rust/compiler/rustc_mir_build/src/build/expr/as_constant.rs

//! See docs in build/expr/mod.rs

use crate::build::{parse_float_into_constval, Builder};
use rustc_ast as ast;
use rustc_hir::def_id::DefId;
use rustc_middle::mir::interpret::{
    Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
};
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt};
use rustc_target::abi::Size;

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr`, yielding a compile-time constant. Assumes that
    /// `expr` is a valid compile-time constant!
    pub(crate) fn as_constant(&mut self, expr: &Expr<'tcx>) -> Constant<'tcx> {
        let create_uneval_from_def_id =
            |tcx: TyCtxt<'tcx>, def_id: DefId, ty: Ty<'tcx>, substs: SubstsRef<'tcx>| {
                let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
                tcx.mk_const(ty::ConstS { kind: ty::ConstKind::Unevaluated(uneval), ty })
            };

        let this = self;
        let tcx = this.tcx;
        let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
        match *kind {
            ExprKind::Scope { region_scope: _, lint_level: _, value } => {
                this.as_constant(&this.thir[value])
            }
            ExprKind::Literal { lit, neg } => {
                let literal =
                    match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
                        Ok(c) => c,
                        Err(LitToConstError::Reported) => ConstantKind::Ty(tcx.const_error(ty)),
                        Err(LitToConstError::TypeError) => {
                            bug!("encountered type error in `lit_to_mir_constant")
                        }
                    };

                Constant { span, user_ty: None, literal }
            }
            ExprKind::NonHirLiteral { lit, ref user_ty } => {
                let user_ty = user_ty.as_ref().map(|user_ty| {
                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span,
                        user_ty: user_ty.clone(),
                        inferred_ty: ty,
                    })
                });
                let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);

                Constant { span, user_ty: user_ty, literal }
            }
            ExprKind::ZstLiteral { ref user_ty } => {
                let user_ty = user_ty.as_ref().map(|user_ty| {
                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span,
                        user_ty: user_ty.clone(),
                        inferred_ty: ty,
                    })
                });
                let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);

                Constant { span, user_ty: user_ty, literal }
            }
            ExprKind::NamedConst { def_id, substs, ref user_ty } => {
                let user_ty = user_ty.as_ref().map(|user_ty| {
                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span,
                        user_ty: user_ty.clone(),
                        inferred_ty: ty,
                    })
                });
                let literal = ConstantKind::Ty(create_uneval_from_def_id(tcx, def_id, ty, substs));

                Constant { user_ty, span, literal }
            }
            ExprKind::ConstParam { param, def_id: _ } => {
                let const_param =
                    tcx.mk_const(ty::ConstS { kind: ty::ConstKind::Param(param), ty: expr.ty });
                let literal = ConstantKind::Ty(const_param);

                Constant { user_ty: None, span, literal }
            }
            ExprKind::ConstBlock { did: def_id, substs } => {
                let literal = ConstantKind::Ty(create_uneval_from_def_id(tcx, def_id, ty, substs));

                Constant { user_ty: None, span, literal }
            }
            ExprKind::StaticRef { alloc_id, ty, .. } => {
                let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
                let literal = ConstantKind::Val(const_val, ty);

                Constant { span, user_ty: None, literal }
            }
            _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
        }
    }
}

#[instrument(skip(tcx, lit_input))]
pub(crate) fn lit_to_mir_constant<'tcx>(
    tcx: TyCtxt<'tcx>,
    lit_input: LitToConstInput<'tcx>,
) -> Result<ConstantKind<'tcx>, LitToConstError> {
    let LitToConstInput { lit, ty, neg } = lit_input;
    let trunc = |n| {
        let param_ty = ty::ParamEnv::reveal_all().and(ty);
        let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
        trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
        let result = width.truncate(n);
        trace!("trunc result: {}", result);
        Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
    };

    let value = match (lit, &ty.kind()) {
        (ast::LitKind::Str(s, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_str() => {
            let s = s.as_str();
            let allocation = Allocation::from_bytes_byte_aligned_immutable(s.as_bytes());
            let allocation = tcx.intern_const_alloc(allocation);
            ConstValue::Slice { data: allocation, start: 0, end: s.len() }
        }
        (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _))
            if matches!(inner_ty.kind(), ty::Slice(_)) =>
        {
            let allocation = Allocation::from_bytes_byte_aligned_immutable(data as &[u8]);
            let allocation = tcx.intern_const_alloc(allocation);
            ConstValue::Slice { data: allocation, start: 0, end: data.len() }
        }
        (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
            let id = tcx.allocate_bytes(data);
            ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
        }
        (ast::LitKind::Byte(n), ty::Uint(ty::UintTy::U8)) => {
            ConstValue::Scalar(Scalar::from_uint(*n, Size::from_bytes(1)))
        }
        (ast::LitKind::Int(n, _), ty::Uint(_)) | (ast::LitKind::Int(n, _), ty::Int(_)) => {
            trunc(if neg { (*n as i128).overflowing_neg().0 as u128 } else { *n })?
        }
        (ast::LitKind::Float(n, _), ty::Float(fty)) => {
            parse_float_into_constval(*n, *fty, neg).ok_or(LitToConstError::Reported)?
        }
        (ast::LitKind::Bool(b), ty::Bool) => ConstValue::Scalar(Scalar::from_bool(*b)),
        (ast::LitKind::Char(c), ty::Char) => ConstValue::Scalar(Scalar::from_char(*c)),
        (ast::LitKind::Err, _) => return Err(LitToConstError::Reported),
        _ => return Err(LitToConstError::TypeError),
    };

    Ok(ConstantKind::Val(value, ty))
}