From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 17 Apr 2024 14:02:58 +0200
Subject: Adding upstream version 1.64.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 compiler/rustc_ast_lowering/src/expr.rs | 1914 +++++++++++++++++++++++++++++++
 1 file changed, 1914 insertions(+)
 create mode 100644 compiler/rustc_ast_lowering/src/expr.rs

(limited to 'compiler/rustc_ast_lowering/src/expr.rs')

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
new file mode 100644
index 000000000..fb6715ff1
--- /dev/null
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -0,0 +1,1914 @@
+use super::ResolverAstLoweringExt;
+use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
+use crate::{FnDeclKind, ImplTraitPosition};
+
+use rustc_ast::attr;
+use rustc_ast::ptr::P as AstP;
+use rustc_ast::*;
+use rustc_data_structures::stack::ensure_sufficient_stack;
+use rustc_data_structures::thin_vec::ThinVec;
+use rustc_errors::struct_span_err;
+use rustc_hir as hir;
+use rustc_hir::def::Res;
+use rustc_hir::definitions::DefPathData;
+use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
+use rustc_span::symbol::{sym, Ident};
+use rustc_span::DUMMY_SP;
+
+impl<'hir> LoweringContext<'_, 'hir> {
+    fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
+        self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
+    }
+
+    pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.lower_expr_mut(e))
+    }
+
+    pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
+        ensure_sufficient_stack(|| {
+            let kind = match e.kind {
+                ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
+                ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
+                ExprKind::ConstBlock(ref anon_const) => {
+                    let anon_const = self.lower_anon_const(anon_const);
+                    hir::ExprKind::ConstBlock(anon_const)
+                }
+                ExprKind::Repeat(ref expr, ref count) => {
+                    let expr = self.lower_expr(expr);
+                    let count = self.lower_array_length(count);
+                    hir::ExprKind::Repeat(expr, count)
+                }
+                ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
+                ExprKind::Call(ref f, ref args) => {
+                    if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
+                        if let [inner] = &args[..] && e.attrs.len() == 1 {
+                            let kind = hir::ExprKind::Box(self.lower_expr(&inner));
+                            let hir_id = self.lower_node_id(e.id);
+                            return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
+                        } else {
+                            self.tcx.sess
+                                .struct_span_err(
+                                    e.span,
+                                    "#[rustc_box] requires precisely one argument \
+                                    and no other attributes are allowed",
+                                )
+                                .emit();
+                            hir::ExprKind::Err
+                        }
+                    } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
+                        self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
+                    } else {
+                        let f = self.lower_expr(f);
+                        hir::ExprKind::Call(f, self.lower_exprs(args))
+                    }
+                }
+                ExprKind::MethodCall(ref seg, ref args, span) => {
+                    let hir_seg = self.arena.alloc(self.lower_path_segment(
+                        e.span,
+                        seg,
+                        ParamMode::Optional,
+                        ParenthesizedGenericArgs::Err,
+                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    ));
+                    let args = self.lower_exprs(args);
+                    hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
+                }
+                ExprKind::Binary(binop, ref lhs, ref rhs) => {
+                    let binop = self.lower_binop(binop);
+                    let lhs = self.lower_expr(lhs);
+                    let rhs = self.lower_expr(rhs);
+                    hir::ExprKind::Binary(binop, lhs, rhs)
+                }
+                ExprKind::Unary(op, ref ohs) => {
+                    let op = self.lower_unop(op);
+                    let ohs = self.lower_expr(ohs);
+                    hir::ExprKind::Unary(op, ohs)
+                }
+                ExprKind::Lit(ref l) => {
+                    hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
+                }
+                ExprKind::Cast(ref expr, ref ty) => {
+                    let expr = self.lower_expr(expr);
+                    let ty =
+                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    hir::ExprKind::Cast(expr, ty)
+                }
+                ExprKind::Type(ref expr, ref ty) => {
+                    let expr = self.lower_expr(expr);
+                    let ty =
+                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    hir::ExprKind::Type(expr, ty)
+                }
+                ExprKind::AddrOf(k, m, ref ohs) => {
+                    let ohs = self.lower_expr(ohs);
+                    hir::ExprKind::AddrOf(k, m, ohs)
+                }
+                ExprKind::Let(ref pat, ref scrutinee, span) => {
+                    hir::ExprKind::Let(self.arena.alloc(hir::Let {
+                        hir_id: self.next_id(),
+                        span: self.lower_span(span),
+                        pat: self.lower_pat(pat),
+                        ty: None,
+                        init: self.lower_expr(scrutinee),
+                    }))
+                }
+                ExprKind::If(ref cond, ref then, ref else_opt) => {
+                    self.lower_expr_if(cond, then, else_opt.as_deref())
+                }
+                ExprKind::While(ref cond, ref body, opt_label) => {
+                    self.with_loop_scope(e.id, |this| {
+                        let span =
+                            this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
+                        this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
+                    })
+                }
+                ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
+                    hir::ExprKind::Loop(
+                        this.lower_block(body, false),
+                        this.lower_label(opt_label),
+                        hir::LoopSource::Loop,
+                        DUMMY_SP,
+                    )
+                }),
+                ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
+                ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
+                    self.lower_expr(expr),
+                    self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
+                    hir::MatchSource::Normal,
+                ),
+                ExprKind::Async(capture_clause, closure_node_id, ref block) => self
+                    .make_async_expr(
+                        capture_clause,
+                        closure_node_id,
+                        None,
+                        block.span,
+                        hir::AsyncGeneratorKind::Block,
+                        |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
+                    ),
+                ExprKind::Await(ref expr) => {
+                    let span = if expr.span.hi() < e.span.hi() {
+                        expr.span.shrink_to_hi().with_hi(e.span.hi())
+                    } else {
+                        // this is a recovered `await expr`
+                        e.span
+                    };
+                    self.lower_expr_await(span, expr)
+                }
+                ExprKind::Closure(
+                    ref binder,
+                    capture_clause,
+                    asyncness,
+                    movability,
+                    ref decl,
+                    ref body,
+                    fn_decl_span,
+                ) => {
+                    if let Async::Yes { closure_id, .. } = asyncness {
+                        self.lower_expr_async_closure(
+                            binder,
+                            capture_clause,
+                            e.id,
+                            closure_id,
+                            decl,
+                            body,
+                            fn_decl_span,
+                        )
+                    } else {
+                        self.lower_expr_closure(
+                            binder,
+                            capture_clause,
+                            e.id,
+                            movability,
+                            decl,
+                            body,
+                            fn_decl_span,
+                        )
+                    }
+                }
+                ExprKind::Block(ref blk, opt_label) => {
+                    let opt_label = self.lower_label(opt_label);
+                    hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
+                }
+                ExprKind::Assign(ref el, ref er, span) => {
+                    self.lower_expr_assign(el, er, span, e.span)
+                }
+                ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
+                    self.lower_binop(op),
+                    self.lower_expr(el),
+                    self.lower_expr(er),
+                ),
+                ExprKind::Field(ref el, ident) => {
+                    hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
