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_hir as hir;
   6 use rustc_middle::mir::*;
   7 use rustc_session::lint::builtin::UNSAFE_OP_IN_UNSAFE_FN;
   8 use rustc_session::lint::Level;
   9 use rustc_span::Span;
  10
  11 impl<'a, 'tcx> Builder<'a, 'tcx> {
  12     crate fn ast_block(
  13         &mut self,
  14         destination: Place<'tcx>,
  15         block: BasicBlock,
  16         ast_block: &'tcx hir::Block<'tcx>,
  17         source_info: SourceInfo,
  18     ) -> BlockAnd<()> {
  19         let Block {
  20             region_scope,
  21             opt_destruction_scope,
  22             span,
  23             stmts,
  24             expr,
  25             targeted_by_break,
  26             safety_mode,
  27         } = self.hir.mirror(ast_block);
  28         self.in_opt_scope(opt_destruction_scope.map(|de| (de, source_info)), move |this| {
  29             this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
  30                 if targeted_by_break {
  31                     this.in_breakable_scope(None, destination, span, |this| {
  32                         Some(this.ast_block_stmts(
  33                             destination,
  34                             block,
  35                             span,
  36                             stmts,
  37                             expr,
  38                             safety_mode,
  39                         ))
  40                     })
  41                 } else {
  42                     this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
  43                 }
  44             })
  45         })
  46     }
  47
  48     fn ast_block_stmts(
  49         &mut self,
  50         destination: Place<'tcx>,
  51         mut block: BasicBlock,
  52         span: Span,
  53         stmts: Vec<StmtRef<'tcx>>,
  54         expr: Option<ExprRef<'tcx>>,
  55         safety_mode: BlockSafety,
  56     ) -> BlockAnd<()> {
  57         let this = self;
  58
  59         // This convoluted structure is to avoid using recursion as we walk down a list
  60         // of statements. Basically, the structure we get back is something like:
  61         //
  62         //    let x = <init> in {
  63         //       expr1;
  64         //       let y = <init> in {
  65         //           expr2;
  66         //           expr3;
  67         //           ...
  68         //       }
  69         //    }
  70         //
  71         // The let bindings are valid till the end of block so all we have to do is to pop all
  72         // the let-scopes at the end.
  73         //
  74         // First we build all the statements in the block.
  75         let mut let_scope_stack = Vec::with_capacity(8);
  76         let outer_source_scope = this.source_scope;
  77         let outer_push_unsafe_count = this.push_unsafe_count;
  78         let outer_unpushed_unsafe = this.unpushed_unsafe;
  79         this.update_source_scope_for_safety_mode(span, safety_mode);
  80
  81         let source_info = this.source_info(span);
  82         for stmt in stmts {
  83             let Stmt { kind, opt_destruction_scope } = this.hir.mirror(stmt);
  84             match kind {
  85                 StmtKind::Expr { scope, expr } => {
  86                     this.block_context.push(BlockFrame::Statement { ignores_expr_result: true });
  87                     unpack!(
  88                         block = this.in_opt_scope(
  89                             opt_destruction_scope.map(|de| (de, source_info)),
  90                             |this| {
  91                                 let si = (scope, source_info);
  92                                 this.in_scope(si, LintLevel::Inherited, |this| {
  93                                     let expr = this.hir.mirror(expr);
  94                                     this.stmt_expr(block, expr, Some(scope))
  95                                 })
  96                             }
  97                         )
  98                     );
  99                 }
 100                 StmtKind::Let { remainder_scope, init_scope, pattern, initializer, lint_level } => {
 101                     let ignores_expr_result = matches!(*pattern.kind, PatKind::Wild);
 102                     this.block_context.push(BlockFrame::Statement { ignores_expr_result });
 103
 104                     // Enter the remainder scope, i.e., the bindings' destruction scope.
 105                     this.push_scope((remainder_scope, source_info));
 106                     let_scope_stack.push(remainder_scope);
 107
 108                     // Declare the bindings, which may create a source scope.
 109                     let remainder_span =
 110                         remainder_scope.span(this.hir.tcx(), &this.hir.region_scope_tree);
 111
 112                     let visibility_scope =
 113                         Some(this.new_source_scope(remainder_span, LintLevel::Inherited, None));
 114
 115                     // Evaluate the initializer, if present.
