol;
use std::hash::Hash;

pub use rustc_query_system::dep_graph::{DepContext, DepNodeParams};

macro_rules! define_dep_nodes {
    (
     $($(#[$attr:meta])*
        [$($modifiers:tt)*] fn $variant:ident($($K:tt)*) -> $V:ty,)*) => {

        #[macro_export]
        macro_rules! make_dep_kind_array {
            ($mod:ident) => {[ $($mod::$variant()),* ]};
        }

        /// This enum serves as an index into arrays built by `make_dep_kind_array`.
        // This enum has more than u8::MAX variants so we need some kind of multi-byte
        // encoding. The derived Encodable/Decodable uses leb128 encoding which is
        // dense when only considering this enum. But DepKind is encoded in a larger
        // struct, and there we can take advantage of the unused bits in the u16.
        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
        #[allow(non_camel_case_types)]
        #[repr(u16)]
        pub enum DepKind {
            $( $( #[$attr] )* $variant),*
        }

        impl DepKind {
            // This const implements two things: A bounds check so that we can decode
            // a DepKind from a u16 with just one check, and a const check that the
            // discriminants of the variants have been assigned consecutively from 0
            // so that just the one comparison suffices to check that the u16 can be
            // transmuted to a DepKind.
            pub const VARIANTS: u16 = {
                let deps: &[DepKind] = &[$(DepKind::$variant,)*];
                let mut i = 0;
                while i < deps.len() {
                    if i as u16 != deps[i] as u16 {
                        panic!();
                    }
                    i += 1;
                }
                deps.len() as u16
            };
        }

        impl<S: rustc_serialize::Encoder> rustc_serialize::Encodable<S> for DepKind {
            #[inline]
            fn encode(&self, s: &mut S) {
                s.emit_u16(*self as u16);
            }
        }

        impl<D: rustc_serialize::Decoder> rustc_serialize::Decodable<D> for DepKind {
            #[inline]
            fn decode(d: &mut D) -> DepKind {
                let discrim = d.read_u16();
                assert!(discrim < DepKind::VARIANTS);
                // SAFETY: DepKind::VARIANTS checks that the discriminant values permit
                // this one check to soundly guard the transmute.
                unsafe {
                    std::mem::transmute::<u16, DepKind>(discrim)
                }
            }
        }

        pub(super) fn dep_kind_from_label_string(label: &str) -> Result<DepKind, ()> {
            match label {
                $(stringify!($variant) => Ok(DepKind::$variant),)*
                _ => Err(()),
            }
        }

        /// Contains variant => str representations for constructing
        /// DepNode groups for tests.
        #[allow(dead_code, non_upper_case_globals)]
        pub mod label_strs {
           $(
                pub const $variant: &str = stringify!($variant);
            )*
        }
    };
}

rustc_query_append!(define_dep_nodes![
    /// We use this for most things when incr. comp. is turned off.
    [] fn Null() -> (),
    /// We use this to create a forever-red node.
    [] fn Red() -> (),
    [] fn TraitSelect() -> (),
    [] fn CompileCodegenUnit() -> (),
    [] fn CompileMonoItem() -> (),
]);

static_assert_size!(DepKind, 2);

// WARNING: `construct` is generic and does not know that `CompileCodegenUnit` takes `Symbol`s as keys.
// Be very careful changing this type signature!
pub(crate) fn make_compile_codegen_unit(tcx: TyCtxt<'_>, name: Symbol) -> DepNode {
    DepNode::construct(tcx, DepKind::CompileCodegenUnit, &name)
}

// WARNING: `construct` is generic and does not know that `CompileMonoItem` takes `MonoItem`s as keys.
// Be very careful changing this type signature!
pub(crate) fn make_compile_mono_item<'tcx>(
    tcx: TyCtxt<'tcx>,
    mono_item: &MonoItem<'tcx>,
) -> DepNode {
    DepNode::construct(tcx, DepKind::CompileMonoItem, mono_item)
}

pub type DepNode = rustc_query_system::dep_graph::DepNode<DepKind>;

