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     pub(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             _ => {
 105                 assert!(
 106                     statement_scope.is_some(),
 107                     "Should not be calling `stmt_expr` on a general expression \
 108                      without a statement scope",
 109                 );
 110
 111                 // Issue #54382: When creating temp for the value of
 112                 // expression like:
 113                 //
 114                 // `{ side_effects(); { let l = stuff(); the_value } }`
 115                 //
 116                 // it is usually better to focus on `the_value` rather
 117                 // than the entirety of block(s) surrounding it.
 118                 let adjusted_span = (|| {
 119                     if let ExprKind::Block { body } = &expr.kind && let Some(tail_ex) = body.expr {
 120                         let mut expr = &this.thir[tail_ex];
 121                         while let ExprKind::Block {
 122                             body: Block { expr: Some(nested_expr), .. },
 123                         }
 124                         | ExprKind::Scope { value: nested_expr, .. } = expr.kind
 125                         {
 126                             expr = &this.thir[nested_expr];
 127                         }
 128                         this.block_context.push(BlockFrame::TailExpr {
 129                             tail_result_is_ignored: true,
 130                             span: expr.span,
 131                         });
 132                         return Some(expr.span);
 133                     }
 134                     None
 135                 })();
 136
 137                 let temp =
 138                     unpack!(block = this.as_temp(block, statement_scope, expr, Mutability::Not));
 139
 140                 if let Some(span) = adjusted_span {
 141                     this.local_decls[temp].source_info.span = span;
 142                     this.block_context.pop();
 143                 }
 144
 145                 block.unit()
 146             }
 147         }
 148     }
 149 }