+                }
+                ExprKind::Index(ref el, ref er) => {
+                    hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
+                }
+                ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
+                    self.lower_expr_range_closed(e.span, e1, e2)
+                }
+                ExprKind::Range(ref e1, ref e2, lims) => {
+                    self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
+                }
+                ExprKind::Underscore => {
+                    self.tcx
+                        .sess.struct_span_err(
+                            e.span,
+                            "in expressions, `_` can only be used on the left-hand side of an assignment",
+                        )
+                        .span_label(e.span, "`_` not allowed here")
+                        .emit();
+                    hir::ExprKind::Err
+                }
+                ExprKind::Path(ref qself, ref path) => {
+                    let qpath = self.lower_qpath(
+                        e.id,
+                        qself,
+                        path,
+                        ParamMode::Optional,
+                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    );
+                    hir::ExprKind::Path(qpath)
+                }
+                ExprKind::Break(opt_label, ref opt_expr) => {
+                    let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
+                    hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
+                }
+                ExprKind::Continue(opt_label) => {
+                    hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
+                }
+                ExprKind::Ret(ref e) => {
+                    let e = e.as_ref().map(|x| self.lower_expr(x));
+                    hir::ExprKind::Ret(e)
+                }
+                ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
+                ExprKind::InlineAsm(ref asm) => {
+                    hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
+                }
+                ExprKind::Struct(ref se) => {
+                    let rest = match &se.rest {
+                        StructRest::Base(e) => Some(self.lower_expr(e)),
+                        StructRest::Rest(sp) => {
+                            self.tcx
+                                .sess
+                                .struct_span_err(*sp, "base expression required after `..`")
+                                .span_label(*sp, "add a base expression here")
+                                .emit();
+                            Some(&*self.arena.alloc(self.expr_err(*sp)))
+                        }
+                        StructRest::None => None,
+                    };
+                    hir::ExprKind::Struct(
+                        self.arena.alloc(self.lower_qpath(
+                            e.id,
+                            &se.qself,
+                            &se.path,
+                            ParamMode::Optional,
+                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        )),
+                        self.arena
+                            .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
+                        rest,
+                    )
+                }
+                ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
+                ExprKind::Err => hir::ExprKind::Err,
+                ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
+                ExprKind::Paren(ref ex) => {
+                    let mut ex = self.lower_expr_mut(ex);
+                    // Include parens in span, but only if it is a super-span.
+                    if e.span.contains(ex.span) {
+                        ex.span = self.lower_span(e.span);
+                    }
+                    // Merge attributes into the inner expression.
+                    if !e.attrs.is_empty() {
+                        let old_attrs =
+                            self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
+                        self.attrs.insert(
+                            ex.hir_id.local_id,
+                            &*self.arena.alloc_from_iter(
+                                e.attrs
+                                    .iter()
+                                    .map(|a| self.lower_attr(a))
+                                    .chain(old_attrs.iter().cloned()),
+                            ),
+                        );
+                    }
+                    return ex;
+                }
+
+                // Desugar `ExprForLoop`
+                // from: `[opt_ident]: for <pat> in <head> <body>`
+                ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
+                    return self.lower_expr_for(e, pat, head, body, opt_label);
+                }
+                ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
+            };
+
+            let hir_id = self.lower_node_id(e.id);
+            self.lower_attrs(hir_id, &e.attrs);
+            hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
+        })
+    }
+
+    fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
+        match u {
+            UnOp::Deref => hir::UnOp::Deref,
+            UnOp::Not => hir::UnOp::Not,
+            UnOp::Neg => hir::UnOp::Neg,
+        }
+    }
+
+    fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
+        Spanned {
+            node: match b.node {
+                BinOpKind::Add => hir::BinOpKind::Add,
+                BinOpKind::Sub => hir::BinOpKind::Sub,
+                BinOpKind::Mul => hir::BinOpKind::Mul,
+                BinOpKind::Div => hir::BinOpKind::Div,
+                BinOpKind::Rem => hir::BinOpKind::Rem,
+                BinOpKind::And => hir::BinOpKind::And,
+                BinOpKind::Or => hir::BinOpKind::Or,
+                BinOpKind::BitXor => hir::BinOpKind::BitXor,
+                BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
+                BinOpKind::BitOr => hir::BinOpKind::BitOr,
+                BinOpKind::Shl => hir::BinOpKind::Shl,
+                BinOpKind::Shr => hir::BinOpKind::Shr,
+                BinOpKind::Eq => hir::BinOpKind::Eq,
+                BinOpKind::Lt => hir::BinOpKind::Lt,
+                BinOpKind::Le => hir::BinOpKind::Le,
+                BinOpKind::Ne => hir::BinOpKind::Ne,
+                BinOpKind::Ge => hir::BinOpKind::Ge,
+                BinOpKind::Gt => hir::BinOpKind::Gt,
+            },
+            span: self.lower_span(b.span),
+        }
+    }
+
+    fn lower_legacy_const_generics(
+        &mut self,
+        mut f: Expr,
+        args: Vec<AstP<Expr>>,
+        legacy_args_idx: &[usize],
+    ) -> hir::ExprKind<'hir> {
+        let ExprKind::Path(None, ref mut path) = f.kind else {
+            unreachable!();
+        };
+
+        // Split the arguments into const generics and normal arguments
+        let mut real_args = vec![];
+        let mut generic_args = vec![];
+        for (idx, arg) in args.into_iter().enumerate() {
+            if legacy_args_idx.contains(&idx) {
+                let parent_def_id = self.current_hir_id_owner;
+                let node_id = self.next_node_id();
+
+                // Add a definition for the in-band const def.
+                self.create_def(parent_def_id, node_id, DefPathData::AnonConst);
+
+                let anon_const = AnonConst { id: node_id, value: arg };
+                generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
+            } else {
+                real_args.push(arg);
+            }
+        }
+
+        // Add generic args to the last element of the path.
+        let last_segment = path.segments.last_mut().unwrap();
+        assert!(last_segment.args.is_none());
+        last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
+            span: DUMMY_SP,
+            args: generic_args,
+        })));
+
+        // Now lower everything as normal.
+        let f = self.lower_expr(&f);
+        hir::ExprKind::Call(f, self.lower_exprs(&real_args))
+    }
+
+    fn lower_expr_if(
+        &mut self,
+        cond: &Expr,
+        then: &Block,
+        else_opt: Option<&Expr>,
+    ) -> hir::ExprKind<'hir> {
+        let lowered_cond = self.lower_expr(cond);
+        let new_cond = self.manage_let_cond(lowered_cond);
+        let then_expr = self.lower_block_expr(then);
+        if let Some(rslt) = else_opt {
+            hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
+        } else {
+            hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
+        }
+    }
+
+    // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
+    // in a temporary block.
+    fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
+        fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
+            match expr.kind {
+                hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
+                hir::ExprKind::Let(..) => true,
+                _ => false,
+            }
+        }
+        if has_let_expr(cond) {
+            cond
+        } else {
+            let reason = DesugaringKind::CondTemporary;
+            let span_block = self.mark_span_with_reason(reason, cond.span, None);
+            self.expr_drop_temps(span_block, cond, AttrVec::new())
+        }
+    }
+
+    // We desugar: `'label: while $cond $body` into:
+    //
+    // ```
+    // 'label: loop {
+    //   if { let _t = $cond; _t } {
+    //     $body
+    //   }
+    //   else {
+    //     break;
+    //   }
+    // }
+    // ```
+    //
+    // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+    // to preserve drop semantics since `while $cond { ... }` does not
+    // let temporaries live outside of `cond`.
