src/librustc_typeck/check/generator_interior.rs

   1 //! This calculates the types which has storage which lives across a suspension point in a
   2 //! generator from the perspective of typeck. The actual types used at runtime
   3 //! is calculated in `rustc_mir::transform::generator` and may be a subset of the
   4 //! types computed here.
   5
   6 use super::FnCtxt;
   7 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   8 use rustc_hir as hir;
   9 use rustc_hir::def::{CtorKind, DefKind, Res};
  10 use rustc_hir::def_id::DefId;
  11 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  12 use rustc_hir::{Expr, ExprKind, Pat, PatKind};
  13 use rustc_middle::middle::region::{self, YieldData};
  14 use rustc_middle::ty::{self, Ty};
  15 use rustc_span::Span;
  16
  17 struct InteriorVisitor<'a, 'tcx> {
  18     fcx: &'a FnCtxt<'a, 'tcx>,
  19     types: FxHashMap<ty::GeneratorInteriorTypeCause<'tcx>, usize>,
  20     region_scope_tree: &'tcx region::ScopeTree,
  21     expr_count: usize,
  22     kind: hir::GeneratorKind,
  23     prev_unresolved_span: Option<Span>,
  24 }
  25
  26 impl<'a, 'tcx> InteriorVisitor<'a, 'tcx> {
  27     fn record(
  28         &mut self,
  29         ty: Ty<'tcx>,
  30         scope: Option<region::Scope>,
  31         expr: Option<&'tcx Expr<'tcx>>,
  32         source_span: Span,
  33     ) {
  34         use rustc_span::DUMMY_SP;
  35
  36         debug!(
  37             "generator_interior: attempting to record type {:?} {:?} {:?} {:?}",
  38             ty, scope, expr, source_span
  39         );
  40
  41         let live_across_yield = scope
  42             .map(|s| {
  43                 self.region_scope_tree.yield_in_scope(s).and_then(|yield_data| {
  44                     // If we are recording an expression that is the last yield
  45                     // in the scope, or that has a postorder CFG index larger
  46                     // than the one of all of the yields, then its value can't
  47                     // be storage-live (and therefore live) at any of the yields.
  48                     //
  49                     // See the mega-comment at `yield_in_scope` for a proof.
  50
  51                     debug!(
  52                         "comparing counts yield: {} self: {}, source_span = {:?}",
  53                         yield_data.expr_and_pat_count, self.expr_count, source_span
  54                     );
  55
  56                     if yield_data.expr_and_pat_count >= self.expr_count {
  57                         Some(yield_data)
  58                     } else {
  59                         None
  60                     }
  61                 })
  62             })
  63             .unwrap_or_else(|| {
  64                 Some(YieldData { span: DUMMY_SP, expr_and_pat_count: 0, source: self.kind.into() })
  65             });
  66
  67         if let Some(yield_data) = live_across_yield {
  68             let ty = self.fcx.resolve_vars_if_possible(&ty);
  69             debug!(
  70                 "type in expr = {:?}, scope = {:?}, type = {:?}, count = {}, yield_span = {:?}",
  71                 expr, scope, ty, self.expr_count, yield_data.span
  72             );
  73
  74             if let Some((unresolved_type, unresolved_type_span)) =
  75                 self.fcx.unresolved_type_vars(&ty)
  76             {
  77                 let note = format!(
  78                     "the type is part of the {} because of this {}",
  79                     self.kind, yield_data.source
  80                 );
  81
  82                 // If unresolved type isn't a ty_var then unresolved_type_span is None
  83                 let span = self
  84                     .prev_unresolved_span
  85                     .unwrap_or_else(|| unresolved_type_span.unwrap_or(source_span));
  86                 self.fcx
  87                     .need_type_info_err_in_generator(self.kind, span, unresolved_type)
  88                     .span_note(yield_data.span, &*note)
  89                     .emit();
  90             } else {
  91                 // Map the type to the number of types added before it
  92                 let entries = self.types.len();
  93                 let scope_span = scope.map(|s| s.span(self.fcx.tcx, self.region_scope_tree));
  94                 self.types
  95                     .entry(ty::GeneratorInteriorTypeCause {
