Auto merge of #100944 - nnethercote:shrink-thir-Expr, r=cjgillot

Shrink `thir::Expr`

r? `@cjgillot`

15 files changed, 191 insertions, 150 deletions

diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index 5e26a52900e..0f2504e3d9a 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -15,50 +15,33 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::RangeEnd;
 use rustc_index::newtype_index;
 use rustc_index::vec::IndexVec;
-use rustc_middle::infer::canonical::Canonical;
 use rustc_middle::middle::region;
 use rustc_middle::mir::interpret::AllocId;
 use rustc_middle::mir::{self, BinOp, BorrowKind, FakeReadCause, Field, Mutability, UnOp};
 use rustc_middle::ty::adjustment::PointerCast;
 use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::CanonicalUserTypeAnnotation;
-use rustc_middle::ty::{self, AdtDef, Ty, UpvarSubsts, UserType};
+use rustc_middle::ty::{self, AdtDef, Ty, UpvarSubsts};
+use rustc_middle::ty::{CanonicalUserType, CanonicalUserTypeAnnotation};
+use rustc_span::def_id::LocalDefId;
 use rustc_span::{Span, Symbol, DUMMY_SP};
 use rustc_target::abi::VariantIdx;
 use rustc_target::asm::InlineAsmRegOrRegClass;
-
-use rustc_span::def_id::LocalDefId;
 use std::fmt;
 use std::ops::Index;
 
 pub mod visit;
 
-newtype_index! {
-    /// An index to an [`Arm`] stored in [`Thir::arms`]
-    #[derive(HashStable)]
-    pub struct ArmId {
-        DEBUG_FORMAT = "a{}"
-    }
-}
-
-newtype_index! {
-    /// An index to an [`Expr`] stored in [`Thir::exprs`]
-    #[derive(HashStable)]
-    pub struct ExprId {
-        DEBUG_FORMAT = "e{}"
-    }
-}
-
-newtype_index! {
-    #[derive(HashStable)]
-    /// An index to a [`Stmt`] stored in [`Thir::stmts`]
-    pub struct StmtId {
-        DEBUG_FORMAT = "s{}"
-    }
-}
-
 macro_rules! thir_with_elements {
-    ($($name:ident: $id:ty => $value:ty,)*) => {
+    ($($name:ident: $id:ty => $value:ty => $format:literal,)*) => {
+        $(
+            newtype_index! {
+                #[derive(HashStable)]
+                pub struct $id {
+                    DEBUG_FORMAT = $format
+                }
+            }
+        )*
+
         /// A container for a THIR body.
         ///
         /// This can be indexed directly by any THIR index (e.g. [`ExprId`]).
@@ -91,9 +74,10 @@ macro_rules! thir_with_elements {
 }
 
 thir_with_elements! {
-    arms: ArmId => Arm<'tcx>,
-    exprs: ExprId => Expr<'tcx>,
-    stmts: StmtId => Stmt<'tcx>,
+    arms: ArmId => Arm<'tcx> => "a{}",
+    blocks: BlockId => Block => "b{}",
+    exprs: ExprId => Expr<'tcx> => "e{}",
+    stmts: StmtId => Stmt<'tcx> => "s{}",
 }
 
 #[derive(Copy, Clone, Debug, HashStable)]
@@ -121,8 +105,10 @@ pub struct Block {
     pub safety_mode: BlockSafety,
 }
 
+type UserTy<'tcx> = Option<Box<CanonicalUserType<'tcx>>>;
+
 #[derive(Clone, Debug, HashStable)]
-pub struct Adt<'tcx> {
+pub struct AdtExpr<'tcx> {
     /// The ADT we're constructing.
     pub adt_def: AdtDef<'tcx>,
     /// The variant of the ADT.
@@ -131,13 +117,30 @@ pub struct Adt<'tcx> {
 
