rust/crates/hir-ty/src/consteval.rs

//! Constant evaluation details

use base_db::{salsa::Cycle, CrateId};
use chalk_ir::{cast::Cast, BoundVar, DebruijnIndex};
use hir_def::{
    body::Body,
    hir::{Expr, ExprId},
    path::Path,
    resolver::{Resolver, ValueNs},
    type_ref::LiteralConstRef,
    ConstBlockLoc, EnumVariantId, GeneralConstId, StaticId,
};
use hir_expand::Lookup;
use stdx::never;
use triomphe::Arc;

use crate::{
    db::HirDatabase, infer::InferenceContext, lower::ParamLoweringMode,
    mir::monomorphize_mir_body_bad, to_placeholder_idx, utils::Generics, Const, ConstData,
    ConstScalar, ConstValue, GenericArg, Interner, MemoryMap, Substitution, TraitEnvironment, Ty,
    TyBuilder,
};

use super::mir::{interpret_mir, lower_to_mir, pad16, MirEvalError, MirLowerError};

/// Extension trait for [`Const`]
pub trait ConstExt {
    /// Is a [`Const`] unknown?
    fn is_unknown(&self) -> bool;
}

impl ConstExt for Const {
    fn is_unknown(&self) -> bool {
        match self.data(Interner).value {
            // interned Unknown
            chalk_ir::ConstValue::Concrete(chalk_ir::ConcreteConst {
                interned: ConstScalar::Unknown,
            }) => true,

            // interned concrete anything else
            chalk_ir::ConstValue::Concrete(..) => false,

            _ => {
                tracing::error!(
                    "is_unknown was called on a non-concrete constant value! {:?}",
                    self
                );
                true
            }
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ConstEvalError {
    MirLowerError(MirLowerError),
    MirEvalError(MirEvalError),
}

impl From<MirLowerError> for ConstEvalError {
    fn from(value: MirLowerError) -> Self {
        match value {
            MirLowerError::ConstEvalError(_, e) => *e,
            _ => ConstEvalError::MirLowerError(value),
        }
    }
}

impl From<MirEvalError> for ConstEvalError {
    fn from(value: MirEvalError) -> Self {
        ConstEvalError::MirEvalError(value)
    }
}

pub(crate) fn path_to_const(
    db: &dyn HirDatabase,
    resolver: &Resolver,
    path: &Path,
    mode: ParamLoweringMode,
    args_lazy: impl FnOnce() -> Generics,
    debruijn: DebruijnIndex,
    expected_ty: Ty,
) -> Option<Const> {
    match resolver.resolve_path_in_value_ns_fully(db.upcast(), path) {
        Some(ValueNs::GenericParam(p)) => {
            let ty = db.const_param_ty(p);
            let args = args_lazy();
            let value = match mode {
                ParamLoweringMode::Placeholder => {
                    ConstValue::Placeholder(to_placeholder_idx(db, p.into()))
                }
                ParamLoweringMode::Variable => match args.param_idx(p.into()) {
                    Some(it) => ConstValue::BoundVar(BoundVar::new(debruijn, it)),
                    None => {
                        never!(
                            "Generic list doesn't contain this param: {:?}, {:?}, {:?}",
                            args,
                            path,
                            p
                        );
                        return None;
                    }
                },
            };
            Some(ConstData { ty, value }.intern(Interner))
        }
        Some(ValueNs::ConstId(c)) => Some(intern_const_scalar(
            ConstScalar::UnevaluatedConst(c.into(), Substitution::empty(Interner)),
            expected_ty,
        )),
        _ => None,
    }
}

pub fn unknown_const(ty: Ty) -> Const {
    ConstData {
        ty,
        value: ConstValue::Concrete(chalk_ir::ConcreteConst { interned: ConstScalar::Unknown }),
    }
    .intern(Interner)
}

pub fn unknown_const_as_generic(ty: Ty) -> GenericArg {
    unknown_const(ty).cast(Interner)
}

/// Interns a constant scalar with the given type
pub fn intern_const_scalar(value: ConstScalar, ty: Ty) -> Const {
    ConstData { ty, value: ConstValue::Concrete(chalk_ir::ConcreteConst { interned: value }) }
        .intern(Interner)
}

/// Interns a constant scalar with the given type
pub fn intern_const_ref(
    db: &dyn HirDatabase,
    value: &LiteralConstRef,
    ty: Ty,
    krate: CrateId,
) -> Const {
    let layout = db.layout_of_ty(ty.clone(), TraitEnvironment::empty(krate));
    let bytes = match value {
        LiteralConstRef::Int(i) => {
            // FIXME: We should handle failure of layout better.
            let size = layout.map(|it| it.size.bytes_usize()).unwrap_or(16);
            ConstScalar::Bytes(i.to_le_bytes()[0..size].into(), MemoryMap::default())
        }
        LiteralConstRef::UInt(i) => {
            let size = layout.map(|it| it.size.bytes_usize()).unwrap_or(16);
            ConstScalar::Bytes(i.to_le_bytes()[0..size].into(), MemoryMap::default())
        }
        LiteralConstRef::Bool(b) => ConstScalar::Bytes(Box::new([*b as u8]), MemoryMap::default()),
        LiteralConstRef::Char(c) => {
            ConstScalar::Bytes((*c as u32).to_le_bytes().into(), MemoryMap::default())
        }
        LiteralConstRef::Unknown => ConstScalar::Unknown,
    };
    intern_const_scalar(bytes, ty)
}

