rust/src/librustc_mir/build/expr/as_constant.rs

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

use crate::build::Builder;
use crate::hair::*;
use rustc::mir::*;
use rustc::ty::CanonicalUserTypeAnnotation;

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr`, yielding a compile-time constant. Assumes that
    /// `expr` is a valid compile-time constant!
    pub fn as_constant<M>(&mut self, expr: M) -> Constant<'tcx>
    where
        M: Mirror<'tcx, Output = Expr<'tcx>>,
    {
        let expr = self.hir.mirror(expr);
        self.expr_as_constant(expr)
    }

    fn expr_as_constant(&mut self, expr: Expr<'tcx>) -> Constant<'tcx> {
        let this = self;
        let Expr {
            ty,
            temp_lifetime: _,
            span,
            kind,
        } = expr;
        match kind {
            ExprKind::Scope {
                region_scope: _,
                lint_level: _,
                value,
            } => this.as_constant(value),
            ExprKind::Literal { literal, user_ty } => {
                let user_ty = user_ty.map(|user_ty| {
                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                        span,
                        user_ty,
                        inferred_ty: ty,
                    })
                });
                assert_eq!(literal.ty, ty);
                Constant {
                    span,
                    user_ty,
                    literal,
                }
            },
            _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
        }
    }
}