// We keep a lot of `DepNode`s in memory during compilation. It's not
// required that their size stay the same, but we don't want to change
// it inadvertently. This assert just ensures we're aware of any change.
#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
static_assert_size!(DepNode, 18);

#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
static_assert_size!(DepNode, 24);

pub trait DepNodeExt: Sized {
    /// Extracts the DefId corresponding to this DepNode. This will work
    /// if two conditions are met:
    ///
    /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
    /// 2. the item that the DefPath refers to exists in the current tcx.
    ///
    /// Condition (1) is determined by the DepKind variant of the
    /// DepNode. Condition (2) might not be fulfilled if a DepNode
    /// refers to something from the previous compilation session that
    /// has been removed.
    fn extract_def_id(&self, tcx: TyCtxt<'_>) -> Option<DefId>;

    /// Used in testing
    fn from_label_string(
        tcx: TyCtxt<'_>,
        label: &str,
        def_path_hash: DefPathHash,
    ) -> Result<Self, ()>;

    /// Used in testing
    fn has_label_string(label: &str) -> bool;
}

impl DepNodeExt for DepNode {
    /// Extracts the DefId corresponding to this DepNode. This will work
    /// if two conditions are met:
    ///
    /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
    /// 2. the item that the DefPath refers to exists in the current tcx.
    ///
    /// Condition (1) is determined by the DepKind variant of the
    /// DepNode. Condition (2) might not be fulfilled if a DepNode
    /// refers to something from the previous compilation session that
    /// has been removed.
    fn extract_def_id(&self, tcx: TyCtxt<'_>) -> Option<DefId> {
        if tcx.fingerprint_style(self.kind) == FingerprintStyle::DefPathHash {
            Some(tcx.def_path_hash_to_def_id(DefPathHash(self.hash.into()), &mut || {
                panic!("Failed to extract DefId: {:?} {}", self.kind, self.hash)
            }))
        } else {
            None
        }
    }

    /// Used in testing
    fn from_label_string(
        tcx: TyCtxt<'_>,
        label: &str,
        def_path_hash: DefPathHash,
    ) -> Result<DepNode, ()> {
        let kind = dep_kind_from_label_string(label)?;

        match tcx.fingerprint_style(kind) {
            FingerprintStyle::Opaque | FingerprintStyle::HirId => Err(()),
            FingerprintStyle::Unit => Ok(DepNode::new_no_params(tcx, kind)),
            FingerprintStyle::DefPathHash => {
                Ok(DepNode::from_def_path_hash(tcx, def_path_hash, kind))
            }
        }
    }

    /// Used in testing
    fn has_label_string(label: &str) -> bool {
        dep_kind_from_label_string(label).is_ok()
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for () {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::Unit
    }

    #[inline(always)]
    fn to_fingerprint(&self, _: TyCtxt<'tcx>) -> Fingerprint {
        Fingerprint::ZERO
    }

    #[inline(always)]
    fn recover(_: TyCtxt<'tcx>, _: &DepNode) -> Option<Self> {
        Some(())
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for DefId {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::DefPathHash
    }

    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        tcx.def_path_hash(*self).0
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        tcx.def_path_str(*self)
    }

    #[inline(always)]
    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
        dep_node.extract_def_id(tcx)
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for LocalDefId {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::DefPathHash
    }

    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        self.to_def_id().to_fingerprint(tcx)
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        self.to_def_id().to_debug_str(tcx)
    }

    #[inline(always)]
    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
        dep_node.extract_def_id(tcx).map(|id| id.expect_local())
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for OwnerId {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::DefPathHash
    }

    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        self.to_def_id().to_fingerprint(tcx)
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        self.to_def_id().to_debug_str(tcx)
    }

