src/librustc_mir/build/expr/stmt.rs

   1 use build::scope::BreakableScope;
   2 use build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
   3 use hair::*;
   4 use rustc::mir::*;
   5
   6 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
   7     /// Builds a block of MIR statements to evaluate the HAIR `expr`.
   8     /// If the original expression was an AST statement,
   9     /// (e.g., `some().code(&here());`) then `opt_stmt_span` is the
  10     /// span of that statement (including its semicolon, if any).
  11     /// Diagnostics use this span (which may be larger than that of
  12     /// `expr`) to identify when statement temporaries are dropped.
  13     pub fn stmt_expr(&mut self,
  14                      mut block: BasicBlock,
  15                      expr: Expr<'tcx>,
  16                      opt_stmt_span: Option<StatementSpan>)
  17                      -> BlockAnd<()>
  18     {
  19         let this = self;
  20         let expr_span = expr.span;
  21         let source_info = this.source_info(expr.span);
  22         // Handle a number of expressions that don't need a destination at all. This
  23         // avoids needing a mountain of temporary `()` variables.
  24         let expr2 = expr.clone();
  25         match expr.kind {
  26             ExprKind::Scope {
  27                 region_scope,
  28                 lint_level,
  29                 value,
  30             } => {
  31                 let value = this.hir.mirror(value);
  32                 this.in_scope((region_scope, source_info), lint_level, block, |this| {
  33                     this.stmt_expr(block, value, opt_stmt_span)
  34                 })
  35             }
  36             ExprKind::Assign { lhs, rhs } => {
  37                 let lhs = this.hir.mirror(lhs);
  38                 let rhs = this.hir.mirror(rhs);
  39                 let lhs_span = lhs.span;
  40
  41                 // Note: we evaluate assignments right-to-left. This
  42                 // is better for borrowck interaction with overloaded
  43                 // operators like x[j] = x[i].
  44
  45                 debug!("stmt_expr Assign block_context.push(SubExpr) : {:?}", expr2);
  46                 this.block_context.push(BlockFrame::SubExpr);
  47
  48                 // Generate better code for things that don't need to be
  49                 // dropped.
  50                 if this.hir.needs_drop(lhs.ty) {
  51                     let rhs = unpack!(block = this.as_local_operand(block, rhs));
  52                     let lhs = unpack!(block = this.as_place(block, lhs));
  53                     unpack!(block = this.build_drop_and_replace(block, lhs_span, lhs, rhs));
  54                 } else {
  55                     let rhs = unpack!(block = this.as_local_rvalue(block, rhs));
  56                     let lhs = unpack!(block = this.as_place(block, lhs));
  57                     this.cfg.push_assign(block, source_info, &lhs, rhs);
  58                 }
  59
  60                 this.block_context.pop();
  61                 block.unit()
  62             }
  63             ExprKind::AssignOp { op, lhs, rhs } => {
  64                 // FIXME(#28160) there is an interesting semantics
  65                 // question raised here -- should we "freeze" the
  66                 // value of the lhs here?  I'm inclined to think not,
  67                 // since it seems closer to the semantics of the
  68                 // overloaded version, which takes `&mut self`.  This
  69                 // only affects weird things like `x += {x += 1; x}`
  70                 // -- is that equal to `x + (x + 1)` or `2*(x+1)`?
  71
  72                 let lhs = this.hir.mirror(lhs);
  73                 let lhs_ty = lhs.ty;
  74
  75                 debug!("stmt_expr AssignOp block_context.push(SubExpr) : {:?}", expr2);
  76                 this.block_context.push(BlockFrame::SubExpr);
  77
  78                 // As above, RTL.
  79                 let rhs = unpack!(block = this.as_local_operand(block, rhs));
  80                 let lhs = unpack!(block = this.as_place(block, lhs));
  81
  82                 // we don't have to drop prior contents or anything
  83                 // because AssignOp is only legal for Copy types
  84                 // (overloaded ops should be desugared into a call).
  85                 let result = unpack!(
  86                     block = this.build_binary_op(
  87                         block,
  88                         op,
  89                         expr_span,
  90                         lhs_ty,
  91                         Operand::Copy(lhs.clone()),
  92                         rhs
  93                     )
  94                 );
  95                 this.cfg.push_assign(block, source_info, &lhs, result);
  96
  97                 this.block_context.pop();
  98                 block.unit()
  99             }
 100             ExprKind::Continue { label } => {
 101                 let BreakableScope {
 102                     continue_block,
 103                     region_scope,
 104                     ..
 105                 } = *this.find_breakable_scope(expr_span, label);
 106                 let continue_block = continue_block
 107                     .expect("Attempted to continue in non-continuable breakable block");
 108                 this.exit_scope(
 109                     expr_span,
 110                     (region_scope, source_info),
 111                     block,
 112                     continue_block,
 113                 );
 114                 this.cfg.start_new_block().unit()
 115             }
 116             ExprKind::Break { label, value } => {
 117                 let (break_block, region_scope, destination) = {
 118                     let BreakableScope {
 119                         break_block,
 120                         region_scope,
 121                         ref break_destination,
 122                         ..