/// Interns a possibly-unknown target usize
pub fn usize_const(db: &dyn HirDatabase, value: Option<u128>, krate: CrateId) -> Const {
    intern_const_ref(
        db,
        &value.map_or(LiteralConstRef::Unknown, LiteralConstRef::UInt),
        TyBuilder::usize(),
        krate,
    )
}

pub fn try_const_usize(db: &dyn HirDatabase, c: &Const) -> Option<u128> {
    match &c.data(Interner).value {
        chalk_ir::ConstValue::BoundVar(_) => None,
        chalk_ir::ConstValue::InferenceVar(_) => None,
        chalk_ir::ConstValue::Placeholder(_) => None,
        chalk_ir::ConstValue::Concrete(c) => match &c.interned {
            ConstScalar::Bytes(it, _) => Some(u128::from_le_bytes(pad16(it, false))),
            ConstScalar::UnevaluatedConst(c, subst) => {
                let ec = db.const_eval(*c, subst.clone(), None).ok()?;
                try_const_usize(db, &ec)
            }
            _ => None,
        },
    }
}

pub(crate) fn const_eval_recover(
    _: &dyn HirDatabase,
    _: &Cycle,
    _: &GeneralConstId,
    _: &Substitution,
    _: &Option<Arc<TraitEnvironment>>,
) -> Result<Const, ConstEvalError> {
    Err(ConstEvalError::MirLowerError(MirLowerError::Loop))
}

pub(crate) fn const_eval_static_recover(
    _: &dyn HirDatabase,
    _: &Cycle,
    _: &StaticId,
) -> Result<Const, ConstEvalError> {
    Err(ConstEvalError::MirLowerError(MirLowerError::Loop))
}

pub(crate) fn const_eval_discriminant_recover(
    _: &dyn HirDatabase,
    _: &Cycle,
    _: &EnumVariantId,
) -> Result<i128, ConstEvalError> {
    Err(ConstEvalError::MirLowerError(MirLowerError::Loop))
}

pub(crate) fn const_eval_query(
    db: &dyn HirDatabase,
    def: GeneralConstId,
    subst: Substitution,
    trait_env: Option<Arc<TraitEnvironment>>,
) -> Result<Const, ConstEvalError> {
    let body = match def {
        GeneralConstId::ConstId(c) => {
            db.monomorphized_mir_body(c.into(), subst, db.trait_environment(c.into()))?
        }
        GeneralConstId::ConstBlockId(c) => {
            let ConstBlockLoc { parent, root } = db.lookup_intern_anonymous_const(c);
            let body = db.body(parent);
            let infer = db.infer(parent);
            Arc::new(monomorphize_mir_body_bad(
                db,
                lower_to_mir(db, parent, &body, &infer, root)?,
                subst,
                db.trait_environment_for_body(parent),
            )?)
        }
        GeneralConstId::InTypeConstId(c) => db.mir_body(c.into())?,
    };
    let c = interpret_mir(db, body, false, trait_env).0?;
    Ok(c)
}

pub(crate) fn const_eval_static_query(
    db: &dyn HirDatabase,
    def: StaticId,
) -> Result<Const, ConstEvalError> {
    let body = db.monomorphized_mir_body(
        def.into(),
        Substitution::empty(Interner),
        db.trait_environment_for_body(def.into()),
    )?;
    let c = interpret_mir(db, body, false, None).0?;
    Ok(c)
}

pub(crate) fn const_eval_discriminant_variant(
    db: &dyn HirDatabase,
    variant_id: EnumVariantId,
) -> Result<i128, ConstEvalError> {
    let def = variant_id.into();
    let body = db.body(def);
    if body.exprs[body.body_expr] == Expr::Missing {
        let loc = variant_id.lookup(db.upcast());
        let prev_idx = loc.index.checked_sub(1);
        let value = match prev_idx {
            Some(prev_idx) => {
                1 + db.const_eval_discriminant(
                    db.enum_data(loc.parent).variants[prev_idx as usize].0,
                )?
            }
            _ => 0,
        };
        return Ok(value);
    }
    let mir_body = db.monomorphized_mir_body(
        def,
        Substitution::empty(Interner),
        db.trait_environment_for_body(def),
    )?;
    let c = interpret_mir(db, mir_body, false, None).0?;
    let c = try_const_usize(db, &c).unwrap() as i128;
    Ok(c)
}

// FIXME: Ideally constants in const eval should have separate body (issue #7434), and this function should
// get an `InferenceResult` instead of an `InferenceContext`. And we should remove `ctx.clone().resolve_all()` here
// and make this function private. See the fixme comment on `InferenceContext::resolve_all`.
pub(crate) fn eval_to_const(
    expr: ExprId,
    mode: ParamLoweringMode,
    ctx: &mut InferenceContext<'_>,
    args: impl FnOnce() -> Generics,
    debruijn: DebruijnIndex,
) -> Const {
    let db = ctx.db;
    let infer = ctx.clone().resolve_all();
    fn has_closure(body: &Body, expr: ExprId) -> bool {
        if matches!(body[expr], Expr::Closure { .. }) {
            return true;
        }
        let mut r = false;
        body[expr].walk_child_exprs(|idx| r |= has_closure(body, idx));
        r
    }
    if has_closure(ctx.body, expr) {
        // Type checking clousres need an isolated body (See the above FIXME). Bail out early to prevent panic.
        return unknown_const(infer[expr].clone());
    }
    if let Expr::Path(p) = &ctx.body.exprs[expr] {
        let resolver = &ctx.resolver;
        if let Some(c) = path_to_const(db, resolver, p, mode, args, debruijn, infer[expr].clone()) {
            return c;
        }
    }
    if let Ok(mir_body) = lower_to_mir(ctx.db, ctx.owner, ctx.body, &infer, expr) {
        if let Ok(result) = interpret_mir(db, Arc::new(mir_body), true, None).0 {
            return result;
        }
    }
    unknown_const(infer[expr].clone())
}

#[cfg(test)]
mod tests;