  96                         span: source_span,
  97                         ty: &ty,
  98                         scope_span,
  99                         yield_span: yield_data.span,
 100                         expr: expr.map(|e| e.hir_id),
 101                     })
 102                     .or_insert(entries);
 103             }
 104         } else {
 105             debug!(
 106                 "no type in expr = {:?}, count = {:?}, span = {:?}",
 107                 expr,
 108                 self.expr_count,
 109                 expr.map(|e| e.span)
 110             );
 111             let ty = self.fcx.resolve_vars_if_possible(&ty);
 112             if let Some((unresolved_type, unresolved_type_span)) =
 113                 self.fcx.unresolved_type_vars(&ty)
 114             {
 115                 debug!(
 116                     "remained unresolved_type = {:?}, unresolved_type_span: {:?}",
 117                     unresolved_type, unresolved_type_span
 118                 );
 119                 self.prev_unresolved_span = unresolved_type_span;
 120             }
 121         }
 122     }
 123 }
 124
 125 pub fn resolve_interior<'a, 'tcx>(
 126     fcx: &'a FnCtxt<'a, 'tcx>,
 127     def_id: DefId,
 128     body_id: hir::BodyId,
 129     interior: Ty<'tcx>,
 130     kind: hir::GeneratorKind,
 131 ) {
 132     let body = fcx.tcx.hir().body(body_id);
 133     let mut visitor = InteriorVisitor {
 134         fcx,
 135         types: FxHashMap::default(),
 136         region_scope_tree: fcx.tcx.region_scope_tree(def_id),
 137         expr_count: 0,
 138         kind,
 139         prev_unresolved_span: None,
 140     };
 141     intravisit::walk_body(&mut visitor, body);
 142
 143     // Check that we visited the same amount of expressions and the RegionResolutionVisitor
 144     let region_expr_count = visitor.region_scope_tree.body_expr_count(body_id).unwrap();
 145     assert_eq!(region_expr_count, visitor.expr_count);
 146
 147     let mut types: Vec<_> = visitor.types.drain().collect();
 148
 149     // Sort types by insertion order
 150     types.sort_by_key(|t| t.1);
 151
 152     // The types in the generator interior contain lifetimes local to the generator itself,
 153     // which should not be exposed outside of the generator. Therefore, we replace these
 154     // lifetimes with existentially-bound lifetimes, which reflect the exact value of the
 155     // lifetimes not being known by users.
 156     //
 157     // These lifetimes are used in auto trait impl checking (for example,
 158     // if a Sync generator contains an &'α T, we need to check whether &'α T: Sync),
 159     // so knowledge of the exact relationships between them isn't particularly important.
 160
 161     debug!("types in generator {:?}, span = {:?}", types, body.value.span);
 162
 163     let mut counter = 0;
 164     let mut captured_tys = FxHashSet::default();
 165     let type_causes: Vec<_> = types
 166         .into_iter()
 167         .filter_map(|(mut cause, _)| {
 168             // Erase regions and canonicalize late-bound regions to deduplicate as many types as we
 169             // can.
 170             let erased = fcx.tcx.erase_regions(&cause.ty);
 171             if captured_tys.insert(erased) {
 172                 // Replace all regions inside the generator interior with late bound regions.
 173                 // Note that each region slot in the types gets a new fresh late bound region,
 174                 // which means that none of the regions inside relate to any other, even if
 175                 // typeck had previously found constraints that would cause them to be related.
 176                 let folded = fcx.tcx.fold_regions(&erased, &mut false, |_, current_depth| {
 177                     counter += 1;
 178                     fcx.tcx.mk_region(ty::ReLateBound(current_depth, ty::BrAnon(counter)))
 179                 });
 180
 181                 cause.ty = folded;
 182                 Some(cause)
 183             } else {
 184                 None
 185             }
 186         })
 187         .collect();
 188
 189     // Extract type components to build the witness type.
 190     let type_list = fcx.tcx.mk_type_list(type_causes.iter().map(|cause| cause.ty));
 191     let witness = fcx.tcx.mk_generator_witness(ty::Binder::bind(type_list));
 192
 193     // Store the generator types and spans into the tables for this generator.
