src/librustc_mir/build/expr/as_temp.rs

   1 //! See docs in build/expr/mod.rs
   2
   3 use crate::build::{BlockAnd, BlockAndExtension, Builder};
   4 use crate::build::scope::DropKind;
   5 use crate::hair::*;
   6 use rustc::hir;
   7 use rustc::middle::region;
   8 use rustc::mir::*;
   9
  10 impl<'a, 'tcx> Builder<'a, 'tcx> {
  11     /// Compile `expr` into a fresh temporary. This is used when building
  12     /// up rvalues so as to freeze the value that will be consumed.
  13     pub fn as_temp<M>(
  14         &mut self,
  15         block: BasicBlock,
  16         temp_lifetime: Option<region::Scope>,
  17         expr: M,
  18         mutability: Mutability,
  19     ) -> BlockAnd<Local>
  20     where
  21         M: Mirror<'tcx, Output = Expr<'tcx>>,
  22     {
  23         let expr = self.hir.mirror(expr);
  24         self.expr_as_temp(block, temp_lifetime, expr, mutability)
  25     }
  26
  27     fn expr_as_temp(
  28         &mut self,
  29         mut block: BasicBlock,
  30         temp_lifetime: Option<region::Scope>,
  31         expr: Expr<'tcx>,
  32         mutability: Mutability,
  33     ) -> BlockAnd<Local> {
  34         debug!(
  35             "expr_as_temp(block={:?}, temp_lifetime={:?}, expr={:?}, mutability={:?})",
  36             block, temp_lifetime, expr, mutability
  37         );
  38         let this = self;
  39
  40         let expr_span = expr.span;
  41         let source_info = this.source_info(expr_span);
  42         if let ExprKind::Scope {
  43             region_scope,
  44             lint_level,
  45             value,
  46         } = expr.kind
  47         {
  48             return this.in_scope((region_scope, source_info), lint_level, |this| {
  49                 this.as_temp(block, temp_lifetime, value, mutability)
  50             });
  51         }
  52
  53         let expr_ty = expr.ty;
  54         let temp = {
  55             let mut local_decl = LocalDecl::new_temp(expr_ty, expr_span);
  56             if mutability == Mutability::Not {
  57                 local_decl = local_decl.immutable();
  58             }
  59
  60             debug!("creating temp {:?} with block_context: {:?}", local_decl, this.block_context);
  61             // Find out whether this temp is being created within the
  62             // tail expression of a block whose result is ignored.
  63             if let Some(tail_info) = this.block_context.currently_in_block_tail() {
  64                 local_decl = local_decl.block_tail(tail_info);
  65             }
  66             this.local_decls.push(local_decl)
  67         };
  68         let temp_place = &Place::from(temp);
  69
  70         match expr.kind {
  71             // Don't bother with StorageLive and Dead for these temporaries,
  72             // they are never assigned.
  73             ExprKind::Break { .. } |
  74             ExprKind::Continue { .. } |
  75             ExprKind::Return { .. } => (),
  76             ExprKind::Block {
  77                 body: hir::Block { expr: None, targeted_by_break: false, .. }
  78             } if expr_ty.is_never() => (),
  79             _ => {
  80                 this.cfg.push(
  81                     block,
  82                     Statement {
  83                         source_info,
  84                         kind: StatementKind::StorageLive(temp),
  85                     },
  86                 );
  87
  88                 // In constants, `temp_lifetime` is `None` for temporaries that
  89                 // live for the `'static` lifetime. Thus we do not drop these
  90                 // temporaries and simply leak them.
  91                 // This is equivalent to what `let x = &foo();` does in
  92                 // functions. The temporary is lifted to their surrounding
  93                 // scope. In a function that means the temporary lives until
  94                 // just before the function returns. In constants that means it
  95                 // outlives the constant's initialization value computation.
  96                 // Anything outliving a constant must have the `'static`
  97                 // lifetime and live forever.
  98                 // Anything with a shorter lifetime (e.g the `&foo()` in
  99                 // `bar(&foo())` or anything within a block will keep the
 100                 // regular drops just like runtime code.
 101                 if let Some(temp_lifetime) = temp_lifetime {
 102                     this.schedule_drop(
 103                         expr_span,
 104                         temp_lifetime,
 105                         temp,
 106                         DropKind::Storage,
 107                     );
 108                 }
 109             }
 110         }
 111
 112         unpack!(block = this.into(temp_place, block, expr));
 113
 114         if let Some(temp_lifetime) = temp_lifetime {
 115             this.schedule_drop(
 116                 expr_span,
 117                 temp_lifetime,
 118                 temp,
 119                 DropKind::Value,
 120             );
 121         }
 122
 123         block.and(temp)
 124     }
 125 }