rust/compiler/rustc_mir_transform/src/const_prop.rs

//! Propagates constants for early reporting of statically known
//! assertion failures

use rustc_index::bit_set::BitSet;
use rustc_index::IndexVec;
use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::{ParamEnv, TyCtxt};
use rustc_target::abi::Size;

/// The maximum number of bytes that we'll allocate space for a local or the return value.
/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
/// Severely regress performance.
const MAX_ALLOC_LIMIT: u64 = 1024;

/// Macro for machine-specific `InterpError` without allocation.
/// (These will never be shown to the user, but they help diagnose ICEs.)
pub(crate) macro throw_machine_stop_str($($tt:tt)*) {{
    // We make a new local type for it. The type itself does not carry any information,
    // but its vtable (for the `MachineStopType` trait) does.
    #[derive(Debug)]
    struct Zst;
    // Printing this type shows the desired string.
    impl std::fmt::Display for Zst {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, $($tt)*)
        }
    }

    impl rustc_middle::mir::interpret::MachineStopType for Zst {
        fn diagnostic_message(&self) -> rustc_errors::DiagnosticMessage {
            self.to_string().into()
        }

        fn add_args(
            self: Box<Self>,
            _: &mut dyn FnMut(std::borrow::Cow<'static, str>, rustc_errors::DiagnosticArgValue),
        ) {}
    }
    throw_machine_stop!(Zst)
}}

/// The mode that `ConstProp` is allowed to run in for a given `Local`.
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum ConstPropMode {
    /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
    FullConstProp,
    /// The `Local` can only be propagated into and from its own block.
    OnlyInsideOwnBlock,
    /// The `Local` cannot be part of propagation at all. Any statement
    /// referencing it either for reading or writing will not get propagated.
    NoPropagation,
}

pub struct CanConstProp {
    can_const_prop: IndexVec<Local, ConstPropMode>,
    // False at the beginning. Once set, no more assignments are allowed to that local.
    found_assignment: BitSet<Local>,
}

impl CanConstProp {
    /// Returns true if `local` can be propagated
    pub fn check<'tcx>(
        tcx: TyCtxt<'tcx>,
        param_env: ParamEnv<'tcx>,
        body: &Body<'tcx>,
    ) -> IndexVec<Local, ConstPropMode> {
        let mut cpv = CanConstProp {
            can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
            found_assignment: BitSet::new_empty(body.local_decls.len()),
        };
        for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
            let ty = body.local_decls[local].ty;
            match tcx.layout_of(param_env.and(ty)) {
                Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
                // Either the layout fails to compute, then we can't use this local anyway
                // or the local is too large, then we don't want to.
                _ => {
                    *val = ConstPropMode::NoPropagation;
                    continue;
                }
            }
        }
        // Consider that arguments are assigned on entry.
        for arg in body.args_iter() {
            cpv.found_assignment.insert(arg);
        }
        cpv.visit_body(body);
        cpv.can_const_prop
    }
}

impl<'tcx> Visitor<'tcx> for CanConstProp {
    fn visit_place(&mut self, place: &Place<'tcx>, mut context: PlaceContext, loc: Location) {
        use rustc_middle::mir::visit::PlaceContext::*;

        // Dereferencing just read the addess of `place.local`.
        if place.projection.first() == Some(&PlaceElem::Deref) {
            context = NonMutatingUse(NonMutatingUseContext::Copy);
        }

        self.visit_local(place.local, context, loc);
        self.visit_projection(place.as_ref(), context, loc);
    }

    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
        use rustc_middle::mir::visit::PlaceContext::*;
        match context {
            // These are just stores, where the storing is not propagatable, but there may be later
            // mutations of the same local via `Store`
            | MutatingUse(MutatingUseContext::Call)
            | MutatingUse(MutatingUseContext::AsmOutput)
            | MutatingUse(MutatingUseContext::Deinit)
            // Actual store that can possibly even propagate a value
            | MutatingUse(MutatingUseContext::Store)
            | MutatingUse(MutatingUseContext::SetDiscriminant) => {
                if !self.found_assignment.insert(local) {
                    match &mut self.can_const_prop[local] {
                        // If the local can only get propagated in its own block, then we don't have
                        // to worry about multiple assignments, as we'll nuke the const state at the
                        // end of the block anyway, and inside the block we overwrite previous
                        // states as applicable.
                        ConstPropMode::OnlyInsideOwnBlock => {}
                        ConstPropMode::NoPropagation => {}
                        other @ ConstPropMode::FullConstProp => {
                            trace!(
                                "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
                                local, other,
                            );
                            *other = ConstPropMode::OnlyInsideOwnBlock;
                        }
                    }
                }
            }
            // Reading constants is allowed an arbitrary number of times
            NonMutatingUse(NonMutatingUseContext::Copy)
            | NonMutatingUse(NonMutatingUseContext::Move)
            | NonMutatingUse(NonMutatingUseContext::Inspect)
            | NonMutatingUse(NonMutatingUseContext::PlaceMention)
            | NonUse(_) => {}

            // These could be propagated with a smarter analysis or just some careful thinking about
            // whether they'd be fine right now.
            MutatingUse(MutatingUseContext::Yield)
            | MutatingUse(MutatingUseContext::Drop)
            | MutatingUse(MutatingUseContext::Retag)
            // These can't ever be propagated under any scheme, as we can't reason about indirect
            // mutation.
            | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
            | NonMutatingUse(NonMutatingUseContext::FakeBorrow)
            | NonMutatingUse(NonMutatingUseContext::AddressOf)
            | MutatingUse(MutatingUseContext::Borrow)
            | MutatingUse(MutatingUseContext::AddressOf) => {
                trace!("local {:?} can't be propagated because it's used: {:?}", local, context);
                self.can_const_prop[local] = ConstPropMode::NoPropagation;
            }
            MutatingUse(MutatingUseContext::Projection)
            | NonMutatingUse(NonMutatingUseContext::Projection) => bug!("visit_place should not pass {context:?} for {local:?}"),
        }
    }
}