src/librustc_mir_build/build/block.rs

   1 use crate::build::matches::ArmHasGuard;
   2 use crate::build::ForGuard::OutsideGuard;
   3 use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
   4 use crate::hair::*;
   5 use rustc::mir::*;
   6 use rustc_hir as hir;
   7 use rustc_span::Span;
   8
   9 impl<'a, 'tcx> Builder<'a, 'tcx> {
  10     crate fn ast_block(
  11         &mut self,
  12         destination: &Place<'tcx>,
  13         block: BasicBlock,
  14         ast_block: &'tcx hir::Block<'tcx>,
  15         source_info: SourceInfo,
  16     ) -> BlockAnd<()> {
  17         let Block {
  18             region_scope,
  19             opt_destruction_scope,
  20             span,
  21             stmts,
  22             expr,
  23             targeted_by_break,
  24             safety_mode,
  25         } = self.hir.mirror(ast_block);
  26         self.in_opt_scope(opt_destruction_scope.map(|de| (de, source_info)), move |this| {
  27             this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
  28                 if targeted_by_break {
  29                     // This is a `break`-able block
  30                     let exit_block = this.cfg.start_new_block();
  31                     let block_exit =
  32                         this.in_breakable_scope(None, exit_block, destination.clone(), |this| {
  33                             this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
  34                         });
  35                     this.cfg.goto(unpack!(block_exit), source_info, exit_block);
  36                     exit_block.unit()
  37                 } else {
  38                     this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
  39                 }
  40             })
  41         })
  42     }
  43
  44     fn ast_block_stmts(
  45         &mut self,
  46         destination: &Place<'tcx>,
  47         mut block: BasicBlock,
  48         span: Span,
  49         stmts: Vec<StmtRef<'tcx>>,
  50         expr: Option<ExprRef<'tcx>>,
  51         safety_mode: BlockSafety,
  52     ) -> BlockAnd<()> {
  53         let this = self;
  54
  55         // This convoluted structure is to avoid using recursion as we walk down a list
  56         // of statements. Basically, the structure we get back is something like:
  57         //
  58         //    let x = <init> in {
  59         //       expr1;
  60         //       let y = <init> in {
  61         //           expr2;
  62         //           expr3;
  63         //           ...
  64         //       }
  65         //    }
  66         //
  67         // The let bindings are valid till the end of block so all we have to do is to pop all
  68         // the let-scopes at the end.
  69         //
  70         // First we build all the statements in the block.
  71         let mut let_scope_stack = Vec::with_capacity(8);
  72         let outer_source_scope = this.source_scope;
  73         let outer_push_unsafe_count = this.push_unsafe_count;
  74         let outer_unpushed_unsafe = this.unpushed_unsafe;
  75         this.update_source_scope_for_safety_mode(span, safety_mode);
  76
  77         let source_info = this.source_info(span);
  78         for stmt in stmts {
  79             let Stmt { kind, opt_destruction_scope } = this.hir.mirror(stmt);
  80             match kind {
  81                 StmtKind::Expr { scope, expr } => {
  82                     this.block_context.push(BlockFrame::Statement { ignores_expr_result: true });
  83                     unpack!(
  84                         block = this.in_opt_scope(
  85                             opt_destruction_scope.map(|de| (de, source_info)),
  86                             |this| {
  87                                 let si = (scope, source_info);
  88                                 this.in_scope(si, LintLevel::Inherited, |this| {
  89                                     let expr = this.hir.mirror(expr);
  90                                     this.stmt_expr(block, expr, Some(scope))
  91                                 })
  92                             }
  93                         )
  94                     );
  95                 }
  96                 StmtKind::Let { remainder_scope, init_scope, pattern, initializer, lint_level } => {
  97                     let ignores_expr_result =
  98                         if let PatKind::Wild = *pattern.kind { true } else { false };
  99                     this.block_context.push(BlockFrame::Statement { ignores_expr_result });
 100
 101                     // Enter the remainder scope, i.e., the bindings' destruction scope.
 102                     this.push_scope((remainder_scope, source_info));
 103                     let_scope_stack.push(remainder_scope);
 104
 105                     // Declare the bindings, which may create a source scope.
 106                     let remainder_span =
 107                         remainder_scope.span(this.hir.tcx(), &this.hir.region_scope_tree);
 108
 109                     let visibility_scope =
 110                         Some(this.new_source_scope(remainder_span, LintLevel::Inherited, None));
 111
 112                     // Evaluate the initializer, if present.