     /// Optional user-given substs: for something like `let x =
     /// Bar::<T> { ... }`.
-    pub user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    pub user_ty: UserTy<'tcx>,
 
     pub fields: Box<[FieldExpr]>,
     /// The base, e.g. `Foo {x: 1, .. base}`.
     pub base: Option<FruInfo<'tcx>>,
 }
 
+#[derive(Clone, Debug, HashStable)]
+pub struct ClosureExpr<'tcx> {
+    pub closure_id: LocalDefId,
+    pub substs: UpvarSubsts<'tcx>,
+    pub upvars: Box<[ExprId]>,
+    pub movability: Option<hir::Movability>,
+    pub fake_reads: Vec<(ExprId, FakeReadCause, hir::HirId)>,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct InlineAsmExpr<'tcx> {
+    pub template: &'tcx [InlineAsmTemplatePiece],
+    pub operands: Box<[InlineAsmOperand<'tcx>]>,
+    pub options: InlineAsmOptions,
+    pub line_spans: &'tcx [Span],
+}
+
 #[derive(Copy, Clone, Debug, HashStable)]
 pub enum BlockSafety {
     Safe,
@@ -183,7 +186,7 @@ pub enum StmtKind<'tcx> {
         initializer: Option<ExprId>,
 
         /// `let pat: ty = <INIT> else { <ELSE> }
-        else_block: Option<Block>,
+        else_block: Option<BlockId>,
 
         /// The lint level for this `let` statement.
         lint_level: LintLevel,
@@ -307,7 +310,7 @@ pub enum ExprKind<'tcx> {
     },
     /// A block.
     Block {
-        body: Block,
+        block: BlockId,
     },
     /// An assignment: `lhs = rhs`.
     Assign {
@@ -387,27 +390,21 @@ pub enum ExprKind<'tcx> {
         fields: Box<[ExprId]>,
     },
     /// An ADT constructor, e.g. `Foo {x: 1, y: 2}`.
-    Adt(Box<Adt<'tcx>>),
+    Adt(Box<AdtExpr<'tcx>>),
     /// A type ascription on a place.
     PlaceTypeAscription {
         source: ExprId,
         /// Type that the user gave to this expression
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        user_ty: UserTy<'tcx>,
     },
     /// A type ascription on a value, e.g. `42: i32`.
     ValueTypeAscription {
         source: ExprId,
         /// Type that the user gave to this expression
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        user_ty: UserTy<'tcx>,
     },
     /// A closure definition.
-    Closure {
-        closure_id: LocalDefId,
-        substs: UpvarSubsts<'tcx>,
-        upvars: Box<[ExprId]>,
-        movability: Option<hir::Movability>,
-        fake_reads: Vec<(ExprId, FakeReadCause, hir::HirId)>,
-    },
+    Closure(Box<ClosureExpr<'tcx>>),
     /// A literal.
     Literal {
         lit: &'tcx hir::Lit,
@@ -416,17 +413,17 @@ pub enum ExprKind<'tcx> {
     /// For literals that don't correspond to anything in the HIR
     NonHirLiteral {
         lit: ty::ScalarInt,
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        user_ty: UserTy<'tcx>,
     },
     /// A literal of a ZST type.
     ZstLiteral {
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        user_ty: UserTy<'tcx>,
     },
     /// Associated constants and named constants
     NamedConst {
         def_id: DefId,
         substs: SubstsRef<'tcx>,
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        user_ty: UserTy<'tcx>,
     },
     ConstParam {
         param: ty::ParamConst,
@@ -443,12 +440,7 @@ pub enum ExprKind<'tcx> {
         def_id: DefId,
     },
     /// Inline assembly, i.e. `asm!()`.
-    InlineAsm {
-        template: &'tcx [InlineAsmTemplatePiece],
-        operands: Box<[InlineAsmOperand<'tcx>]>,
-        options: InlineAsmOptions,
-        line_spans: &'tcx [Span],
-    },
+    InlineAsm(Box<InlineAsmExpr<'tcx>>),
     /// An expression taking a reference to a thread local.
     ThreadLocalRef(DefId),
     /// A `yield` expression.
@@ -815,7 +807,10 @@ mod size_asserts {
     use super::*;
     // These are in alphabetical order, which is easy to maintain.
     static_assert_size!(Block, 56);
-    static_assert_size!(Expr<'_>, 104);
+    static_assert_size!(Expr<'_>, 64);
+    static_assert_size!(ExprKind<'_>, 40);
     static_assert_size!(Pat<'_>, 24);
-    static_assert_size!(Stmt<'_>, 120);
+    static_assert_size!(PatKind<'_>, 112);
+    static_assert_size!(Stmt<'_>, 72);
+    static_assert_size!(StmtKind<'_>, 64);
 }
diff --git a/compiler/rustc_middle/src/thir/visit.rs b/compiler/rustc_middle/src/thir/visit.rs
index 97249fdd175..c5c48a63609 100644
--- a/compiler/rustc_middle/src/thir/visit.rs
+++ b/compiler/rustc_middle/src/thir/visit.rs
@@ -1,5 +1,6 @@
 use super::{
-    Arm, Block, Expr, ExprKind, Guard, InlineAsmOperand, Pat, PatKind, Stmt, StmtKind, Thir,
+    AdtExpr, Arm, Block, ClosureExpr, Expr, ExprKind, Guard, InlineAsmExpr, InlineAsmOperand, Pat,
+    PatKind, Stmt, StmtKind, Thir,
 };
 