 123                     } = *this.find_breakable_scope(expr_span, label);
 124                     (break_block, region_scope, break_destination.clone())
 125                 };
 126                 if let Some(value) = value {
 127                     debug!("stmt_expr Break val block_context.push(SubExpr) : {:?}", expr2);
 128                     this.block_context.push(BlockFrame::SubExpr);
 129                     unpack!(block = this.into(&destination, block, value));
 130                     this.block_context.pop();
 131                 } else {
 132                     this.cfg.push_assign_unit(block, source_info, &destination)
 133                 }
 134                 this.exit_scope(expr_span, (region_scope, source_info), block, break_block);
 135                 this.cfg.start_new_block().unit()
 136             }
 137             ExprKind::Return { value } => {
 138                 block = match value {
 139                     Some(value) => {
 140                         debug!("stmt_expr Return val block_context.push(SubExpr) : {:?}", expr2);
 141                         this.block_context.push(BlockFrame::SubExpr);
 142                         let result = unpack!(this.into(&Place::Local(RETURN_PLACE), block, value));
 143                         this.block_context.pop();
 144                         result
 145                     }
 146                     None => {
 147                         this.cfg
 148                             .push_assign_unit(block, source_info, &Place::Local(RETURN_PLACE));
 149                         block
 150                     }
 151                 };
 152                 let region_scope = this.region_scope_of_return_scope();
 153                 let return_block = this.return_block();
 154                 this.exit_scope(expr_span, (region_scope, source_info), block, return_block);
 155                 this.cfg.start_new_block().unit()
 156             }
 157             ExprKind::InlineAsm {
 158                 asm,
 159                 outputs,
 160                 inputs,
 161             } => {
 162                 debug!("stmt_expr InlineAsm block_context.push(SubExpr) : {:?}", expr2);
 163                 this.block_context.push(BlockFrame::SubExpr);
 164                 let outputs = outputs
 165                     .into_iter()
 166                     .map(|output| unpack!(block = this.as_place(block, output)))
 167                     .collect::<Vec<_>>()
 168                     .into_boxed_slice();
 169                 let inputs = inputs
 170                     .into_iter()
 171                     .map(|input| {
 172                         (
 173                             input.span(),
 174                             unpack!(block = this.as_local_operand(block, input)),
 175                         )
 176                     }).collect::<Vec<_>>()
 177                     .into_boxed_slice();
 178                 this.cfg.push(
 179                     block,
 180                     Statement {
 181                         source_info,
 182                         kind: StatementKind::InlineAsm {
 183                             asm: box asm.clone(),
 184                             outputs,
 185                             inputs,
 186                         },
 187                     },
 188                 );
 189                 this.block_context.pop();
 190                 block.unit()
 191             }
 192             _ => {
 193                 let expr_ty = expr.ty;
 194
 195                 // Issue #54382: When creating temp for the value of
 196                 // expression like:
 197                 //
 198                 // `{ side_effects(); { let l = stuff(); the_value } }`
 199                 //
 200                 // it is usually better to focus on `the_value` rather
 201                 // than the entirety of block(s) surrounding it.
 202                 let mut temp_span = expr_span;
 203                 let mut temp_in_tail_of_block = false;
 204                 if let ExprKind::Block { body } = expr.kind {
 205                     if let Some(tail_expr) = &body.expr {
 206                         let mut expr = tail_expr;
 207                         while let rustc::hir::ExprKind::Block(subblock, _label) = &expr.node {
 208                             if let Some(subtail_expr) = &subblock.expr {
 209                                 expr = subtail_expr
 210                             } else {
 211                                 break;
 212                             }
 213                         }
 214                         temp_span = expr.span;
 215                         temp_in_tail_of_block = true;
 216                     }
 217                 }
 218
 219                 let temp = {
 220                     let mut local_decl = LocalDecl::new_temp(expr.ty.clone(), temp_span);
 221                     if temp_in_tail_of_block {
 222                         if this.block_context.currently_ignores_tail_results() {
 223                             local_decl = local_decl.block_tail(BlockTailInfo {
 224                                 tail_result_is_ignored: true
 225                             });
 226                         }
 227                     }
 228                     let temp = this.local_decls.push(local_decl);
 229                     let place = Place::Local(temp);
 230                     debug!("created temp {:?} for expr {:?} in block_context: {:?}",
 231                            temp, expr, this.block_context);
 232                     place
 233                 };
 234                 unpack!(block = this.into(&temp, block, expr));
 235
 236                 // Attribute drops of the statement's temps to the
 237                 // semicolon at the statement's end.
 238                 let drop_point = this.hir.tcx().sess.source_map().end_point(match opt_stmt_span {
 239                     None => expr_span,
 240                     Some(StatementSpan(span)) => span,
 241                 });
 242
 243                 unpack!(block = this.build_drop(block, drop_point, temp, expr_ty));
 244                 block.unit()
 245             }
 246         }
 247     }
 248 }