+    fn lower_expr_while_in_loop_scope(
+        &mut self,
+        span: Span,
+        cond: &Expr,
+        body: &Block,
+        opt_label: Option<Label>,
+    ) -> hir::ExprKind<'hir> {
+        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
+        let new_cond = self.manage_let_cond(lowered_cond);
+        let then = self.lower_block_expr(body);
+        let expr_break = self.expr_break(span, ThinVec::new());
+        let stmt_break = self.stmt_expr(span, expr_break);
+        let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
+        let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
+        let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
+        let if_expr = self.expr(span, if_kind, ThinVec::new());
+        let block = self.block_expr(self.arena.alloc(if_expr));
+        let span = self.lower_span(span.with_hi(cond.span.hi()));
+        let opt_label = self.lower_label(opt_label);
+        hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
+    }
+
+    /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
+    /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
+    /// and save the block id to use it as a break target for desugaring of the `?` operator.
+    fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
+        self.with_catch_scope(body.id, |this| {
+            let mut block = this.lower_block_noalloc(body, true);
+
+            // Final expression of the block (if present) or `()` with span at the end of block
+            let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
+                (
+                    this.mark_span_with_reason(
+                        DesugaringKind::TryBlock,
+                        expr.span,
+                        this.allow_try_trait.clone(),
+                    ),
+                    expr,
+                )
+            } else {
+                let try_span = this.mark_span_with_reason(
+                    DesugaringKind::TryBlock,
+                    this.tcx.sess.source_map().end_point(body.span),
+                    this.allow_try_trait.clone(),
+                );
+
+                (try_span, this.expr_unit(try_span))
+            };
+
+            let ok_wrapped_span =
+                this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
+
+            // `::std::ops::Try::from_output($tail_expr)`
+            block.expr = Some(this.wrap_in_try_constructor(
+                hir::LangItem::TryTraitFromOutput,
+                try_span,
+                tail_expr,
+                ok_wrapped_span,
+            ));
+
+            hir::ExprKind::Block(this.arena.alloc(block), None)
+        })
+    }
+
+    fn wrap_in_try_constructor(
+        &mut self,
+        lang_item: hir::LangItem,
+        method_span: Span,
+        expr: &'hir hir::Expr<'hir>,
+        overall_span: Span,
+    ) -> &'hir hir::Expr<'hir> {
+        let constructor = self.arena.alloc(self.expr_lang_item_path(
+            method_span,
+            lang_item,
+            ThinVec::new(),
+            None,
+        ));
+        self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
+    }
+
+    fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
+        let pat = self.lower_pat(&arm.pat);
+        let guard = arm.guard.as_ref().map(|cond| {
+            if let ExprKind::Let(ref pat, ref scrutinee, span) = cond.kind {
+                hir::Guard::IfLet(self.arena.alloc(hir::Let {
+                    hir_id: self.next_id(),
+                    span: self.lower_span(span),
+                    pat: self.lower_pat(pat),
+                    ty: None,
+                    init: self.lower_expr(scrutinee),
+                }))
+            } else {
+                hir::Guard::If(self.lower_expr(cond))
+            }
+        });
+        let hir_id = self.next_id();
+        self.lower_attrs(hir_id, &arm.attrs);
+        hir::Arm {
+            hir_id,
+            pat,
+            guard,
+            body: self.lower_expr(&arm.body),
+            span: self.lower_span(arm.span),
+        }
+    }
+
+    /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
+    ///
+    /// This results in:
+    ///
+    /// ```text
+    /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
+    ///     <body>
+    /// })
+    /// ```
+    pub(super) fn make_async_expr(
+        &mut self,
+        capture_clause: CaptureBy,
+        closure_node_id: NodeId,
+        ret_ty: Option<AstP<Ty>>,
+        span: Span,
+        async_gen_kind: hir::AsyncGeneratorKind,
+        body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
+    ) -> hir::ExprKind<'hir> {
+        let output = match ret_ty {
+            Some(ty) => hir::FnRetTy::Return(
+                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
+            ),
+            None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
+        };
+
+        // Resume argument type. We let the compiler infer this to simplify the lowering. It is
+        // fully constrained by `future::from_generator`.
+        let input_ty = hir::Ty {
+            hir_id: self.next_id(),
+            kind: hir::TyKind::Infer,
+            span: self.lower_span(span),
+        };
+
+        // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
+        let fn_decl = self.arena.alloc(hir::FnDecl {
+            inputs: arena_vec![self; input_ty],
+            output,
+            c_variadic: false,
+            implicit_self: hir::ImplicitSelfKind::None,
+        });
+
+        // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
+        let (pat, task_context_hid) = self.pat_ident_binding_mode(
+            span,
+            Ident::with_dummy_span(sym::_task_context),
+            hir::BindingAnnotation::Mutable,
+        );
+        let param = hir::Param {
+            hir_id: self.next_id(),
+            pat,
+            ty_span: self.lower_span(span),
+            span: self.lower_span(span),
+        };
+        let params = arena_vec![self; param];
+
+        let body = self.lower_body(move |this| {
+            this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
+
+            let old_ctx = this.task_context;
+            this.task_context = Some(task_context_hid);
+            let res = body(this);
+            this.task_context = old_ctx;
+            (params, res)
+        });
+
+        // `static |_task_context| -> <ret_ty> { body }`:
+        let generator_kind = {
+            let c = self.arena.alloc(hir::Closure {
+                binder: hir::ClosureBinder::Default,
+                capture_clause,
+                bound_generic_params: &[],
+                fn_decl,
+                body,
+                fn_decl_span: self.lower_span(span),
+                movability: Some(hir::Movability::Static),
+            });
+
+            hir::ExprKind::Closure(c)
+        };
+        let generator = hir::Expr {
+            hir_id: self.lower_node_id(closure_node_id),
+            kind: generator_kind,
+            span: self.lower_span(span),
+        };
+
+        // `future::from_generator`:
+        let unstable_span =
+            self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
+        let gen_future = self.expr_lang_item_path(
+            unstable_span,
+            hir::LangItem::FromGenerator,
+            ThinVec::new(),
+            None,
+        );
+
+        // `future::from_generator(generator)`:
+        hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
+    }
+
+    /// Desugar `<expr>.await` into:
+    /// ```ignore (pseudo-rust)
+    /// match ::std::future::IntoFuture::into_future(<expr>) {
+    ///     mut __awaitee => loop {
+    ///         match unsafe { ::std::future::Future::poll(
+    ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
+    ///             ::std::future::get_context(task_context),
+    ///         ) } {
+    ///             ::std::task::Poll::Ready(result) => break result,
+    ///             ::std::task::Poll::Pending => {}
+    ///         }
+    ///         task_context = yield ();
+    ///     }
+    /// }
+    /// ```
+    fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
+        let full_span = expr.span.to(dot_await_span);
+        match self.generator_kind {
+            Some(hir::GeneratorKind::Async(_)) => {}
+            Some(hir::GeneratorKind::Gen) | None => {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    dot_await_span,
+                    E0728,
+                    "`await` is only allowed inside `async` functions and blocks"
+                );
+                err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
+                if let Some(item_sp) = self.current_item {
+                    err.span_label(item_sp, "this is not `async`");
+                }
+                err.emit();
+            }
+        }
+        let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
+        let gen_future_span = self.mark_span_with_reason(
+            DesugaringKind::Await,
+            full_span,
+            self.allow_gen_future.clone(),
+        );
+        let expr = self.lower_expr_mut(expr);
+        let expr_hir_id = expr.hir_id;
+
+        // Note that the name of this binding must not be changed to something else because
+        // debuggers and debugger extensions expect it to be called `__awaitee`. They use
+        // this name to identify what is being awaited by a suspended async functions.
