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 hair::*;
  13 use rustc::mir::*;
  14 use rustc::hir;
  15 use syntax_pos::Span;
  16
  17 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  18     pub fn ast_block(&mut self,
  19                      destination: &Place<'tcx>,
  20                      block: BasicBlock,
  21                      ast_block: &'tcx hir::Block,
  22                      source_info: SourceInfo)
  23                      -> BlockAnd<()> {
  24         let Block {
  25             region_scope,
  26             opt_destruction_scope,
  27             span,
  28             stmts,
  29             expr,
  30             targeted_by_break,
  31             safety_mode
  32         } =
  33             self.hir.mirror(ast_block);
  34         self.in_opt_scope(opt_destruction_scope.map(|de|(de, source_info)), block, move |this| {
  35             this.in_scope((region_scope, source_info), LintLevel::Inherited, block, move |this| {
  36                 if targeted_by_break {
  37                     // This is a `break`-able block (currently only `catch { ... }`)
  38                     let exit_block = this.cfg.start_new_block();
  39                     let block_exit = this.in_breakable_scope(
  40                         None, exit_block, destination.clone(), |this| {
  41                             this.ast_block_stmts(destination, block, span, stmts, expr,
  42                                                  safety_mode)
  43                         });
  44                     this.cfg.terminate(unpack!(block_exit), source_info,
  45                                        TerminatorKind::Goto { target: exit_block });
  46                     exit_block.unit()
  47                 } else {
  48                     this.ast_block_stmts(destination, block, span, stmts, expr,
  49                                          safety_mode)
  50                 }
  51             })
  52         })
  53     }
  54
  55     fn ast_block_stmts(&mut self,
  56                        destination: &Place<'tcx>,
  57                        mut block: BasicBlock,
  58                        span: Span,
  59                        stmts: Vec<StmtRef<'tcx>>,
  60                        expr: Option<ExprRef<'tcx>>,
  61                        safety_mode: BlockSafety)
  62                        -> BlockAnd<()> {
  63         let this = self;
  64
  65         // This convoluted structure is to avoid using recursion as we walk down a list
  66         // of statements. Basically, the structure we get back is something like:
  67         //
  68         //    let x = <init> in {
  69         //       expr1;
  70         //       let y = <init> in {
  71         //           expr2;
  72         //           expr3;
  73         //           ...
  74         //       }
  75         //    }
  76         //
  77         // The let bindings are valid till the end of block so all we have to do is to pop all
  78         // the let-scopes at the end.
  79         //
  80         // First we build all the statements in the block.
  81         let mut let_scope_stack = Vec::with_capacity(8);
  82         let outer_visibility_scope = this.visibility_scope;
  83         let outer_push_unsafe_count = this.push_unsafe_count;
  84         let outer_unpushed_unsafe = this.unpushed_unsafe;
  85         this.update_visibility_scope_for_safety_mode(span, safety_mode);
  86
  87         let source_info = this.source_info(span);
  88         for stmt in stmts {
  89             let Stmt { kind, opt_destruction_scope } = this.hir.mirror(stmt);
  90             match kind {
  91                 StmtKind::Expr { scope, expr } => {
  92                     unpack!(block = this.in_opt_scope(
  93                         opt_destruction_scope.map(|de|(de, source_info)), block, |this| {
  94                             let si = (scope, source_info);
  95                             this.in_scope(si, LintLevel::Inherited, block, |this| {
  96                                 let expr = this.hir.mirror(expr);
  97                                 this.stmt_expr(block, expr)
  98                             })
  99                         }));
 100                 }
 101                 StmtKind::Let {
 102                     remainder_scope,
 103                     init_scope,
 104                     pattern,
 105                     initializer,
 106                     lint_level
 107                 } => {
 108                     // Enter the remainder scope, i.e. the bindings' destruction scope.
 109                     this.push_scope((remainder_scope, source_info));
 110                     let_scope_stack.push(remainder_scope);
 111
 112                     // Declare the bindings, which may create a visibility scope.
