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

//! See docs in build/expr/mod.rs

use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::HirId;
use rustc_middle::middle::region::{Scope, ScopeData, TempLifetime};
use rustc_middle::mir::*;
use rustc_middle::thir::*;
use tracing::{debug, instrument};

use crate::builder::scope::{DropKind, LintLevel};
use crate::builder::{BlockAnd, BlockAndExtension, Builder};

impl<'a, 'tcx> Builder<'a, 'tcx> {
    /// Compile `expr` into a fresh temporary. This is used when building
    /// up rvalues so as to freeze the value that will be consumed.
    pub(crate) fn as_temp(
        &mut self,
        block: BasicBlock,
        temp_lifetime: TempLifetime,
        expr_id: ExprId,
        mutability: Mutability,
    ) -> BlockAnd<Local> {
        // this is the only place in mir building that we need to truly need to worry about
        // infinite recursion. Everything else does recurse, too, but it always gets broken up
        // at some point by inserting an intermediate temporary
        ensure_sufficient_stack(|| self.as_temp_inner(block, temp_lifetime, expr_id, mutability))
    }

    #[instrument(skip(self), level = "debug")]
    fn as_temp_inner(
        &mut self,
        mut block: BasicBlock,
        temp_lifetime: TempLifetime,
        expr_id: ExprId,
        mutability: Mutability,
    ) -> BlockAnd<Local> {
        let this = self; // See "LET_THIS_SELF".

        let expr = &this.thir[expr_id];
        let expr_span = expr.span;
        let source_info = this.source_info(expr_span);
        if let ExprKind::Scope { region_scope, hir_id, value } = expr.kind {
            return this.in_scope(
                (region_scope, source_info),
                LintLevel::Explicit(hir_id),
                |this| this.as_temp(block, temp_lifetime, value, mutability),
            );
        }

        let expr_ty = expr.ty;
        let deduplicate_temps = this.fixed_temps_scope.is_some()
            && this.fixed_temps_scope == temp_lifetime.temp_lifetime;
        let temp = if deduplicate_temps && let Some(temp_index) = this.fixed_temps.get(&expr_id) {
            *temp_index
        } else {
            let mut local_decl = LocalDecl::new(expr_ty, expr_span);
            if mutability.is_not() {
                local_decl = local_decl.immutable();
            }

            debug!("creating temp {:?} with block_context: {:?}", local_decl, this.block_context);
            let local_info = match expr.kind {
                ExprKind::StaticRef { def_id, .. } => {
                    assert!(!this.tcx.is_thread_local_static(def_id));
                    LocalInfo::StaticRef { def_id, is_thread_local: false }
                }
                ExprKind::ThreadLocalRef(def_id) => {
                    assert!(this.tcx.is_thread_local_static(def_id));
                    LocalInfo::StaticRef { def_id, is_thread_local: true }
                }
                ExprKind::NamedConst { def_id, .. } | ExprKind::ConstParam { def_id, .. } => {
                    LocalInfo::ConstRef { def_id }
                }
                // Find out whether this temp is being created within the
                // tail expression of a block whose result is ignored.
                _ if let Some(tail_info) = this.block_context.currently_in_block_tail() => {
                    LocalInfo::BlockTailTemp(tail_info)
                }

                _ if let Some(Scope { data: ScopeData::IfThenRescope, local_id }) =
                    temp_lifetime.temp_lifetime =>
                {
                    LocalInfo::IfThenRescopeTemp {
                        if_then: HirId { owner: this.hir_id.owner, local_id },
                    }
                }

                _ => LocalInfo::Boring,
            };
            **local_decl.local_info.as_mut().unwrap_crate_local() = local_info;
            this.local_decls.push(local_decl)
        };
        debug!(?temp);
        if deduplicate_temps {
            this.fixed_temps.insert(expr_id, temp);
        }
        let temp_place = Place::from(temp);

        match expr.kind {
            // Don't bother with StorageLive and Dead for these temporaries,
            // they are never assigned.
            ExprKind::Break { .. } | ExprKind::Continue { .. } | ExprKind::Return { .. } => (),
            ExprKind::Block { block }
                if let Block { expr: None, targeted_by_break: false, .. } = this.thir[block]
                    && expr_ty.is_never() => {}
            _ => {
                this.cfg.push(block, Statement::new(source_info, StatementKind::StorageLive(temp)));

                // In constants, `temp_lifetime` is `None` for temporaries that
                // live for the `'static` lifetime. Thus we do not drop these
                // temporaries and simply leak them.
                // This is equivalent to what `let x = &foo();` does in
                // functions. The temporary is lifted to their surrounding
                // scope. In a function that means the temporary lives until
                // just before the function returns. In constants that means it
                // outlives the constant's initialization value computation.
                // Anything outliving a constant must have the `'static`
                // lifetime and live forever.
                // Anything with a shorter lifetime (e.g the `&foo()` in
                // `bar(&foo())` or anything within a block will keep the
                // regular drops just like runtime code.
                if let Some(temp_lifetime) = temp_lifetime.temp_lifetime {
                    this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Storage);
                }
            }
        }

        block = this.expr_into_dest(temp_place, block, expr_id).into_block();

        if let Some(temp_lifetime) = temp_lifetime.temp_lifetime {
            this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Value);
        }

        if let Some(backwards_incompatible) = temp_lifetime.backwards_incompatible {
            this.schedule_backwards_incompatible_drop(expr_span, backwards_incompatible, temp);
        }

        block.and(temp)
    }
}