+        let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
+        let (awaitee_pat, awaitee_pat_hid) =
+            self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
+
+        let task_context_ident = Ident::with_dummy_span(sym::_task_context);
+
+        // unsafe {
+        //     ::std::future::Future::poll(
+        //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
+        //         ::std::future::get_context(task_context),
+        //     )
+        // }
+        let poll_expr = {
+            let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
+            let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
+            let task_context = if let Some(task_context_hid) = self.task_context {
+                self.expr_ident_mut(span, task_context_ident, task_context_hid)
+            } else {
+                // Use of `await` outside of an async context, we cannot use `task_context` here.
+                self.expr_err(span)
+            };
+            let new_unchecked = self.expr_call_lang_item_fn_mut(
+                span,
+                hir::LangItem::PinNewUnchecked,
+                arena_vec![self; ref_mut_awaitee],
+                Some(expr_hir_id),
+            );
+            let get_context = self.expr_call_lang_item_fn_mut(
+                gen_future_span,
+                hir::LangItem::GetContext,
+                arena_vec![self; task_context],
+                Some(expr_hir_id),
+            );
+            let call = self.expr_call_lang_item_fn(
+                span,
+                hir::LangItem::FuturePoll,
+                arena_vec![self; new_unchecked, get_context],
+                Some(expr_hir_id),
+            );
+            self.arena.alloc(self.expr_unsafe(call))
+        };
+
+        // `::std::task::Poll::Ready(result) => break result`
+        let loop_node_id = self.next_node_id();
+        let loop_hir_id = self.lower_node_id(loop_node_id);
+        let ready_arm = {
+            let x_ident = Ident::with_dummy_span(sym::result);
+            let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
+            let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
+            let ready_field = self.single_pat_field(gen_future_span, x_pat);
+            let ready_pat = self.pat_lang_item_variant(
+                span,
+                hir::LangItem::PollReady,
+                ready_field,
+                Some(expr_hir_id),
+            );
+            let break_x = self.with_loop_scope(loop_node_id, move |this| {
+                let expr_break =
+                    hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
+                this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
+            });
+            self.arm(ready_pat, break_x)
+        };
+
+        // `::std::task::Poll::Pending => {}`
+        let pending_arm = {
+            let pending_pat = self.pat_lang_item_variant(
+                span,
+                hir::LangItem::PollPending,
+                &[],
+                Some(expr_hir_id),
+            );
+            let empty_block = self.expr_block_empty(span);
+            self.arm(pending_pat, empty_block)
+        };
+
+        let inner_match_stmt = {
+            let match_expr = self.expr_match(
+                span,
+                poll_expr,
+                arena_vec![self; ready_arm, pending_arm],
+                hir::MatchSource::AwaitDesugar,
+            );
+            self.stmt_expr(span, match_expr)
+        };
+
+        // task_context = yield ();
+        let yield_stmt = {
+            let unit = self.expr_unit(span);
+            let yield_expr = self.expr(
+                span,
+                hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
+                ThinVec::new(),
+            );
+            let yield_expr = self.arena.alloc(yield_expr);
+
+            if let Some(task_context_hid) = self.task_context {
+                let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
+                let assign = self.expr(
+                    span,
+                    hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
+                    AttrVec::new(),
+                );
+                self.stmt_expr(span, assign)
+            } else {
+                // Use of `await` outside of an async context. Return `yield_expr` so that we can
+                // proceed with type checking.
+                self.stmt(span, hir::StmtKind::Semi(yield_expr))
+            }
+        };
+
+        let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
+
+        // loop { .. }
+        let loop_expr = self.arena.alloc(hir::Expr {
+            hir_id: loop_hir_id,
+            kind: hir::ExprKind::Loop(
+                loop_block,
+                None,
+                hir::LoopSource::Loop,
+                self.lower_span(span),
+            ),
+            span: self.lower_span(span),
+        });
+
+        // mut __awaitee => loop { ... }
+        let awaitee_arm = self.arm(awaitee_pat, loop_expr);
+
+        // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
+        let into_future_span = self.mark_span_with_reason(
+            DesugaringKind::Await,
+            dot_await_span,
+            self.allow_into_future.clone(),
+        );
+        let into_future_expr = self.expr_call_lang_item_fn(
+            into_future_span,
+            hir::LangItem::IntoFutureIntoFuture,
+            arena_vec![self; expr],
+            Some(expr_hir_id),
+        );
+
+        // match <into_future_expr> {
+        //     mut __awaitee => loop { .. }
+        // }
+        hir::ExprKind::Match(
+            into_future_expr,
+            arena_vec![self; awaitee_arm],
+            hir::MatchSource::AwaitDesugar,
+        )
+    }
+
+    fn lower_expr_closure(
+        &mut self,
+        binder: &ClosureBinder,
+        capture_clause: CaptureBy,
+        closure_id: NodeId,
+        movability: Movability,
+        decl: &FnDecl,
+        body: &Expr,
+        fn_decl_span: Span,
+    ) -> hir::ExprKind<'hir> {
+        let (binder_clause, generic_params) = self.lower_closure_binder(binder);
+
+        let (body_id, generator_option) = self.with_new_scopes(move |this| {
+            let prev = this.current_item;
+            this.current_item = Some(fn_decl_span);
+            let mut generator_kind = None;
+            let body_id = this.lower_fn_body(decl, |this| {
+                let e = this.lower_expr_mut(body);
+                generator_kind = this.generator_kind;
+                e
+            });
+            let generator_option =
+                this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
+            this.current_item = prev;
+            (body_id, generator_option)
+        });
+
+        let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
+        // Lower outside new scope to preserve `is_in_loop_condition`.