    #[inline(always)]
    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
        dep_node.extract_def_id(tcx).map(|id| OwnerId { def_id: id.expect_local() })
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for CrateNum {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::DefPathHash
    }

    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        let def_id = self.as_def_id();
        def_id.to_fingerprint(tcx)
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        tcx.crate_name(*self).to_string()
    }

    #[inline(always)]
    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
        dep_node.extract_def_id(tcx).map(|id| id.krate)
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for (DefId, DefId) {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::Opaque
    }

    // We actually would not need to specialize the implementation of this
    // method but it's faster to combine the hashes than to instantiate a full
    // hashing context and stable-hashing state.
    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        let (def_id_0, def_id_1) = *self;

        let def_path_hash_0 = tcx.def_path_hash(def_id_0);
        let def_path_hash_1 = tcx.def_path_hash(def_id_1);

        def_path_hash_0.0.combine(def_path_hash_1.0)
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        let (def_id_0, def_id_1) = *self;

        format!("({}, {})", tcx.def_path_debug_str(def_id_0), tcx.def_path_debug_str(def_id_1))
    }
}

impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for HirId {
    #[inline(always)]
    fn fingerprint_style() -> FingerprintStyle {
        FingerprintStyle::HirId
    }

    // We actually would not need to specialize the implementation of this
    // method but it's faster to combine the hashes than to instantiate a full
    // hashing context and stable-hashing state.
    #[inline(always)]
    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
        let HirId { owner, local_id } = *self;
        let def_path_hash = tcx.def_path_hash(owner.to_def_id());
        Fingerprint::new(
            // `owner` is local, so is completely defined by the local hash
            def_path_hash.local_hash(),
            local_id.as_u32() as u64,
        )
    }

    #[inline(always)]
    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
        let HirId { owner, local_id } = *self;
        format!("{}.{}", tcx.def_path_str(owner), local_id.as_u32())
    }

    #[inline(always)]
    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
        if tcx.fingerprint_style(dep_node.kind) == FingerprintStyle::HirId {
            let (local_hash, local_id) = Fingerprint::from(dep_node.hash).split();
            let def_path_hash = DefPathHash::new(tcx.stable_crate_id(LOCAL_CRATE), local_hash);
            let def_id = tcx
                .def_path_hash_to_def_id(def_path_hash, &mut || {
                    panic!("Failed to extract HirId: {:?} {}", dep_node.kind, dep_node.hash)
                })
                .expect_local();
            let local_id = local_id
                .as_u64()
                .try_into()
                .unwrap_or_else(|_| panic!("local id should be u32, found {local_id:?}"));
            Some(HirId { owner: OwnerId { def_id }, local_id: ItemLocalId::from_u32(local_id) })
        } else {
            None
        }
    }
}

macro_rules! impl_for_typed_def_id {
    ($Name:ident, $LocalName:ident) => {
        impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for $Name {
            #[inline(always)]
            fn fingerprint_style() -> FingerprintStyle {
                FingerprintStyle::DefPathHash
            }

            #[inline(always)]
            fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
                self.to_def_id().to_fingerprint(tcx)
            }

            #[inline(always)]
            fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
                self.to_def_id().to_debug_str(tcx)
            }

            #[inline(always)]
            fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
                DefId::recover(tcx, dep_node).map($Name::new_unchecked)
            }
        }

        impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for $LocalName {
            #[inline(always)]
            fn fingerprint_style() -> FingerprintStyle {
                FingerprintStyle::DefPathHash
            }

            #[inline(always)]
            fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
                self.to_def_id().to_fingerprint(tcx)
            }

            #[inline(always)]
            fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
                self.to_def_id().to_debug_str(tcx)
            }

            #[inline(always)]
            fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
                LocalDefId::recover(tcx, dep_node).map($LocalName::new_unchecked)
            }
        }
    };
}

impl_for_typed_def_id! { ModDefId, LocalModDefId }