 116                     if let Some(init) = initializer {
 117                         let initializer_span = init.span();
 118
 119                         unpack!(
 120                             block = this.in_opt_scope(
 121                                 opt_destruction_scope.map(|de| (de, source_info)),
 122                                 |this| {
 123                                     let scope = (init_scope, source_info);
 124                                     this.in_scope(scope, lint_level, |this| {
 125                                         this.declare_bindings(
 126                                             visibility_scope,
 127                                             remainder_span,
 128                                             &pattern,
 129                                             ArmHasGuard(false),
 130                                             Some((None, initializer_span)),
 131                                         );
 132                                         this.expr_into_pattern(block, pattern, init)
 133                                     })
 134                                 }
 135                             )
 136                         );
 137                     } else {
 138                         let scope = (init_scope, source_info);
 139                         unpack!(this.in_scope(scope, lint_level, |this| {
 140                             this.declare_bindings(
 141                                 visibility_scope,
 142                                 remainder_span,
 143                                 &pattern,
 144                                 ArmHasGuard(false),
 145                                 None,
 146                             );
 147                             block.unit()
 148                         }));
 149
 150                         debug!("ast_block_stmts: pattern={:?}", pattern);
 151                         this.visit_primary_bindings(
 152                             &pattern,
 153                             UserTypeProjections::none(),
 154                             &mut |this, _, _, _, node, span, _, _| {
 155                                 this.storage_live_binding(block, node, span, OutsideGuard, true);
 156                                 this.schedule_drop_for_binding(node, span, OutsideGuard);
 157                             },
 158                         )
 159                     }
 160
 161                     // Enter the visibility scope, after evaluating the initializer.
 162                     if let Some(source_scope) = visibility_scope {
 163                         this.source_scope = source_scope;
 164                     }
 165                 }
 166             }
 167
 168             let popped = this.block_context.pop();
 169             assert!(popped.map_or(false, |bf| bf.is_statement()));
 170         }
 171
 172         // Then, the block may have an optional trailing expression which is a “return” value
 173         // of the block, which is stored into `destination`.
 174         let tcx = this.hir.tcx();
 175         let destination_ty = destination.ty(&this.local_decls, tcx).ty;
 176         if let Some(expr) = expr {
 177             let tail_result_is_ignored =
 178                 destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
 179             let span = match expr {
 180                 ExprRef::Thir(expr) => expr.span,
 181                 ExprRef::Mirror(ref expr) => expr.span,
 182             };
 183             this.block_context.push(BlockFrame::TailExpr { tail_result_is_ignored, span });
 184
 185             unpack!(block = this.into(destination, block, expr));
 186             let popped = this.block_context.pop();
 187
 188             assert!(popped.map_or(false, |bf| bf.is_tail_expr()));
 189         } else {
 190             // If a block has no trailing expression, then it is given an implicit return type.
 191             // This return type is usually `()`, unless the block is diverging, in which case the
 192             // return type is `!`. For the unit type, we need to actually return the unit, but in
 193             // the case of `!`, no return value is required, as the block will never return.
 194             if destination_ty.is_unit() {
 195                 // We only want to assign an implicit `()` as the return value of the block if the
 196                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
 197                 this.cfg.push_assign_unit(block, source_info, destination, this.hir.tcx());
 198             }
 199         }
 200         // Finally, we pop all the let scopes before exiting out from the scope of block
 201         // itself.
 202         for scope in let_scope_stack.into_iter().rev() {
 203             unpack!(block = this.pop_scope((scope, source_info), block));
 204         }
 205         // Restore the original source scope.
 206         this.source_scope = outer_source_scope;
 207         this.push_unsafe_count = outer_push_unsafe_count;
 208         this.unpushed_unsafe = outer_unpushed_unsafe;
 209         block.unit()
 210     }
 211
 212     /// If we are changing the safety mode, create a new source scope
 213     fn update_source_scope_for_safety_mode(&mut self, span: Span, safety_mode: BlockSafety) {
 214         debug!("update_source_scope_for({:?}, {:?})", span, safety_mode);
 215         let new_unsafety = match safety_mode {
 216             BlockSafety::Safe => None,
 217             BlockSafety::ExplicitUnsafe(hir_id) => {
 218                 assert_eq!(self.push_unsafe_count, 0);
 219                 match self.unpushed_unsafe {
 220                     Safety::Safe => {}
 221                     // no longer treat `unsafe fn`s as `unsafe` contexts (see RFC #2585)
 222                     Safety::FnUnsafe
 223                         if self.hir.tcx().lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, hir_id).0
 224                             != Level::Allow => {}
 225                     _ => return,
 226                 }
 227                 self.unpushed_unsafe = Safety::ExplicitUnsafe(hir_id);
 228                 Some(Safety::ExplicitUnsafe(hir_id))
 229             }
 230             BlockSafety::PushUnsafe => {
 231                 self.push_unsafe_count += 1;
 232                 Some(Safety::BuiltinUnsafe)
 233             }
 234             BlockSafety::PopUnsafe => {
 235                 self.push_unsafe_count = self
 236                     .push_unsafe_count
 237                     .checked_sub(1)
 238                     .unwrap_or_else(|| span_bug!(span, "unsafe count underflow"));
 239                 if self.push_unsafe_count == 0 { Some(self.unpushed_unsafe) } else { None }
 240             }
 241         };
 242
 243         if let Some(unsafety) = new_unsafety {
 244             self.source_scope = self.new_source_scope(span, LintLevel::Inherited, Some(unsafety));
 245         }
 246     }
 247 }