+        let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
+
+        let c = self.arena.alloc(hir::Closure {
+            binder: binder_clause,
+            capture_clause,
+            bound_generic_params,
+            fn_decl,
+            body: body_id,
+            fn_decl_span: self.lower_span(fn_decl_span),
+            movability: generator_option,
+        });
+
+        hir::ExprKind::Closure(c)
+    }
+
+    fn generator_movability_for_fn(
+        &mut self,
+        decl: &FnDecl,
+        fn_decl_span: Span,
+        generator_kind: Option<hir::GeneratorKind>,
+        movability: Movability,
+    ) -> Option<hir::Movability> {
+        match generator_kind {
+            Some(hir::GeneratorKind::Gen) => {
+                if decl.inputs.len() > 1 {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        fn_decl_span,
+                        E0628,
+                        "too many parameters for a generator (expected 0 or 1 parameters)"
+                    )
+                    .emit();
+                }
+                Some(movability)
+            }
+            Some(hir::GeneratorKind::Async(_)) => {
+                panic!("non-`async` closure body turned `async` during lowering");
+            }
+            None => {
+                if movability == Movability::Static {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        fn_decl_span,
+                        E0697,
+                        "closures cannot be static"
+                    )
+                    .emit();
+                }
+                None
+            }
+        }
+    }
+
+    fn lower_closure_binder<'c>(
+        &mut self,
+        binder: &'c ClosureBinder,
+    ) -> (hir::ClosureBinder, &'c [GenericParam]) {
+        let (binder, params) = match binder {
+            ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
+            &ClosureBinder::For { span, ref generic_params } => {
+                let span = self.lower_span(span);
+                (hir::ClosureBinder::For { span }, &**generic_params)
+            }
+        };
+
+        (binder, params)
+    }
+
+    fn lower_expr_async_closure(
+        &mut self,
+        binder: &ClosureBinder,
+        capture_clause: CaptureBy,
+        closure_id: NodeId,
+        inner_closure_id: NodeId,
+        decl: &FnDecl,
+        body: &Expr,
+        fn_decl_span: Span,
+    ) -> hir::ExprKind<'hir> {
+        if let &ClosureBinder::For { span, .. } = binder {
+            self.tcx.sess.span_err(
+                span,
+                "`for<...>` binders on `async` closures are not currently supported",
+            );
+        }
+
+        let (binder_clause, generic_params) = self.lower_closure_binder(binder);
+
+        let outer_decl =
+            FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
+
+        let body = self.with_new_scopes(|this| {
+            // FIXME(cramertj): allow `async` non-`move` closures with arguments.
+            if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
+                struct_span_err!(
+                    this.tcx.sess,
+                    fn_decl_span,
+                    E0708,
+                    "`async` non-`move` closures with parameters are not currently supported",
+                )
+                .help(
+                    "consider using `let` statements to manually capture \
+                    variables by reference before entering an `async move` closure",
+                )
+                .emit();
+            }
+
+            // Transform `async |x: u8| -> X { ... }` into
+            // `|x: u8| future_from_generator(|| -> X { ... })`.
+            let body_id = this.lower_fn_body(&outer_decl, |this| {
+                let async_ret_ty =
+                    if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
+                let async_body = this.make_async_expr(
+                    capture_clause,
+                    inner_closure_id,
+                    async_ret_ty,
+                    body.span,
+                    hir::AsyncGeneratorKind::Closure,
+                    |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
+                );
+                this.expr(fn_decl_span, async_body, ThinVec::new())
+            });
+            body_id
+        });
+
+        let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
+
+        // We need to lower the declaration outside the new scope, because we
+        // have to conserve the state of being inside a loop condition for the
+        // closure argument types.
+        let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
+
+        let c = self.arena.alloc(hir::Closure {
+            binder: binder_clause,
+            capture_clause,
+            bound_generic_params,
+            fn_decl,
+            body,
+            fn_decl_span: self.lower_span(fn_decl_span),
+            movability: None,
+        });
+        hir::ExprKind::Closure(c)
+    }
+
+    /// Destructure the LHS of complex assignments.
+    /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
+    fn lower_expr_assign(
+        &mut self,
+        lhs: &Expr,
+        rhs: &Expr,
+        eq_sign_span: Span,
+        whole_span: Span,
+    ) -> hir::ExprKind<'hir> {
+        // Return early in case of an ordinary assignment.
+        fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
+            match &lhs.kind {
+                ExprKind::Array(..)
+                | ExprKind::Struct(..)
+                | ExprKind::Tup(..)
+                | ExprKind::Underscore => false,
+                // Check for tuple struct constructor.
+                ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
+                ExprKind::Paren(e) => {
+                    match e.kind {
+                        // We special-case `(..)` for consistency with patterns.
+                        ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
+                        _ => is_ordinary(lower_ctx, e),
+                    }
+                }
+                _ => true,
+            }
+        }
+        if is_ordinary(self, lhs) {
+            return hir::ExprKind::Assign(
+                self.lower_expr(lhs),
+                self.lower_expr(rhs),
+                self.lower_span(eq_sign_span),
+            );
+        }
+
+        let mut assignments = vec![];
+
+        // The LHS becomes a pattern: `(lhs1, lhs2)`.
+        let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
+        let rhs = self.lower_expr(rhs);
+
+        // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
+        let destructure_let = self.stmt_let_pat(
+            None,
+            whole_span,
+            Some(rhs),
+            pat,
+            hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
+        );
+
+        // `a = lhs1; b = lhs2;`.
+        let stmts = self
+            .arena
+            .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
+
+        // Wrap everything in a block.
+        hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
+    }
+
+    /// If the given expression is a path to a tuple struct, returns that path.
+    /// It is not a complete check, but just tries to reject most paths early
+    /// if they are not tuple structs.
+    /// Type checking will take care of the full validation later.
+    fn extract_tuple_struct_path<'a>(
+        &mut self,
+        expr: &'a Expr,
+    ) -> Option<(&'a Option<QSelf>, &'a Path)> {
+        if let ExprKind::Path(qself, path) = &expr.kind {
+            // Does the path resolve to something disallowed in a tuple struct/variant pattern?
+            if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
+                if partial_res.unresolved_segments() == 0
+                    && !partial_res.base_res().expected_in_tuple_struct_pat()
+                {
+                    return None;
+                }
+            }
+            return Some((qself, path));
+        }
+        None
+    }
+
+    /// If the given expression is a path to a unit struct, returns that path.
+    /// It is not a complete check, but just tries to reject most paths early
+    /// if they are not unit structs.
+    /// Type checking will take care of the full validation later.
+    fn extract_unit_struct_path<'a>(
+        &mut self,
+        expr: &'a Expr,
+    ) -> Option<(&'a Option<QSelf>, &'a Path)> {
+        if let ExprKind::Path(qself, path) = &expr.kind {
+            // Does the path resolve to something disallowed in a unit struct/variant pattern?
+            if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
+                if partial_res.unresolved_segments() == 0
+                    && !partial_res.base_res().expected_in_unit_struct_pat()
+                {
+                    return None;
+                }
+            }
+            return Some((qself, path));
+        }
+        None
+    }
+
+    /// Convert the LHS of a destructuring assignment to a pattern.
+    /// Each sub-assignment is recorded in `assignments`.
+    fn destructure_assign(
+        &mut self,
+        lhs: &Expr,
+        eq_sign_span: Span,
+        assignments: &mut Vec<hir::Stmt<'hir>>,
+    ) -> &'hir hir::Pat<'hir> {
+        self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
+    }
+
+    fn destructure_assign_mut(
+        &mut self,
+        lhs: &Expr,
+        eq_sign_span: Span,
+        assignments: &mut Vec<hir::Stmt<'hir>>,
+    ) -> hir::Pat<'hir> {
+        match &lhs.kind {
+            // Underscore pattern.