 113                     if let Some(init) = initializer {
 114                         let initializer_span = init.span();
 115
 116                         unpack!(
 117                             block = this.in_opt_scope(
 118                                 opt_destruction_scope.map(|de| (de, source_info)),
 119                                 |this| {
 120                                     let scope = (init_scope, source_info);
 121                                     this.in_scope(scope, lint_level, |this| {
 122                                         this.declare_bindings(
 123                                             visibility_scope,
 124                                             remainder_span,
 125                                             &pattern,
 126                                             ArmHasGuard(false),
 127                                             Some((None, initializer_span)),
 128                                         );
 129                                         this.expr_into_pattern(block, pattern, init)
 130                                     })
 131                                 }
 132                             )
 133                         );
 134                     } else {
 135                         let scope = (init_scope, source_info);
 136                         unpack!(this.in_scope(scope, lint_level, |this| {
 137                             this.declare_bindings(
 138                                 visibility_scope,
 139                                 remainder_span,
 140                                 &pattern,
 141                                 ArmHasGuard(false),
 142                                 None,
 143                             );
 144                             block.unit()
 145                         }));
 146
 147                         debug!("ast_block_stmts: pattern={:?}", pattern);
 148                         this.visit_bindings(
 149                             &pattern,
 150                             UserTypeProjections::none(),
 151                             &mut |this, _, _, _, node, span, _, _| {
 152                                 this.storage_live_binding(block, node, span, OutsideGuard);
 153                                 this.schedule_drop_for_binding(node, span, OutsideGuard);
 154                             },
 155                         )
 156                     }
 157
 158                     // Enter the visibility scope, after evaluating the initializer.
 159                     if let Some(source_scope) = visibility_scope {
 160                         this.source_scope = source_scope;
 161                     }
 162                 }
 163             }
 164
 165             let popped = this.block_context.pop();
 166             assert!(popped.map_or(false, |bf| bf.is_statement()));
 167         }
 168
 169         // Then, the block may have an optional trailing expression which is a “return” value
 170         // of the block, which is stored into `destination`.
 171         let tcx = this.hir.tcx();
 172         let destination_ty = destination.ty(&this.local_decls, tcx).ty;
 173         if let Some(expr) = expr {
 174             let tail_result_is_ignored =
 175                 destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
 176             this.block_context.push(BlockFrame::TailExpr { tail_result_is_ignored });
 177
 178             unpack!(block = this.into(destination, block, expr));
 179             let popped = this.block_context.pop();
 180
 181             assert!(popped.map_or(false, |bf| bf.is_tail_expr()));
 182         } else {
 183             // If a block has no trailing expression, then it is given an implicit return type.
 184             // This return type is usually `()`, unless the block is diverging, in which case the
 185             // return type is `!`. For the unit type, we need to actually return the unit, but in
 186             // the case of `!`, no return value is required, as the block will never return.
 187             if destination_ty.is_unit() {
 188                 // We only want to assign an implicit `()` as the return value of the block if the
 189                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
 190                 this.cfg.push_assign_unit(block, source_info, destination);
 191             }
 192         }
 193         // Finally, we pop all the let scopes before exiting out from the scope of block
 194         // itself.
 195         for scope in let_scope_stack.into_iter().rev() {
 196             unpack!(block = this.pop_scope((scope, source_info), block));
 197         }
 198         // Restore the original source scope.
 199         this.source_scope = outer_source_scope;
 200         this.push_unsafe_count = outer_push_unsafe_count;
 201         this.unpushed_unsafe = outer_unpushed_unsafe;
 202         block.unit()
 203     }
 204
 205     /// If we are changing the safety mode, create a new source scope
 206     fn update_source_scope_for_safety_mode(&mut self, span: Span, safety_mode: BlockSafety) {
 207         debug!("update_source_scope_for({:?}, {:?})", span, safety_mode);
 208         let new_unsafety = match safety_mode {
 209             BlockSafety::Safe => None,
 210             BlockSafety::ExplicitUnsafe(hir_id) => {
 211                 assert_eq!(self.push_unsafe_count, 0);
 212                 match self.unpushed_unsafe {
 213                     Safety::Safe => {}
 214                     _ => return,
 215                 }
 216                 self.unpushed_unsafe = Safety::ExplicitUnsafe(hir_id);
 217                 Some(Safety::ExplicitUnsafe(hir_id))
 218             }
 219             BlockSafety::PushUnsafe => {
 220                 self.push_unsafe_count += 1;
 221                 Some(Safety::BuiltinUnsafe)
 222             }
 223             BlockSafety::PopUnsafe => {
 224                 self.push_unsafe_count = self
 225                     .push_unsafe_count
 226                     .checked_sub(1)
 227                     .unwrap_or_else(|| span_bug!(span, "unsafe count underflow"));
 228                 if self.push_unsafe_count == 0 { Some(self.unpushed_unsafe) } else { None }
 229             }
 230         };
 231
 232         if let Some(unsafety) = new_unsafety {
 233             self.source_scope = self.new_source_scope(span, LintLevel::Inherited, Some(unsafety));
 234         }
 235     }
 236 }