src/librustc_mir/build/block.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use build::{BlockAnd, BlockAndExtension, Builder};
  12 use build::ForGuard::OutsideGuard;
  13 use build::matches::ArmHasGuard;
  14 use hair::*;
  15 use rustc::mir::*;
  16 use rustc::hir;
  17 use syntax_pos::Span;
  18
  19 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  20     pub fn ast_block(&mut self,
  21                      destination: &Place<'tcx>,
  22                      block: BasicBlock,
  23                      ast_block: &'tcx hir::Block,
  24                      source_info: SourceInfo)
  25                      -> BlockAnd<()> {
  26         let Block {
  27             region_scope,
  28             opt_destruction_scope,
  29             span,
  30             stmts,
  31             expr,
  32             targeted_by_break,
  33             safety_mode
  34         } =
  35             self.hir.mirror(ast_block);
  36         self.in_opt_scope(opt_destruction_scope.map(|de|(de, source_info)), block, move |this| {
  37             this.in_scope((region_scope, source_info), LintLevel::Inherited, block, move |this| {
  38                 if targeted_by_break {
  39                     // This is a `break`-able block
  40                     let exit_block = this.cfg.start_new_block();
  41                     let block_exit = this.in_breakable_scope(
  42                         None, exit_block, destination.clone(), |this| {
  43                             this.ast_block_stmts(destination, block, span, stmts, expr,
  44                                                  safety_mode)
  45                         });
  46                     this.cfg.terminate(unpack!(block_exit), source_info,
  47                                        TerminatorKind::Goto { target: exit_block });
  48                     exit_block.unit()
  49                 } else {
  50                     this.ast_block_stmts(destination, block, span, stmts, expr,
  51                                          safety_mode)
  52                 }
  53             })
  54         })
  55     }
  56
  57     fn ast_block_stmts(&mut self,
  58                        destination: &Place<'tcx>,
  59                        mut block: BasicBlock,
  60                        span: Span,
  61                        stmts: Vec<StmtRef<'tcx>>,
  62                        expr: Option<ExprRef<'tcx>>,
  63                        safety_mode: BlockSafety)
  64                        -> BlockAnd<()> {
  65         let this = self;
  66
  67         // This convoluted structure is to avoid using recursion as we walk down a list
  68         // of statements. Basically, the structure we get back is something like:
  69         //
  70         //    let x = <init> in {
  71         //       expr1;
  72         //       let y = <init> in {
  73         //           expr2;
  74         //           expr3;
  75         //           ...
  76         //       }
  77         //    }
  78         //
  79         // The let bindings are valid till the end of block so all we have to do is to pop all
  80         // the let-scopes at the end.
  81         //
  82         // First we build all the statements in the block.
  83         let mut let_scope_stack = Vec::with_capacity(8);
  84         let outer_source_scope = this.source_scope;
  85         let outer_push_unsafe_count = this.push_unsafe_count;
  86         let outer_unpushed_unsafe = this.unpushed_unsafe;
  87         this.update_source_scope_for_safety_mode(span, safety_mode);
  88
  89         let source_info = this.source_info(span);
  90         for stmt in stmts {
  91             let Stmt { kind, opt_destruction_scope } = this.hir.mirror(stmt);
  92             match kind {
  93                 StmtKind::Expr { scope, expr } => {
  94                     unpack!(block = this.in_opt_scope(
  95                         opt_destruction_scope.map(|de|(de, source_info)), block, |this| {
  96                             let si = (scope, source_info);
  97                             this.in_scope(si, LintLevel::Inherited, block, |this| {
  98                                 let expr = this.hir.mirror(expr);
  99                                 this.stmt_expr(block, expr)
 100                             })
 101                         }));
 102                 }
 103                 StmtKind::Let {
 104                     remainder_scope,
 105                     init_scope,
 106                     pattern,
 107                     ty,
 108                     initializer,
 109                     lint_level
 110                 } => {
 111                     // Enter the remainder scope, i.e. the bindings' destruction scope.
 112                     this.push_scope((remainder_scope, source_info));
 113                     let_scope_stack.push(remainder_scope);
 114
 115                     // Declare the bindings, which may create a source scope.
 116                     let remainder_span = remainder_scope.span(this.hir.tcx(),
 117                                                               &this.hir.region_scope_tree);
 118                     let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern,
 119                                                       ArmHasGuard(false));
 120
 121                     // Evaluate the initializer, if present.
