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

   1 //! See docs in build/expr/mod.rs
   2
   3 use crate::build::scope::DropKind;
   4 use crate::build::{BlockAnd, BlockAndExtension, Builder};
   5 use rustc_data_structures::stack::ensure_sufficient_stack;
   6 use rustc_middle::middle::region;
   7 use rustc_middle::mir::*;
   8 use rustc_middle::thir::*;
   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     crate fn as_temp(
  14         &mut self,
  15         block: BasicBlock,
  16         temp_lifetime: Option<region::Scope>,
  17         expr: &Expr<'tcx>,
  18         mutability: Mutability,
  19     ) -> BlockAnd<Local> {
  20         // this is the only place in mir building that we need to truly need to worry about
  21         // infinite recursion. Everything else does recurse, too, but it always gets broken up
  22         // at some point by inserting an intermediate temporary
  23         ensure_sufficient_stack(|| self.as_temp_inner(block, temp_lifetime, expr, mutability))
  24     }
  25
  26     fn as_temp_inner(
  27         &mut self,
  28         mut block: BasicBlock,
  29         temp_lifetime: Option<region::Scope>,
  30         expr: &Expr<'tcx>,
  31         mutability: Mutability,
  32     ) -> BlockAnd<Local> {
  33         debug!(
  34             "as_temp(block={:?}, temp_lifetime={:?}, expr={:?}, mutability={:?})",
  35             block, temp_lifetime, expr, mutability
  36         );
  37         let this = self;
  38
  39         let expr_span = expr.span;
  40         let source_info = this.source_info(expr_span);
  41         if let ExprKind::Scope { region_scope, lint_level, value } = expr.kind {
  42             return this.in_scope((region_scope, source_info), lint_level, |this| {
  43                 this.as_temp(block, temp_lifetime, &this.thir[value], mutability)
  44             });
  45         }
  46
  47         let expr_ty = expr.ty;
  48         let temp = {
  49             let mut local_decl = LocalDecl::new(expr_ty, expr_span);
  50             if mutability == Mutability::Not {
  51                 local_decl = local_decl.immutable();
  52             }
  53
  54             debug!("creating temp {:?} with block_context: {:?}", local_decl, this.block_context);
  55             // Find out whether this temp is being created within the
  56             // tail expression of a block whose result is ignored.
  57             if let Some(tail_info) = this.block_context.currently_in_block_tail() {
  58                 local_decl = local_decl.block_tail(tail_info);
  59             }
  60             match expr.kind {
  61                 ExprKind::StaticRef { def_id, .. } => {
  62                     assert!(!this.tcx.is_thread_local_static(def_id));
  63                     local_decl.internal = true;
  64                     local_decl.local_info =
  65                         Some(Box::new(LocalInfo::StaticRef { def_id, is_thread_local: false }));
  66                 }
  67                 ExprKind::ThreadLocalRef(def_id) => {
  68                     assert!(this.tcx.is_thread_local_static(def_id));
  69                     local_decl.internal = true;
  70                     local_decl.local_info =
  71                         Some(Box::new(LocalInfo::StaticRef { def_id, is_thread_local: true }));
  72                 }
  73                 ExprKind::NamedConst { def_id, .. } | ExprKind::ConstParam { def_id, .. } => {
  74                     local_decl.local_info = Some(Box::new(LocalInfo::ConstRef { def_id }));
  75                 }
  76                 _ => {}
  77             }
  78             this.local_decls.push(local_decl)
  79         };
  80         let temp_place = Place::from(temp);
  81
  82         match expr.kind {
  83             // Don't bother with StorageLive and Dead for these temporaries,
  84             // they are never assigned.
  85             ExprKind::Break { .. } | ExprKind::Continue { .. } | ExprKind::Return { .. } => (),
  86             ExprKind::Block { body: Block { expr: None, targeted_by_break: false, .. } }
  87                 if expr_ty.is_never() => {}
  88             _ => {
  89                 this.cfg
  90                     .push(block, Statement { source_info, kind: StatementKind::StorageLive(temp) });
  91
  92                 // In constants, `temp_lifetime` is `None` for temporaries that
  93                 // live for the `'static` lifetime. Thus we do not drop these
  94                 // temporaries and simply leak them.
  95                 // This is equivalent to what `let x = &foo();` does in
  96                 // functions. The temporary is lifted to their surrounding
  97                 // scope. In a function that means the temporary lives until
  98                 // just before the function returns. In constants that means it
  99                 // outlives the constant's initialization value computation.
 100                 // Anything outliving a constant must have the `'static`
 101                 // lifetime and live forever.
 102                 // Anything with a shorter lifetime (e.g the `&foo()` in
 103                 // `bar(&foo())` or anything within a block will keep the
 104                 // regular drops just like runtime code.
 105                 if let Some(temp_lifetime) = temp_lifetime {
 106                     this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Storage);
 107                 }
 108             }
 109         }
 110
 111         unpack!(block = this.expr_into_dest(temp_place, block, expr));
 112
 113         if let Some(temp_lifetime) = temp_lifetime {
 114             this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Value);
 115         }
 116
 117         block.and(temp)
 118     }
 119 }