 113                     let remainder_span = remainder_scope.span(this.hir.tcx(),
 114                                                               &this.hir.region_scope_tree);
 115                     let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern);
 116
 117                     // Evaluate the initializer, if present.
 118                     if let Some(init) = initializer {
 119                         unpack!(block = this.in_opt_scope(
 120                             opt_destruction_scope.map(|de|(de, source_info)), block, move |this| {
 121                                 let scope = (init_scope, source_info);
 122                                 this.in_scope(scope, lint_level, block, move |this| {
 123                                     // FIXME #30046                             ^~~~
 124                                     this.expr_into_pattern(block, pattern, init)
 125                                 })
 126                             }));
 127                     } else {
 128                         this.visit_bindings(&pattern, &mut |this, _, _, node, span, _| {
 129                             this.storage_live_binding(block, node, span);
 130                             this.schedule_drop_for_binding(node, span);
 131                         })
 132                     }
 133
 134                     // Enter the visibility scope, after evaluating the initializer.
 135                     if let Some(visibility_scope) = scope {
 136                         this.visibility_scope = visibility_scope;
 137                     }
 138                 }
 139             }
 140         }
 141         // Then, the block may have an optional trailing expression which is a “return” value
 142         // of the block.
 143         if let Some(expr) = expr {
 144             unpack!(block = this.into(destination, block, expr));
 145         } else {
 146             // If a block has no trailing expression, then it is given an implicit return type.
 147             // This return type is usually `()`, unless the block is diverging, in which case the
 148             // return type is `!`. For the unit type, we need to actually return the unit, but in
 149             // the case of `!`, no return value is required, as the block will never return.
 150             let tcx = this.hir.tcx();
 151             let ty = destination.ty(&this.local_decls, tcx).to_ty(tcx);
 152             if ty.is_nil() {
 153                 // We only want to assign an implicit `()` as the return value of the block if the
 154                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
 155                 this.cfg.push_assign_unit(block, source_info, destination);
 156             }
 157         }
 158         // Finally, we pop all the let scopes before exiting out from the scope of block
 159         // itself.
 160         for scope in let_scope_stack.into_iter().rev() {
 161             unpack!(block = this.pop_scope((scope, source_info), block));
 162         }
 163         // Restore the original visibility scope.
 164         this.visibility_scope = outer_visibility_scope;
 165         this.push_unsafe_count = outer_push_unsafe_count;
 166         this.unpushed_unsafe = outer_unpushed_unsafe;
 167         block.unit()
 168     }
 169
 170     /// If we are changing the safety mode, create a new visibility scope
 171     fn update_visibility_scope_for_safety_mode(&mut self,
 172                                                span: Span,
 173                                                safety_mode: BlockSafety)
 174     {
 175         debug!("update_visibility_scope_for({:?}, {:?})", span, safety_mode);
 176         let new_unsafety = match safety_mode {
 177             BlockSafety::Safe => None,
 178             BlockSafety::ExplicitUnsafe(node_id) => {
 179                 assert_eq!(self.push_unsafe_count, 0);
 180                 match self.unpushed_unsafe {
 181                     Safety::Safe => {}
 182                     _ => return
 183                 }
 184                 self.unpushed_unsafe = Safety::ExplicitUnsafe(node_id);
 185                 Some(Safety::ExplicitUnsafe(node_id))
 186             }
 187             BlockSafety::PushUnsafe => {
 188                 self.push_unsafe_count += 1;
 189                 Some(Safety::BuiltinUnsafe)
 190             }
 191             BlockSafety::PopUnsafe => {
 192                 self.push_unsafe_count =
 193                     self.push_unsafe_count.checked_sub(1).unwrap_or_else(|| {
 194                         span_bug!(span, "unsafe count underflow")
 195                     });
 196                 if self.push_unsafe_count == 0 {
 197                     Some(self.unpushed_unsafe)
 198                 } else {
 199                     None
 200                 }
 201             }
 202         };
 203
 204         if let Some(unsafety) = new_unsafety {
 205             self.visibility_scope = self.new_visibility_scope(
 206                 span, LintLevel::Inherited, Some(unsafety));
 207         }
 208     }
 209 }