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