+            ExprKind::Underscore => {
+                return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
+            }
+            // Slice patterns.
+            ExprKind::Array(elements) => {
+                let (pats, rest) =
+                    self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
+                let slice_pat = if let Some((i, span)) = rest {
+                    let (before, after) = pats.split_at(i);
+                    hir::PatKind::Slice(
+                        before,
+                        Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
+                        after,
+                    )
+                } else {
+                    hir::PatKind::Slice(pats, None, &[])
+                };
+                return self.pat_without_dbm(lhs.span, slice_pat);
+            }
+            // Tuple structs.
+            ExprKind::Call(callee, args) => {
+                if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
+                    let (pats, rest) = self.destructure_sequence(
+                        args,
+                        "tuple struct or variant",
+                        eq_sign_span,
+                        assignments,
+                    );
+                    let qpath = self.lower_qpath(
+                        callee.id,
+                        qself,
+                        path,
+                        ParamMode::Optional,
+                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    );
+                    // Destructure like a tuple struct.
+                    let tuple_struct_pat =
+                        hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
+                    return self.pat_without_dbm(lhs.span, tuple_struct_pat);
+                }
+            }
+            // Unit structs and enum variants.
+            ExprKind::Path(..) => {
+                if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
+                    let qpath = self.lower_qpath(
+                        lhs.id,
+                        qself,
+                        path,
+                        ParamMode::Optional,
+                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    );
+                    // Destructure like a unit struct.
+                    let unit_struct_pat = hir::PatKind::Path(qpath);
+                    return self.pat_without_dbm(lhs.span, unit_struct_pat);
+                }
+            }
+            // Structs.
+            ExprKind::Struct(se) => {
+                let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
+                    let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
+                    hir::PatField {
+                        hir_id: self.next_id(),
+                        ident: self.lower_ident(f.ident),
+                        pat,
+                        is_shorthand: f.is_shorthand,
+                        span: self.lower_span(f.span),
+                    }
+                }));
+                let qpath = self.lower_qpath(
+                    lhs.id,
+                    &se.qself,
+                    &se.path,
+                    ParamMode::Optional,
+                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                );
+                let fields_omitted = match &se.rest {
+                    StructRest::Base(e) => {
+                        self.tcx
+                            .sess
+                            .struct_span_err(
+                                e.span,
+                                "functional record updates are not allowed in destructuring \
+                                    assignments",
+                            )
+                            .span_suggestion(
+                                e.span,
+                                "consider removing the trailing pattern",
+                                "",
+                                rustc_errors::Applicability::MachineApplicable,
+                            )
+                            .emit();
+                        true
+                    }
+                    StructRest::Rest(_) => true,
+                    StructRest::None => false,
+                };
+                let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
+                return self.pat_without_dbm(lhs.span, struct_pat);
+            }
+            // Tuples.
+            ExprKind::Tup(elements) => {
+                let (pats, rest) =
+                    self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
+                let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
+                return self.pat_without_dbm(lhs.span, tuple_pat);
+            }
+            ExprKind::Paren(e) => {
+                // We special-case `(..)` for consistency with patterns.
+                if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
+                    let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
+                    return self.pat_without_dbm(lhs.span, tuple_pat);
+                } else {
+                    return self.destructure_assign_mut(e, eq_sign_span, assignments);
+                }
+            }
+            _ => {}
+        }
+        // Treat all other cases as normal lvalue.
+        let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
+        let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
+        let ident = self.expr_ident(lhs.span, ident, binding);
+        let assign =
+            hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
+        let expr = self.expr(lhs.span, assign, ThinVec::new());
+        assignments.push(self.stmt_expr(lhs.span, expr));
+        pat
+    }
+
+    /// Destructure a sequence of expressions occurring on the LHS of an assignment.
+    /// Such a sequence occurs in a tuple (struct)/slice.
+    /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
+    /// exists.
+    /// Each sub-assignment is recorded in `assignments`.
+    fn destructure_sequence(
+        &mut self,
+        elements: &[AstP<Expr>],
+        ctx: &str,
+        eq_sign_span: Span,
+        assignments: &mut Vec<hir::Stmt<'hir>>,
+    ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
+        let mut rest = None;
+        let elements =
+            self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
+                // Check for `..` pattern.
+                if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
+                    if let Some((_, prev_span)) = rest {
+                        self.ban_extra_rest_pat(e.span, prev_span, ctx);
+                    } else {
+                        rest = Some((i, e.span));
+                    }
+                    None
+                } else {
+                    Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
+                }
+            }));
+        (elements, rest)
+    }
+
+    /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
+    fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
+        let e1 = self.lower_expr_mut(e1);
+        let e2 = self.lower_expr_mut(e2);
+        let fn_path =
+            hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
+        let fn_expr =
+            self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
+        hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
+    }
+
+    fn lower_expr_range(
+        &mut self,
+        span: Span,
+        e1: Option<&Expr>,
+        e2: Option<&Expr>,
+        lims: RangeLimits,
+    ) -> hir::ExprKind<'hir> {
+        use rustc_ast::RangeLimits::*;
+
+        let lang_item = match (e1, e2, lims) {
+            (None, None, HalfOpen) => hir::LangItem::RangeFull,
+            (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
+            (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
+            (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
+            (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
+            (Some(..), Some(..), Closed) => unreachable!(),
+            (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
+        };
+
+        let fields = self.arena.alloc_from_iter(
+            e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
+                |(s, e)| {
+                    let expr = self.lower_expr(&e);
+                    let ident = Ident::new(s, self.lower_span(e.span));
+                    self.expr_field(ident, expr, e.span)
+                },
+            ),
+        );
+
+        hir::ExprKind::Struct(
+            self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
+            fields,
+            None,
+        )
+    }
+
+    fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
+        let label = opt_label?;
+        Some(Label { ident: self.lower_ident(label.ident) })
+    }
+
+    fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
+        let target_id = match destination {
+            Some((id, _)) => {
+                if let Some(loop_id) = self.resolver.get_label_res(id) {
+                    Ok(self.lower_node_id(loop_id))
+                } else {
+                    Err(hir::LoopIdError::UnresolvedLabel)
+                }
+            }
+            None => self
+                .loop_scope
+                .map(|id| Ok(self.lower_node_id(id)))
+                .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
+        };
+        let label = self.lower_label(destination.map(|(_, label)| label));
+        hir::Destination { label, target_id }
+    }
+
+    fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
+        if self.is_in_loop_condition && opt_label.is_none() {
+            hir::Destination {
+                label: None,
+                target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
+            }
+        } else {
+            self.lower_loop_destination(opt_label.map(|label| (id, label)))
+        }
+    }
+
+    fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
+        let old_scope = self.catch_scope.replace(catch_id);
+        let result = f(self);
+        self.catch_scope = old_scope;
+        result
+    }
+
+    fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
+        // We're no longer in the base loop's condition; we're in another loop.
