src/librustc_mir/util/aggregate.rs

   1 use rustc_index::vec::Idx;
   2 use rustc_middle::mir::*;
   3 use rustc_middle::ty::{Ty, TyCtxt};
   4 use rustc_target::abi::VariantIdx;
   5
   6 use std::convert::TryFrom;
   7 use std::iter::TrustedLen;
   8
   9 /// Expand `lhs = Rvalue::Aggregate(kind, operands)` into assignments to the fields.
  10 ///
  11 /// Produces something like
  12 ///
  13 /// (lhs as Variant).field0 = arg0;     // We only have a downcast if this is an enum
  14 /// (lhs as Variant).field1 = arg1;
  15 /// discriminant(lhs) = variant_index;  // If lhs is an enum or generator.
  16 pub fn expand_aggregate<'tcx>(
  17     mut lhs: Place<'tcx>,
  18     operands: impl Iterator<Item = (Operand<'tcx>, Ty<'tcx>)> + TrustedLen,
  19     kind: AggregateKind<'tcx>,
  20     source_info: SourceInfo,
  21     tcx: TyCtxt<'tcx>,
  22 ) -> impl Iterator<Item = Statement<'tcx>> + TrustedLen {
  23     let mut set_discriminant = None;
  24     let active_field_index = match kind {
  25         AggregateKind::Adt(adt_def, variant_index, _, _, active_field_index) => {
  26             if adt_def.is_enum() {
  27                 set_discriminant = Some(Statement {
  28                     kind: StatementKind::SetDiscriminant { place: box (lhs), variant_index },
  29                     source_info,
  30                 });
  31                 lhs = tcx.mk_place_downcast(lhs, adt_def, variant_index);
  32             }
  33             active_field_index
  34         }
  35         AggregateKind::Generator(..) => {
  36             // Right now we only support initializing generators to
  37             // variant 0 (Unresumed).
  38             let variant_index = VariantIdx::new(0);
  39             set_discriminant = Some(Statement {
  40                 kind: StatementKind::SetDiscriminant { place: box (lhs), variant_index },
  41                 source_info,
  42             });
  43
  44             // Operands are upvars stored on the base place, so no
  45             // downcast is necessary.
  46
  47             None
  48         }
  49         _ => None,
  50     };
  51
  52     operands
  53         .enumerate()
  54         .map(move |(i, (op, ty))| {
  55             let lhs_field = if let AggregateKind::Array(_) = kind {
  56                 let offset = u64::try_from(i).unwrap();
  57                 tcx.mk_place_elem(
  58                     lhs,
  59                     ProjectionElem::ConstantIndex {
  60                         offset,
  61                         min_length: offset + 1,
  62                         from_end: false,
  63                     },
  64                 )
  65             } else {
  66                 let field = Field::new(active_field_index.unwrap_or(i));
  67                 tcx.mk_place_field(lhs, field, ty)
  68             };
  69             Statement { source_info, kind: StatementKind::Assign(box (lhs_field, Rvalue::Use(op))) }
  70         })
  71         .chain(set_discriminant)
  72 }