src/librustc_mir_build/hair/pattern/const_to_pat.rs

   1 use rustc::lint;
   2 use rustc::mir::Field;
   3 use rustc::ty::{self, Ty, TyCtxt};
   4 use rustc_hir as hir;
   5 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
   6 use rustc_infer::traits::predicate_for_trait_def;
   7 use rustc_infer::traits::{self, ObligationCause, PredicateObligation};
   8
   9 use rustc_index::vec::Idx;
  10
  11 use rustc_span::Span;
  12
  13 use std::cell::Cell;
  14
  15 use super::{FieldPat, Pat, PatCtxt, PatKind};
  16
  17 impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
  18     /// Converts an evaluated constant to a pattern (if possible).
  19     /// This means aggregate values (like structs and enums) are converted
  20     /// to a pattern that matches the value (as if you'd compared via structural equality).
  21     pub(super) fn const_to_pat(
  22         &self,
  23         cv: &'tcx ty::Const<'tcx>,
  24         id: hir::HirId,
  25         span: Span,
  26     ) -> Pat<'tcx> {
  27         debug!("const_to_pat: cv={:#?} id={:?}", cv, id);
  28         debug!("const_to_pat: cv.ty={:?} span={:?}", cv.ty, span);
  29
  30         self.tcx.infer_ctxt().enter(|infcx| {
  31             let mut convert = ConstToPat::new(self, id, span, infcx);
  32             convert.to_pat(cv)
  33         })
  34     }
  35 }
  36
  37 struct ConstToPat<'a, 'tcx> {
  38     id: hir::HirId,
  39     span: Span,
  40     param_env: ty::ParamEnv<'tcx>,
  41
  42     // This tracks if we signal some hard error for a given const value, so that
  43     // we will not subsequently issue an irrelevant lint for the same const
  44     // value.
  45     saw_const_match_error: Cell<bool>,
  46
  47     // inference context used for checking `T: Structural` bounds.
  48     infcx: InferCtxt<'a, 'tcx>,
  49
  50     include_lint_checks: bool,
  51 }
  52
  53 impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
  54     fn new(
  55         pat_ctxt: &PatCtxt<'_, 'tcx>,
  56         id: hir::HirId,
  57         span: Span,
  58         infcx: InferCtxt<'a, 'tcx>,
  59     ) -> Self {
  60         ConstToPat {
  61             id,
  62             span,
  63             infcx,
  64             param_env: pat_ctxt.param_env,
  65             include_lint_checks: pat_ctxt.include_lint_checks,
  66             saw_const_match_error: Cell::new(false),
  67         }
  68     }
  69
  70     fn tcx(&self) -> TyCtxt<'tcx> {
  71         self.infcx.tcx
  72     }
  73
  74     fn search_for_structural_match_violation(
  75         &self,
  76         ty: Ty<'tcx>,
  77     ) -> Option<traits::NonStructuralMatchTy<'tcx>> {
  78         traits::search_for_structural_match_violation(self.id, self.span, self.tcx(), ty)
  79     }
  80
  81     fn type_marked_structural(&self, ty: Ty<'tcx>) -> bool {
  82         traits::type_marked_structural(self.id, self.span, &self.infcx, ty)
  83     }
  84
  85     fn to_pat(&mut self, cv: &'tcx ty::Const<'tcx>) -> Pat<'tcx> {
  86         // This method is just a wrapper handling a validity check; the heavy lifting is
  87         // performed by the recursive `recur` method, which is not meant to be
  88         // invoked except by this method.
  89         //
  90         // once indirect_structural_match is a full fledged error, this
  91         // level of indirection can be eliminated
  92
  93         let inlined_const_as_pat = self.recur(cv);
  94
  95         if self.include_lint_checks && !self.saw_const_match_error.get() {
  96             // If we were able to successfully convert the const to some pat,
  97             // double-check that all types in the const implement `Structural`.
  98
  99             let structural = self.search_for_structural_match_violation(cv.ty);
 100             debug!(
 101                 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
 102                 cv.ty, structural
 103             );
 104             if let Some(non_sm_ty) = structural {
 105                 let adt_def = match non_sm_ty {
 106                     traits::NonStructuralMatchTy::Adt(adt_def) => adt_def,
 107                     traits::NonStructuralMatchTy::Param => {
 108                         bug!("use of constant whose type is a parameter inside a pattern")
 109                     }
 110                 };
 111                 let path = self.tcx().def_path_str(adt_def.did);
 112
 113                 let make_msg = || -> String {
 114                     format!(
 115                         "to use a constant of type `{}` in a pattern, \
 116                          `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 117                         path, path,
 118                     )
 119                 };
 120
 121                 // double-check there even *is* a semantic `PartialEq` to dispatch to.
 122                 //
 123                 // (If there isn't, then we can safely issue a hard
 124                 // error, because that's never worked, due to compiler
 125                 // using `PartialEq::eq` in this scenario in the past.)
 126                 //
 127                 // Note: To fix rust-lang/rust#65466, one could lift this check
 128                 // *before* any structural-match checking, and unconditionally error
 129                 // if `PartialEq` is not implemented. However, that breaks stable
 130                 // code at the moment, because types like `for <'a> fn(&'a ())` do
 131                 // not *yet* implement `PartialEq`. So for now we leave this here.