+        let was_in_loop_condition = self.is_in_loop_condition;
+        self.is_in_loop_condition = false;
+
+        let old_scope = self.loop_scope.replace(loop_id);
+        let result = f(self);
+        self.loop_scope = old_scope;
+
+        self.is_in_loop_condition = was_in_loop_condition;
+
+        result
+    }
+
+    fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
+        let was_in_loop_condition = self.is_in_loop_condition;
+        self.is_in_loop_condition = true;
+
+        let result = f(self);
+
+        self.is_in_loop_condition = was_in_loop_condition;
+
+        result
+    }
+
+    fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
+        hir::ExprField {
+            hir_id: self.next_id(),
+            ident: self.lower_ident(f.ident),
+            expr: self.lower_expr(&f.expr),
+            span: self.lower_span(f.span),
+            is_shorthand: f.is_shorthand,
+        }
+    }
+
+    fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
+        match self.generator_kind {
+            Some(hir::GeneratorKind::Gen) => {}
+            Some(hir::GeneratorKind::Async(_)) => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0727,
+                    "`async` generators are not yet supported"
+                )
+                .emit();
+            }
+            None => self.generator_kind = Some(hir::GeneratorKind::Gen),
+        }
+
+        let expr =
+            opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
+
+        hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
+    }
+
+    /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
+    /// ```ignore (pseudo-rust)
+    /// {
+    ///     let result = match IntoIterator::into_iter(<head>) {
+    ///         mut iter => {
+    ///             [opt_ident]: loop {
+    ///                 match Iterator::next(&mut iter) {
+    ///                     None => break,
+    ///                     Some(<pat>) => <body>,
+    ///                 };
+    ///             }
+    ///         }
+    ///     };
+    ///     result
+    /// }
+    /// ```
+    fn lower_expr_for(
+        &mut self,
+        e: &Expr,
+        pat: &Pat,
+        head: &Expr,
+        body: &Block,
+        opt_label: Option<Label>,
+    ) -> hir::Expr<'hir> {
+        let head = self.lower_expr_mut(head);
+        let pat = self.lower_pat(pat);
+        let for_span =
+            self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
+        let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
+        let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
+
+        // `None => break`
+        let none_arm = {
+            let break_expr =
+                self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
+            let pat = self.pat_none(for_span);
+            self.arm(pat, break_expr)
+        };
+
+        // Some(<pat>) => <body>,
+        let some_arm = {
+            let some_pat = self.pat_some(pat_span, pat);
+            let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
+            let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
+            self.arm(some_pat, body_expr)
+        };
+
+        // `mut iter`
+        let iter = Ident::with_dummy_span(sym::iter);
+        let (iter_pat, iter_pat_nid) =
+            self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
+
+        // `match Iterator::next(&mut iter) { ... }`
+        let match_expr = {
+            let iter = self.expr_ident(head_span, iter, iter_pat_nid);
+            let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
+            let next_expr = self.expr_call_lang_item_fn(
+                head_span,
+                hir::LangItem::IteratorNext,
+                arena_vec![self; ref_mut_iter],
+                None,
+            );
+            let arms = arena_vec![self; none_arm, some_arm];
+
+            self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
+        };
+        let match_stmt = self.stmt_expr(for_span, match_expr);
+
+        let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
+
+        // `[opt_ident]: loop { ... }`
+        let kind = hir::ExprKind::Loop(
+            loop_block,
+            self.lower_label(opt_label),
+            hir::LoopSource::ForLoop,
+            self.lower_span(for_span.with_hi(head.span.hi())),
+        );
+        let loop_expr =
+            self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
+
+        // `mut iter => { ... }`
+        let iter_arm = self.arm(iter_pat, loop_expr);
+
+        // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
+        let into_iter_expr = {
+            self.expr_call_lang_item_fn(
+                head_span,
+                hir::LangItem::IntoIterIntoIter,
+                arena_vec![self; head],
+                None,
+            )
+        };
+
+        let match_expr = self.arena.alloc(self.expr_match(
+            for_span,
+            into_iter_expr,
+            arena_vec![self; iter_arm],
+            hir::MatchSource::ForLoopDesugar,
+        ));
+
+        let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
+
+        // This is effectively `{ let _result = ...; _result }`.
+        // The construct was introduced in #21984 and is necessary to make sure that
+        // temporaries in the `head` expression are dropped and do not leak to the
+        // surrounding scope of the `match` since the `match` is not a terminating scope.
+        //
+        // Also, add the attributes to the outer returned expr node.
+        self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
+    }
+
+    /// Desugar `ExprKind::Try` from: `<expr>?` into:
+    /// ```ignore (pseudo-rust)
+    /// match Try::branch(<expr>) {
+    ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
+    ///     ControlFlow::Break(residual) =>
+    ///         #[allow(unreachable_code)]
+    ///         // If there is an enclosing `try {...}`:
+    ///         break 'catch_target Try::from_residual(residual),
+    ///         // Otherwise:
+    ///         return Try::from_residual(residual),
+    /// }
+    /// ```
+    fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
+        let unstable_span = self.mark_span_with_reason(
+            DesugaringKind::QuestionMark,
+            span,
+            self.allow_try_trait.clone(),
+        );
+        let try_span = self.tcx.sess.source_map().end_point(span);
+        let try_span = self.mark_span_with_reason(
+            DesugaringKind::QuestionMark,
+            try_span,
+            self.allow_try_trait.clone(),
+        );
+
+        // `Try::branch(<expr>)`
+        let scrutinee = {
+            // expand <expr>
+            let sub_expr = self.lower_expr_mut(sub_expr);
+
+            self.expr_call_lang_item_fn(
+                unstable_span,
+                hir::LangItem::TryTraitBranch,
+                arena_vec![self; sub_expr],
+                None,
+            )
+        };
+
+        // `#[allow(unreachable_code)]`
+        let attr = {
+            // `allow(unreachable_code)`
+            let allow = {
+                let allow_ident = Ident::new(sym::allow, self.lower_span(span));
+                let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
+                let uc_nested = attr::mk_nested_word_item(uc_ident);
+                attr::mk_list_item(allow_ident, vec![uc_nested])
+            };
+            attr::mk_attr_outer(allow)
+        };
+        let attrs = vec![attr];
+
+        // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
+        let continue_arm = {
+            let val_ident = Ident::with_dummy_span(sym::val);
+            let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
+            let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
+                span,
+                val_ident,
+                val_pat_nid,
+                ThinVec::from(attrs.clone()),
+            ));
+            let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
+            self.arm(continue_pat, val_expr)
+        };
+
+        // `ControlFlow::Break(residual) =>
+        //     #[allow(unreachable_code)]
+        //     return Try::from_residual(residual),`
+        let break_arm = {
+            let residual_ident = Ident::with_dummy_span(sym::residual);
+            let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
+            let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
+            let from_residual_expr = self.wrap_in_try_constructor(
+                hir::LangItem::TryTraitFromResidual,
+                try_span,
+                self.arena.alloc(residual_expr),
+                unstable_span,
+            );
+            let thin_attrs = ThinVec::from(attrs);
+            let ret_expr = if let Some(catch_node) = self.catch_scope {
+                let target_id = Ok(self.lower_node_id(catch_node));
+                self.arena.alloc(self.expr(
+                    try_span,
+                    hir::ExprKind::Break(
+                        hir::Destination { label: None, target_id },
+                        Some(from_residual_expr),
+                    ),
+                    thin_attrs,
+                ))
+            } else {
+                self.arena.alloc(self.expr(
+                    try_span,
+                    hir::ExprKind::Ret(Some(from_residual_expr)),
+                    thin_attrs,
+                ))
+            };
+
+            let break_pat = self.pat_cf_break(try_span, residual_local);
+            self.arm(break_pat, ret_expr)
+        };
+
+        hir::ExprKind::Match(
+            scrutinee,
+            arena_vec![self; break_arm, continue_arm],
+            hir::MatchSource::TryDesugar,
+        )
+    }
+
+    /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
+    /// ```rust
+    /// // If there is an enclosing `try {...}`:
+    /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
+    /// // Otherwise:
+    /// return FromResidual::from_residual(Yeet(residual)),
+    /// ```
+    /// But to simplify this, there's a `from_yeet` lang item function which
+    /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
+    fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
+        // The expression (if present) or `()` otherwise.
