rustc_mir_build/src/builder/expr/as_constant.rs

Source
//! See docs in builder/expr/mod.rs

use rustc_abi::Size;
use rustc_ast as ast;
use rustc_hir::LangItem;
use rustc_middle::mir::interpret::{CTFE_ALLOC_SALT, Scalar};
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use rustc_middle::ty::{
    self, CanonicalUserType, CanonicalUserTypeAnnotation, LitToConstInput, Ty, TyCtxt,
    TypeVisitableExt as _, UserTypeAnnotationIndex,
};
use rustc_middle::{bug, mir, span_bug};
use tracing::{instrument, trace};

use crate::builder::{Builder, parse_float_into_constval};

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; // See "LET_THIS_SELF".
        let tcx = this.tcx;
        let Expr { ty, temp_scope_id: _, span, ref kind } = *expr;
        match kind {
            ExprKind::Scope { region_scope: _, hir_id: _, 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_scope_id: _, span, ref kind } = *expr;

    match *kind {
        ExprKind::Literal { lit, neg } => {
            let const_ =
                lit_to_mir_constant(tcx, LitToConstInput { lit: lit.node, ty: Some(ty), neg });

            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);
            if tcx.is_type_const(def_id) {
                let uneval = ty::UnevaluatedConst::new(def_id, args);
                let ct = ty::Const::new_unevaluated(tcx, uneval);

                let const_ = Const::Ty(ty, ct);
                return ConstOperand { span, user_ty, const_ };
            }

            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>) -> Const<'tcx> {
    let LitToConstInput { lit, ty, neg } = lit_input;

    let ty = ty.expect("type of literal must be known at this point");

    if let Err(guar) = ty.error_reported() {
        return Const::Ty(Ty::new_error(tcx, guar), ty::Const::new_error(tcx, guar));
    }

    let lit_ty = match *ty.kind() {
        ty::Pat(base, _) => base,
        _ => ty,
    };

    let trunc = |n| {
        let width = lit_ty.primitive_size(tcx);
        trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
        let result = width.truncate(n);
        trace!("trunc result: {}", result);
        ConstValue::Scalar(Scalar::from_uint(result, width))
    };

    let value = match (lit, lit_ty.kind()) {
        (ast::LitKind::Str(s, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_str() => {
            let s = s.as_str().as_bytes();
            let len = s.len();
            let allocation = tcx.allocate_bytes_dedup(s, CTFE_ALLOC_SALT);
            ConstValue::Slice { alloc_id: allocation, meta: len.try_into().unwrap() }
        }
        (ast::LitKind::ByteStr(byte_sym, _), ty::Ref(_, inner_ty, _))
            if matches!(inner_ty.kind(), ty::Slice(_)) =>
        {
            let data = byte_sym.as_byte_str();
            let len = data.len();
            let allocation = tcx.allocate_bytes_dedup(data, CTFE_ALLOC_SALT);
            ConstValue::Slice { alloc_id: allocation, meta: len.try_into().unwrap() }
        }
        (ast::LitKind::ByteStr(byte_sym, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
            let id = tcx.allocate_bytes_dedup(byte_sym.as_byte_str(), CTFE_ALLOC_SALT);
            ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
        }
        (ast::LitKind::CStr(byte_sym, _), ty::Ref(_, inner_ty, _)) if matches!(inner_ty.kind(), ty::Adt(def, _) if tcx.is_lang_item(def.did(), LangItem::CStr)) =>
        {
            let data = byte_sym.as_byte_str();
            let len = data.len();
            let allocation = tcx.allocate_bytes_dedup(data, CTFE_ALLOC_SALT);
            ConstValue::Slice { alloc_id: allocation, meta: len.try_into().unwrap() }
        }
        (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(_)) if !neg => trunc(n.get()),
        (ast::LitKind::Int(n, _), ty::Int(_)) => {
            // Unsigned "negation" has the same bitwise effect as signed negation,
            // which gets the result we want without additional casts.
            trunc(if neg { u128::wrapping_neg(n.get()) } else { n.get() })
        }
        (ast::LitKind::Float(n, _), ty::Float(fty)) => {
            parse_float_into_constval(n, *fty, neg).unwrap()
        }
        (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 Const::Ty(Ty::new_error(tcx, guar), ty::Const::new_error(tcx, guar));
        }
        _ => bug!("invalid lit/ty combination in `lit_to_mir_constant`: {lit:?}: {ty:?}"),
    };

    Const::Val(value, ty)
}

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Created: 2026-07-20 10:58