compiler/rustc_mir_build/src/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::thir::*;
   5 use rustc_middle::mir::*;
   6 use rustc_session::lint::builtin::UNSAFE_OP_IN_UNSAFE_FN;
   7 use rustc_session::lint::Level;
   8 use rustc_span::Span;
   9
  10 impl<'a, 'tcx> Builder<'a, 'tcx> {
  11     crate fn ast_block(
  12         &mut self,
  13         destination: Place<'tcx>,
  14         block: BasicBlock,
  15         ast_block: &Block<'_, 'tcx>,
  16         source_info: SourceInfo,
  17     ) -> BlockAnd<()> {
  18         let Block {
  19             region_scope,
  20             opt_destruction_scope,
  21             span,
  22             stmts,
  23             expr,
  24             targeted_by_break,
  25             safety_mode,
  26         } = *ast_block;
  27         self.in_opt_scope(opt_destruction_scope.map(|de| (de, source_info)), move |this| {
  28             this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
  29                 if targeted_by_break {
  30                     this.in_breakable_scope(None, destination, span, |this| {
  31                         Some(this.ast_block_stmts(
  32                             destination,
  33                             block,
  34                             span,
  35                             stmts,
  36                             expr,
  37                             safety_mode,
  38                         ))
  39                     })
  40                 } else {
  41                     this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
  42                 }
  43             })
  44         })
  45     }
  46
  47     fn ast_block_stmts(
  48         &mut self,
  49         destination: Place<'tcx>,
  50         mut block: BasicBlock,
  51         span: Span,
  52         stmts: &[Stmt<'_, 'tcx>],
  53         expr: Option<&Expr<'_, 'tcx>>,
  54         safety_mode: BlockSafety,
  55     ) -> BlockAnd<()> {
  56         let this = self;
  57
  58         // This convoluted structure is to avoid using recursion as we walk down a list
  59         // of statements. Basically, the structure we get back is something like:
  60         //
  61         //    let x = <init> in {
  62         //       expr1;
  63         //       let y = <init> in {
  64         //           expr2;
  65         //           expr3;
  66         //           ...
  67         //       }
  68         //    }
  69         //
  70         // The let bindings are valid till the end of block so all we have to do is to pop all
  71         // the let-scopes at the end.
  72         //
  73         // First we build all the statements in the block.
  74         let mut let_scope_stack = Vec::with_capacity(8);
  75         let outer_source_scope = this.source_scope;
  76         let outer_push_unsafe_count = this.push_unsafe_count;
  77         let outer_unpushed_unsafe = this.unpushed_unsafe;
  78         this.update_source_scope_for_safety_mode(span, safety_mode);
  79
  80         let source_info = this.source_info(span);
  81         for Stmt { kind, opt_destruction_scope } in stmts {
  82             match kind {
  83                 &StmtKind::Expr { scope, expr } => {
  84                     this.block_context.push(BlockFrame::Statement { ignores_expr_result: true });
  85                     unpack!(
  86                         block = this.in_opt_scope(
  87                             opt_destruction_scope.map(|de| (de, source_info)),
  88                             |this| {
  89                                 let si = (scope, source_info);
  90                                 this.in_scope(si, LintLevel::Inherited, |this| {
  91                                     this.stmt_expr(block, expr, Some(scope))
  92                                 })
  93                             }
  94                         )
  95                     );
  96                 }
  97                 StmtKind::Let { remainder_scope, init_scope, pattern, initializer, lint_level } => {
  98                     let ignores_expr_result = matches!(*pattern.kind, PatKind::Wild);
  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 = remainder_scope.span(this.tcx, this.region_scope_tree);
 107
 108                     let visibility_scope =
 109                         Some(this.new_source_scope(remainder_span, LintLevel::Inherited, None));
 110
 111                     // Evaluate the initializer, if present.
 112                     if let Some(init) = initializer {
 113                         let initializer_span = init.span;
 114
 115                         unpack!(
 116                             block = this.in_opt_scope(
 117                                 opt_destruction_scope.map(|de| (de, source_info)),
 118                                 |this| {
 119                                     let scope = (*init_scope, source_info);
 120                                     this.in_scope(scope, *lint_level, |this| {
 121                                         this.declare_bindings(
 122                                             visibility_scope,
 123                                             remainder_span,
 124                                             pattern,
 125                                             ArmHasGuard(false),
 126                                             Some((None, initializer_span)),
 127                                         );
 128                                         this.expr_into_pattern(block, pattern.clone(), init)
 129                                     })
 130                                 }
 131                             )
 132                         );
 133                     } else {
 134                         let scope = (*init_scope, source_info);
 135                         unpack!(this.in_scope(scope, *lint_level, |this| {
 136                             this.declare_bindings(
 137                                 visibility_scope,
 138                                 remainder_span,
 139                                 pattern,
 140                                 ArmHasGuard(false),
 141                                 None,
 142                             );
 143                             block.unit()
 144                         }));
 145
 146                         debug!("ast_block_stmts: pattern={:?}", pattern);
 147                         this.visit_primary_bindings(
 148                             &pattern,
 149                             UserTypeProjections::none(),
 150                             &mut |this, _, _, _, node, span, _, _| {
 151                                 this.storage_live_binding(block, node, span, OutsideGuard, true);
 152                                 this.schedule_drop_for_binding(node, span, OutsideGuard);
 153                             },
 154                         )
 155                     }
 156
 157                     // Enter the visibility scope, after evaluating the initializer.
 158                     if let Some(source_scope) = visibility_scope {
 159                         this.source_scope = source_scope;
 160                     }
 161                 }
 162             }
 163
 164             let popped = this.block_context.pop();
 165             assert!(popped.map_or(false, |bf| bf.is_statement()));
 166         }
 167
 168         // Then, the block may have an optional trailing expression which is a “return” value
 169         // of the block, which is stored into `destination`.
 170         let tcx = this.tcx;
 171         let destination_ty = destination.ty(&this.local_decls, tcx).ty;
 172         if let Some(expr) = expr {
 173             let tail_result_is_ignored =
 174                 destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
 175             this.block_context
 176                 .push(BlockFrame::TailExpr { tail_result_is_ignored, span: expr.span });
 177
 178             unpack!(block = this.expr_into_dest(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, this.tcx);
 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                     // no longer treat `unsafe fn`s as `unsafe` contexts (see RFC #2585)
 215                     Safety::FnUnsafe
 216                         if self.tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, hir_id).0
 217                             != Level::Allow => {}
 218                     _ => return,
 219                 }
 220                 self.unpushed_unsafe = Safety::ExplicitUnsafe(hir_id);
 221                 Some(Safety::ExplicitUnsafe(hir_id))
 222             }
 223             BlockSafety::PushUnsafe => {
 224                 self.push_unsafe_count += 1;
 225                 Some(Safety::BuiltinUnsafe)
 226             }
 227             BlockSafety::PopUnsafe => {
 228                 self.push_unsafe_count = self
 229                     .push_unsafe_count
 230                     .checked_sub(1)
 231                     .unwrap_or_else(|| span_bug!(span, "unsafe count underflow"));
 232                 if self.push_unsafe_count == 0 { Some(self.unpushed_unsafe) } else { None }
 233             }
 234         };
 235
 236         if let Some(unsafety) = new_unsafety {
 237             self.source_scope = self.new_source_scope(span, LintLevel::Inherited, Some(unsafety));
 238         }
 239     }
 240 }