+        let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
+            (sub_expr.span, self.lower_expr(sub_expr))
+        } else {
+            (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
+        };
+
+        let unstable_span = self.mark_span_with_reason(
+            DesugaringKind::YeetExpr,
+            span,
+            self.allow_try_trait.clone(),
+        );
+
+        let from_yeet_expr = self.wrap_in_try_constructor(
+            hir::LangItem::TryTraitFromYeet,
+            unstable_span,
+            yeeted_expr,
+            yeeted_span,
+        );
+
+        if let Some(catch_node) = self.catch_scope {
+            let target_id = Ok(self.lower_node_id(catch_node));
+            hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
+        } else {
+            hir::ExprKind::Ret(Some(from_yeet_expr))
+        }
+    }
+
+    // =========================================================================
+    // Helper methods for building HIR.
+    // =========================================================================
+
+    /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
+    ///
+    /// In terms of drop order, it has the same effect as wrapping `expr` in
+    /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
+    ///
+    /// The drop order can be important in e.g. `if expr { .. }`.
+    pub(super) fn expr_drop_temps(
+        &mut self,
+        span: Span,
+        expr: &'hir hir::Expr<'hir>,
+        attrs: AttrVec,
+    ) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
+    }
+
+    pub(super) fn expr_drop_temps_mut(
+        &mut self,
+        span: Span,
+        expr: &'hir hir::Expr<'hir>,
+        attrs: AttrVec,
+    ) -> hir::Expr<'hir> {
+        self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
+    }
+
+    fn expr_match(
+        &mut self,
+        span: Span,
+        arg: &'hir hir::Expr<'hir>,
+        arms: &'hir [hir::Arm<'hir>],
+        source: hir::MatchSource,
+    ) -> hir::Expr<'hir> {
+        self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
+    }
+
+    fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
+        let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
+        self.expr(span, expr_break, attrs)
+    }
+
+    fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
+        let expr_break = self.expr_break(span, attrs);
+        self.arena.alloc(expr_break)
+    }
+
+    fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
+        self.expr(
+            span,
+            hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
+            ThinVec::new(),
+        )
+    }
+
+    fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
+    }
+
+    fn expr_call_mut(
+        &mut self,
+        span: Span,
+        e: &'hir hir::Expr<'hir>,
+        args: &'hir [hir::Expr<'hir>],
+    ) -> hir::Expr<'hir> {
+        self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
+    }
+
+    fn expr_call(
+        &mut self,
+        span: Span,
+        e: &'hir hir::Expr<'hir>,
+        args: &'hir [hir::Expr<'hir>],
+    ) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.expr_call_mut(span, e, args))
+    }
+
+    fn expr_call_lang_item_fn_mut(
+        &mut self,
+        span: Span,
+        lang_item: hir::LangItem,
+        args: &'hir [hir::Expr<'hir>],
+        hir_id: Option<hir::HirId>,
+    ) -> hir::Expr<'hir> {
+        let path =
+            self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
+        self.expr_call_mut(span, path, args)
+    }
+
+    fn expr_call_lang_item_fn(
+        &mut self,
+        span: Span,
+        lang_item: hir::LangItem,
+        args: &'hir [hir::Expr<'hir>],
+        hir_id: Option<hir::HirId>,
+    ) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
+    }
+
+    fn expr_lang_item_path(
+        &mut self,
+        span: Span,
+        lang_item: hir::LangItem,
+        attrs: AttrVec,
+        hir_id: Option<hir::HirId>,
+    ) -> hir::Expr<'hir> {
+        self.expr(
+            span,
+            hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
+            attrs,
+        )
+    }
+
+    pub(super) fn expr_ident(
+        &mut self,
+        sp: Span,
+        ident: Ident,
+        binding: hir::HirId,
+    ) -> &'hir hir::Expr<'hir> {
+        self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
+    }
+
+    pub(super) fn expr_ident_mut(
+        &mut self,
+        sp: Span,
+        ident: Ident,
+        binding: hir::HirId,
+    ) -> hir::Expr<'hir> {
+        self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
+    }
+
+    fn expr_ident_with_attrs(
+        &mut self,
+        span: Span,
+        ident: Ident,
+        binding: hir::HirId,
+        attrs: AttrVec,
+    ) -> hir::Expr<'hir> {
+        let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
+            None,
+            self.arena.alloc(hir::Path {
+                span: self.lower_span(span),
+                res: Res::Local(binding),
+                segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
+            }),
+        ));
+
+        self.expr(span, expr_path, attrs)
+    }
+
+    fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
+        let hir_id = self.next_id();
+        let span = expr.span;
+        self.expr(
+            span,
+            hir::ExprKind::Block(
+                self.arena.alloc(hir::Block {
+                    stmts: &[],
+                    expr: Some(expr),
+                    hir_id,
+                    rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
+                    span: self.lower_span(span),
+                    targeted_by_break: false,
+                }),
+                None,
+            ),
+            ThinVec::new(),
+        )
+    }
+
+    fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
+        let blk = self.block_all(span, &[], None);
+        let expr = self.expr_block(blk, ThinVec::new());
+        self.arena.alloc(expr)
+    }
+
+    pub(super) fn expr_block(
+        &mut self,
+        b: &'hir hir::Block<'hir>,
+        attrs: AttrVec,
+    ) -> hir::Expr<'hir> {
+        self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
+    }
+
+    pub(super) fn expr(
+        &mut self,
+        span: Span,
+        kind: hir::ExprKind<'hir>,
+        attrs: AttrVec,
+    ) -> hir::Expr<'hir> {
+        let hir_id = self.next_id();
+        self.lower_attrs(hir_id, &attrs);
+        hir::Expr { hir_id, kind, span: self.lower_span(span) }
+    }
+
+    fn expr_field(
+        &mut self,
+        ident: Ident,
+        expr: &'hir hir::Expr<'hir>,
+        span: Span,
+    ) -> hir::ExprField<'hir> {
+        hir::ExprField {
+            hir_id: self.next_id(),
+            ident,
+            span: self.lower_span(span),
+            expr,
+            is_shorthand: false,
+        }
+    }
+
+    fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
+        hir::Arm {
+            hir_id: self.next_id(),
+            pat,
+            guard: None,
+            span: self.lower_span(expr.span),
+            body: expr,
+        }
+    }
+}
-- 
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