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

use crate::build::{parse_float_into_constval, Builder};
use rustc_ast as ast;
use rustc_hir::LangItem;
use rustc_middle::mir;
use rustc_middle::mir::interpret::{Allocation, LitToConstError, LitToConstInput, Scalar};
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::{
    self, CanonicalUserType, CanonicalUserTypeAnnotation, Ty, TyCtxt, UserTypeAnnotationIndex,
};
use rustc_middle::{bug, span_bug};
use rustc_target::abi::Size;
use tracing::{instrument, trace};

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>) -> ConstOperand<'tcx> {
        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])
            }
            _ => as_constant_inner(
                expr,
                |user_ty| {
                    Some(this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span,
                        user_ty: user_ty.clone(),
                        inferred_ty: ty,
                    }))
                },
                tcx,
            ),
        }
    }
}

pub(crate) fn as_constant_inner<'tcx>(
    expr: &Expr<'tcx>,
    push_cuta: impl FnMut(&Box<CanonicalUserType<'tcx>>) -> Option<UserTypeAnnotationIndex>,
    tcx: TyCtxt<'tcx>,
) -> ConstOperand<'tcx> {
    let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
    match *kind {
        ExprKind::Literal { lit, neg } => {
            let const_ = match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg })
            {
                Ok(c) => c,
                Err(LitToConstError::Reported(guar)) => {
                    Const::Ty(Ty::new_error(tcx, guar), ty::Const::new_error(tcx, guar))
                }
                Err(LitToConstError::TypeError) => {
                    bug!("encountered type error in `lit_to_mir_constant`")
                }
            };

            ConstOperand { span, user_ty: None, const_ }
        }
        ExprKind::NonHirLiteral { lit, ref user_ty } => {
            let user_ty = user_ty.as_ref().and_then(push_cuta);

            let const_ = Const::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);

            ConstOperand { span, user_ty, const_ }
        }
        ExprKind::ZstLiteral { ref user_ty } => {
            let user_ty = user_ty.as_ref().and_then(push_cuta);

            let const_ = Const::Val(ConstValue::ZeroSized, ty);

            ConstOperand { span, user_ty, const_ }
        }
        ExprKind::NamedConst { def_id, args, ref user_ty } => {
            let user_ty = user_ty.as_ref().and_then(push_cuta);

            let uneval = mir::UnevaluatedConst::new(def_id, args);
            let const_ = Const::Unevaluated(uneval, ty);

            ConstOperand { user_ty, span, const_ }
        }
        ExprKind::ConstParam { param, def_id: _ } => {
            let const_param = ty::Const::new_param(tcx, param);
            let const_ = Const::Ty(expr.ty, const_param);

            ConstOperand { user_ty: None, span, const_ }
        }
        ExprKind::ConstBlock { did: def_id, args } => {
            let uneval = mir::UnevaluatedConst::new(def_id, args);
            let const_ = Const::Unevaluated(uneval, ty);

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

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

#[instrument(skip(tcx, lit_input))]
fn lit_to_mir_constant<'tcx>(
    tcx: TyCtxt<'tcx>,
    lit_input: LitToConstInput<'tcx>,
) -> Result<Const<'tcx>, LitToConstError> {
    let LitToConstInput { lit, ty, neg } = lit_input;
    let trunc = |n| {
        let param_ty = ty::ParamEnv::reveal_all().and(ty);
        let width = match tcx.layout_of(param_ty) {
            Ok(layout) => layout.size,
            Err(_) => {
                tcx.dcx().bug(format!("couldn't compute width of literal: {:?}", lit_input.lit))
            }
        };
        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.mk_const_alloc(allocation);
            ConstValue::Slice { data: allocation, meta: allocation.inner().size().bytes() }
        }
        (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.mk_const_alloc(allocation);
            ConstValue::Slice { data: allocation, meta: allocation.inner().size().bytes() }
        }
        (ast::LitKind::ByteStr(data, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
            let id = tcx.allocate_bytes_dedup(data);
            ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
        }
        (ast::LitKind::CStr(data, _), ty::Ref(_, inner_ty, _)) if matches!(inner_ty.kind(), ty::Adt(def, _) if tcx.is_lang_item(def.did(), LangItem::CStr)) =>
        {
            let allocation = Allocation::from_bytes_byte_aligned_immutable(data as &[u8]);
            let allocation = tcx.mk_const_alloc(allocation);
            ConstValue::Slice { data: allocation, meta: allocation.inner().size().bytes() }
        }
        (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.get() as i128).overflowing_neg().0 as u128 } else { n.get() })?
        }
        (ast::LitKind::Float(n, _), ty::Float(fty)) => parse_float_into_constval(*n, *fty, neg)
            .ok_or_else(|| {
                LitToConstError::Reported(
                    tcx.dcx()
                        .delayed_bug(format!("couldn't parse float literal: {:?}", lit_input.lit)),
                )
            })?,
        (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(guar), _) => return Err(LitToConstError::Reported(*guar)),
        _ => return Err(LitToConstError::TypeError),
    };

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