 132                 let ty_is_partial_eq: bool = {
 133                     let partial_eq_trait_id = self.tcx().lang_items().eq_trait().unwrap();
 134                     let obligation: PredicateObligation<'_> = predicate_for_trait_def(
 135                         self.tcx(),
 136                         self.param_env,
 137                         ObligationCause::misc(self.span, self.id),
 138                         partial_eq_trait_id,
 139                         0,
 140                         cv.ty,
 141                         &[],
 142                     );
 143                     // FIXME: should this call a `predicate_must_hold` variant instead?
 144                     self.infcx.predicate_may_hold(&obligation)
 145                 };
 146
 147                 if !ty_is_partial_eq {
 148                     // span_fatal avoids ICE from resolution of non-existent method (rare case).
 149                     self.tcx().sess.span_fatal(self.span, &make_msg());
 150                 } else {
 151                     self.tcx().struct_span_lint_hir(
 152                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 153                         self.id,
 154                         self.span,
 155                         |lint| lint.build(&make_msg()).emit(),
 156                     );
 157                 }
 158             }
 159         }
 160
 161         inlined_const_as_pat
 162     }
 163
 164     // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
 165     fn recur(&self, cv: &'tcx ty::Const<'tcx>) -> Pat<'tcx> {
 166         let id = self.id;
 167         let span = self.span;
 168         let tcx = self.tcx();
 169         let param_env = self.param_env;
 170
 171         let field_pats = |vals: &[&'tcx ty::Const<'tcx>]| {
 172             vals.iter()
 173                 .enumerate()
 174                 .map(|(idx, val)| {
 175                     let field = Field::new(idx);
 176                     FieldPat { field, pattern: self.recur(val) }
 177                 })
 178                 .collect()
 179         };
 180
 181         let kind = match cv.ty.kind {
 182             ty::Float(_) => {
 183                 tcx.struct_span_lint_hir(
 184                     ::rustc::lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
 185                     id,
 186                     span,
 187                     |lint| lint.build("floating-point types cannot be used in patterns").emit(),
 188                 );
 189                 PatKind::Constant { value: cv }
 190             }
 191             ty::Adt(adt_def, _) if adt_def.is_union() => {
 192                 // Matching on union fields is unsafe, we can't hide it in constants
 193                 self.saw_const_match_error.set(true);
 194                 tcx.sess.span_err(span, "cannot use unions in constant patterns");
 195                 PatKind::Wild
 196             }
 197             // keep old code until future-compat upgraded to errors.
 198             ty::Adt(adt_def, _) if !self.type_marked_structural(cv.ty) => {
 199                 debug!("adt_def {:?} has !type_marked_structural for cv.ty: {:?}", adt_def, cv.ty);
 200                 let path = tcx.def_path_str(adt_def.did);
 201                 let msg = format!(
 202                     "to use a constant of type `{}` in a pattern, \
 203                      `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 204                     path, path,
 205                 );
 206                 self.saw_const_match_error.set(true);
 207                 tcx.sess.span_err(span, &msg);
 208                 PatKind::Wild
 209             }
 210             // keep old code until future-compat upgraded to errors.
 211             ty::Ref(_, adt_ty @ ty::TyS { kind: ty::Adt(_, _), .. }, _)
 212                 if !self.type_marked_structural(adt_ty) =>
 213             {
 214                 let adt_def =
 215                     if let ty::Adt(adt_def, _) = adt_ty.kind { adt_def } else { unreachable!() };
 216
 217                 debug!(
 218                     "adt_def {:?} has !type_marked_structural for adt_ty: {:?}",
 219                     adt_def, adt_ty
 220                 );
 221
 222                 // HACK(estebank): Side-step ICE #53708, but anything other than erroring here
 223                 // would be wrong. Returnging `PatKind::Wild` is not technically correct.
 224                 let path = tcx.def_path_str(adt_def.did);
 225                 let msg = format!(
 226                     "to use a constant of type `{}` in a pattern, \
 227                      `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 228                     path, path,
 229                 );
 230                 self.saw_const_match_error.set(true);
 231                 tcx.sess.span_err(span, &msg);
 232                 PatKind::Wild
 233             }
 234             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
 235                 let destructured = tcx.destructure_const(param_env.and(cv));
 236                 PatKind::Variant {
 237                     adt_def,
 238                     substs,
 239                     variant_index: destructured.variant,
 240                     subpatterns: field_pats(destructured.fields),
 241                 }
 242             }
 243             ty::Adt(_, _) => {
 244                 let destructured = tcx.destructure_const(param_env.and(cv));
 245                 PatKind::Leaf { subpatterns: field_pats(destructured.fields) }
 246             }
 247             ty::Tuple(_) => {
 248                 let destructured = tcx.destructure_const(param_env.and(cv));
 249                 PatKind::Leaf { subpatterns: field_pats(destructured.fields) }
 250             }
 251             ty::Array(..) => PatKind::Array {
 252                 prefix: tcx
 253                     .destructure_const(param_env.and(cv))
 254                     .fields
 255                     .iter()
 256                     .map(|val| self.recur(val))
 257                     .collect(),
 258                 slice: None,
 259                 suffix: Vec::new(),
 260             },
 261             _ => PatKind::Constant { value: cv },
 262         };
 263
 264         Pat { span, ty: cv.ty, kind: Box::new(kind) }
 265     }
 266 }