compiler/rustc_mir_build/src/build/expr/stmt.rs

   1 use crate::build::scope::BreakableTarget;
   2 use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
   3 use rustc_middle::middle::region;
   4 use rustc_middle::mir::*;
   5 use rustc_middle::thir::*;
   6
   7 impl<'a, 'tcx> Builder<'a, 'tcx> {
   8     /// Builds a block of MIR statements to evaluate the THIR `expr`.
   9     /// If the original expression was an AST statement,
  10     /// (e.g., `some().code(&here());`) then `opt_stmt_span` is the
  11     /// span of that statement (including its semicolon, if any).
  12     /// The scope is used if a statement temporary must be dropped.
  13     crate fn stmt_expr(
  14         &mut self,
  15         mut block: BasicBlock,
  16         expr: &Expr<'tcx>,
  17         statement_scope: Option<region::Scope>,
  18     ) -> BlockAnd<()> {
  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         match expr.kind {
  25             ExprKind::Scope { region_scope, lint_level, value } => {
  26                 this.in_scope((region_scope, source_info), lint_level, |this| {
  27                     this.stmt_expr(block, &this.thir[value], statement_scope)
  28                 })
  29             }
  30             ExprKind::Assign { lhs, rhs } => {
  31                 let lhs = &this.thir[lhs];
  32                 let rhs = &this.thir[rhs];
  33                 let lhs_span = lhs.span;
  34
  35                 // Note: we evaluate assignments right-to-left. This
  36                 // is better for borrowck interaction with overloaded
  37                 // operators like x[j] = x[i].
  38
  39                 debug!("stmt_expr Assign block_context.push(SubExpr) : {:?}", expr);
  40                 this.block_context.push(BlockFrame::SubExpr);
  41
  42                 // Generate better code for things that don't need to be
  43                 // dropped.
  44                 if lhs.ty.needs_drop(this.tcx, this.param_env) {
  45                     let rhs = unpack!(block = this.as_local_operand(block, rhs));
  46                     let lhs = unpack!(block = this.as_place(block, lhs));
  47                     unpack!(block = this.build_drop_and_replace(block, lhs_span, lhs, rhs));
  48                 } else {
  49                     let rhs = unpack!(block = this.as_local_rvalue(block, rhs));
  50                     let lhs = unpack!(block = this.as_place(block, lhs));
  51                     this.cfg.push_assign(block, source_info, lhs, rhs);
  52                 }
  53
  54                 this.block_context.pop();
  55                 block.unit()
  56             }
  57             ExprKind::AssignOp { op, lhs, rhs } => {
  58                 // FIXME(#28160) there is an interesting semantics
  59                 // question raised here -- should we "freeze" the
  60                 // value of the lhs here?  I'm inclined to think not,
  61                 // since it seems closer to the semantics of the
  62                 // overloaded version, which takes `&mut self`.  This
  63                 // only affects weird things like `x += {x += 1; x}`
  64                 // -- is that equal to `x + (x + 1)` or `2*(x+1)`?
  65
  66                 let lhs = &this.thir[lhs];
  67                 let rhs = &this.thir[rhs];
  68                 let lhs_ty = lhs.ty;
  69
  70                 debug!("stmt_expr AssignOp block_context.push(SubExpr) : {:?}", expr);
  71                 this.block_context.push(BlockFrame::SubExpr);
  72
  73                 // As above, RTL.
  74                 let rhs = unpack!(block = this.as_local_operand(block, rhs));
  75                 let lhs = unpack!(block = this.as_place(block, lhs));
  76
  77                 // we don't have to drop prior contents or anything
  78                 // because AssignOp is only legal for Copy types
  79                 // (overloaded ops should be desugared into a call).
  80                 let result = unpack!(
  81                     block =
  82                         this.build_binary_op(block, op, expr_span, lhs_ty, Operand::Copy(lhs), rhs)
  83                 );
  84                 this.cfg.push_assign(block, source_info, lhs, result);
  85
  86                 this.block_context.pop();
  87                 block.unit()
  88             }
  89             ExprKind::Continue { label } => {
  90                 this.break_scope(block, None, BreakableTarget::Continue(label), source_info)
  91             }
  92             ExprKind::Break { label, value } => this.break_scope(
  93                 block,
  94                 value.map(|value| &this.thir[value]),
  95                 BreakableTarget::Break(label),
  96                 source_info,
  97             ),
  98             ExprKind::Return { value } => this.break_scope(
  99                 block,
 100                 value.map(|value| &this.thir[value]),
 101                 BreakableTarget::Return,
 102                 source_info,
 103             ),
 104             ExprKind::LlvmInlineAsm { asm, ref outputs, ref inputs } => {
 105                 debug!("stmt_expr LlvmInlineAsm block_context.push(SubExpr) : {:?}", expr);
 106                 this.block_context.push(BlockFrame::SubExpr);
 107                 let outputs = outputs
 108                     .into_iter()
 109                     .copied()
 110                     .map(|output| unpack!(block = this.as_place(block, &this.thir[output])))
 111                     .collect::<Vec<_>>()
 112                     .into_boxed_slice();
 113                 let inputs = inputs
 114                     .into_iter()
 115                     .copied()
 116                     .map(|input| {
 117                         let input = &this.thir[input];
 118                         (input.span, unpack!(block = this.as_local_operand(block, &input)))
 119                     })
 120                     .collect::<Vec<_>>()
 121                     .into_boxed_slice();
 122                 this.cfg.push(
 123                     block,
 124                     Statement {
 125                         source_info,
 126                         kind: StatementKind::LlvmInlineAsm(Box::new(LlvmInlineAsm {
 127                             asm: asm.clone(),
 128                             outputs,
 129                             inputs,
 130                         })),
 131                     },
 132                 );
 133                 this.block_context.pop();
 134                 block.unit()
 135             }
 136             _ => {
 137                 assert!(
 138                     statement_scope.is_some(),
 139                     "Should not be calling `stmt_expr` on a general expression \
 140                      without a statement scope",
 141                 );
 142
 143                 // Issue #54382: When creating temp for the value of
 144                 // expression like:
 145                 //
 146                 // `{ side_effects(); { let l = stuff(); the_value } }`
 147                 //
 148                 // it is usually better to focus on `the_value` rather
 149                 // than the entirety of block(s) surrounding it.
 150                 let adjusted_span = (|| {
 151                     if let ExprKind::Block { body } = &expr.kind {
 152                         if let Some(tail_expr) = body.expr {
 153                             let mut expr = &this.thir[tail_expr];
 154                             while let ExprKind::Block {
 155                                 body: Block { expr: Some(nested_expr), .. },
 156                             }
 157                             | ExprKind::Scope { value: nested_expr, .. } = expr.kind
 158                             {
 159                                 expr = &this.thir[nested_expr];
 160                             }
 161                             this.block_context.push(BlockFrame::TailExpr {
 162                                 tail_result_is_ignored: true,
 163                                 span: expr.span,
 164                             });
 165                             return Some(expr.span);
 166                         }
 167                     }
 168                     None
 169                 })();
 170
 171                 let temp =
 172                     unpack!(block = this.as_temp(block, statement_scope, expr, Mutability::Not));
 173
 174                 if let Some(span) = adjusted_span {
 175                     this.local_decls[temp].source_info.span = span;
 176                     this.block_context.pop();
 177                 }
 178
 179                 block.unit()
 180             }
 181         }
 182     }
 183 }