 194     visitor.fcx.inh.tables.borrow_mut().generator_interior_types = type_causes;
 195
 196     debug!(
 197         "types in generator after region replacement {:?}, span = {:?}",
 198         witness, body.value.span
 199     );
 200
 201     // Unify the type variable inside the generator with the new witness
 202     match fcx.at(&fcx.misc(body.value.span), fcx.param_env).eq(interior, witness) {
 203         Ok(ok) => fcx.register_infer_ok_obligations(ok),
 204         _ => bug!(),
 205     }
 206 }
 207
 208 // This visitor has to have the same visit_expr calls as RegionResolutionVisitor in
 209 // librustc_middle/middle/region.rs since `expr_count` is compared against the results
 210 // there.
 211 impl<'a, 'tcx> Visitor<'tcx> for InteriorVisitor<'a, 'tcx> {
 212     type Map = intravisit::ErasedMap<'tcx>;
 213
 214     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 215         NestedVisitorMap::None
 216     }
 217
 218     fn visit_pat(&mut self, pat: &'tcx Pat<'tcx>) {
 219         intravisit::walk_pat(self, pat);
 220
 221         self.expr_count += 1;
 222
 223         if let PatKind::Binding(..) = pat.kind {
 224             let scope = self.region_scope_tree.var_scope(pat.hir_id.local_id);
 225             let ty = self.fcx.tables.borrow().pat_ty(pat);
 226             self.record(ty, Some(scope), None, pat.span);
 227         }
 228     }
 229
 230     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
 231         match &expr.kind {
 232             ExprKind::Call(callee, args) => match &callee.kind {
 233                 ExprKind::Path(qpath) => {
 234                     let res = self.fcx.tables.borrow().qpath_res(qpath, callee.hir_id);
 235                     match res {
 236                         // Direct calls never need to keep the callee `ty::FnDef`
 237                         // ZST in a temporary, so skip its type, just in case it
 238                         // can significantly complicate the generator type.
 239                         Res::Def(
 240                             DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(_, CtorKind::Fn),
 241                             _,
 242                         ) => {
 243                             // NOTE(eddyb) this assumes a path expression has
 244                             // no nested expressions to keep track of.
 245                             self.expr_count += 1;
 246
 247                             // Record the rest of the call expression normally.
 248                             for arg in *args {
 249                                 self.visit_expr(arg);
 250                             }
 251                         }
 252                         _ => intravisit::walk_expr(self, expr),
 253                     }
 254                 }
 255                 _ => intravisit::walk_expr(self, expr),
 256             },
 257             _ => intravisit::walk_expr(self, expr),
 258         }
 259
 260         self.expr_count += 1;
 261
 262         let scope = self.region_scope_tree.temporary_scope(expr.hir_id.local_id);
 263
 264         // If there are adjustments, then record the final type --
 265         // this is the actual value that is being produced.
 266         if let Some(adjusted_ty) = self.fcx.tables.borrow().expr_ty_adjusted_opt(expr) {
 267             self.record(adjusted_ty, scope, Some(expr), expr.span);
 268         }
 269
 270         // Also record the unadjusted type (which is the only type if
 271         // there are no adjustments). The reason for this is that the
 272         // unadjusted value is sometimes a "temporary" that would wind
 273         // up in a MIR temporary.
 274         //
 275         // As an example, consider an expression like `vec![].push()`.
 276         // Here, the `vec![]` would wind up MIR stored into a
 277         // temporary variable `t` which we can borrow to invoke
 278         // `<Vec<_>>::push(&mut t)`.
 279         //
 280         // Note that an expression can have many adjustments, and we
 281         // are just ignoring those intermediate types. This is because
 282         // those intermediate values are always linearly "consumed" by
 283         // the other adjustments, and hence would never be directly
 284         // captured in the MIR.
 285         //
 286         // (Note that this partly relies on the fact that the `Deref`
 287         // traits always return references, which means their content
 288         // can be reborrowed without needing to spill to a temporary.
 289         // If this were not the case, then we could conceivably have
 290         // to create intermediate temporaries.)
 291         //
 292         // The type table might not have information for this expression
 293         // if it is in a malformed scope. (#66387)
 294         if let Some(ty) = self.fcx.tables.borrow().expr_ty_opt(expr) {
 295             self.record(ty, scope, Some(expr), expr.span);
 296         } else {
 297             self.fcx.tcx.sess.delay_span_bug(expr.span, "no type for node");
 298         }
 299     }
 300 }