 122                     if let Some(init) = initializer {
 123                         unpack!(block = this.in_opt_scope(
 124                             opt_destruction_scope.map(|de|(de, source_info)), block, |this| {
 125                                 let scope = (init_scope, source_info);
 126                                 this.in_scope(scope, lint_level, block, |this| {
 127                                     this.expr_into_pattern(block, ty, pattern, init)
 128                                 })
 129                             }));
 130                     } else {
 131                         // FIXME(#47184): We currently only insert `UserAssertTy` statements for
 132                         // patterns that are bindings, this is as we do not want to deconstruct
 133                         // the type being assertion to match the pattern.
 134                         if let PatternKind::Binding { var, .. } = *pattern.kind {
 135                             if let Some(ty) = ty {
 136                                 this.user_assert_ty(block, ty, var, span);
 137                             }
 138                         }
 139
 140                         this.visit_bindings(&pattern, &mut |this, _, _, _, node, span, _| {
 141                             this.storage_live_binding(block, node, span, OutsideGuard);
 142                             this.schedule_drop_for_binding(node, span, OutsideGuard);
 143                         })
 144                     }
 145
 146                     // Enter the source scope, after evaluating the initializer.
 147                     if let Some(source_scope) = scope {
 148                         this.source_scope = source_scope;
 149                     }
 150                 }
 151             }
 152         }
 153         // Then, the block may have an optional trailing expression which is a “return” value
 154         // of the block.
 155         if let Some(expr) = expr {
 156             unpack!(block = this.into(destination, block, expr));
 157         } else {
 158             // If a block has no trailing expression, then it is given an implicit return type.
 159             // This return type is usually `()`, unless the block is diverging, in which case the
 160             // return type is `!`. For the unit type, we need to actually return the unit, but in
 161             // the case of `!`, no return value is required, as the block will never return.
 162             let tcx = this.hir.tcx();
 163             let ty = destination.ty(&this.local_decls, tcx).to_ty(tcx);
 164             if ty.is_nil() {
 165                 // We only want to assign an implicit `()` as the return value of the block if the
 166                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
 167                 this.cfg.push_assign_unit(block, source_info, destination);
 168             }
 169         }
 170         // Finally, we pop all the let scopes before exiting out from the scope of block
 171         // itself.
 172         for scope in let_scope_stack.into_iter().rev() {
 173             unpack!(block = this.pop_scope((scope, source_info), block));
 174         }
 175         // Restore the original source scope.
 176         this.source_scope = outer_source_scope;
 177         this.push_unsafe_count = outer_push_unsafe_count;
 178         this.unpushed_unsafe = outer_unpushed_unsafe;
 179         block.unit()
 180     }
 181
 182     /// If we are changing the safety mode, create a new source scope
 183     fn update_source_scope_for_safety_mode(&mut self,
 184                                                span: Span,
 185                                                safety_mode: BlockSafety)
 186     {
 187         debug!("update_source_scope_for({:?}, {:?})", span, safety_mode);
 188         let new_unsafety = match safety_mode {
 189             BlockSafety::Safe => None,
 190             BlockSafety::ExplicitUnsafe(node_id) => {
 191                 assert_eq!(self.push_unsafe_count, 0);
 192                 match self.unpushed_unsafe {
 193                     Safety::Safe => {}
 194                     _ => return
 195                 }
 196                 self.unpushed_unsafe = Safety::ExplicitUnsafe(node_id);
 197                 Some(Safety::ExplicitUnsafe(node_id))
 198             }
 199             BlockSafety::PushUnsafe => {
 200                 self.push_unsafe_count += 1;
 201                 Some(Safety::BuiltinUnsafe)
 202             }
 203             BlockSafety::PopUnsafe => {
 204                 self.push_unsafe_count =
 205                     self.push_unsafe_count.checked_sub(1).unwrap_or_else(|| {
 206                         span_bug!(span, "unsafe count underflow")
 207                     });
 208                 if self.push_unsafe_count == 0 {
 209                     Some(self.unpushed_unsafe)
 210                 } else {
 211                     None
 212                 }
 213             }
 214         };
 215
 216         if let Some(unsafety) = new_unsafety {
 217             self.source_scope = self.new_source_scope(
 218                 span, LintLevel::Inherited, Some(unsafety));
 219         }
 220     }
 221 }