 pub trait Visitor<'a, 'tcx: 'a>: Sized {
@@ -75,7 +76,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_arm(&visitor.thir()[arm]);
             }
         }
-        Block { ref body } => visitor.visit_block(body),
+        Block { block } => visitor.visit_block(&visitor.thir()[block]),
         Assign { lhs, rhs } | AssignOp { lhs, rhs, op: _ } => {
             visitor.visit_expr(&visitor.thir()[lhs]);
             visitor.visit_expr(&visitor.thir()[rhs]);
@@ -108,7 +109,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[field]);
             }
         }
-        Adt(box crate::thir::Adt {
+        Adt(box AdtExpr {
             ref fields,
             ref base,
             adt_def: _,
@@ -126,14 +127,20 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
         PlaceTypeAscription { source, user_ty: _ } | ValueTypeAscription { source, user_ty: _ } => {
             visitor.visit_expr(&visitor.thir()[source])
         }
-        Closure { closure_id: _, substs: _, upvars: _, movability: _, fake_reads: _ } => {}
+        Closure(box ClosureExpr {
+            closure_id: _,
+            substs: _,
+            upvars: _,
+            movability: _,
+            fake_reads: _,
+        }) => {}
         Literal { lit: _, neg: _ } => {}
         NonHirLiteral { lit: _, user_ty: _ } => {}
         ZstLiteral { user_ty: _ } => {}
         NamedConst { def_id: _, substs: _, user_ty: _ } => {}
         ConstParam { param: _, def_id: _ } => {}
         StaticRef { alloc_id: _, ty: _, def_id: _ } => {}
-        InlineAsm { ref operands, template: _, options: _, line_spans: _ } => {
+        InlineAsm(box InlineAsmExpr { ref operands, template: _, options: _, line_spans: _ }) => {
             for op in &**operands {
                 use InlineAsmOperand::*;
                 match op {
@@ -174,7 +181,7 @@ pub fn walk_stmt<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, stmt: &Stm
             }
             visitor.visit_pat(pattern);
             if let Some(block) = else_block {
-                visitor.visit_block(block)
+                visitor.visit_block(&visitor.thir()[*block])
             }
         }
     }
diff --git a/compiler/rustc_mir_build/src/build/block.rs b/compiler/rustc_mir_build/src/build/block.rs
index 280b6aad12c..d5213dc0e04 100644
--- a/compiler/rustc_mir_build/src/build/block.rs
+++ b/compiler/rustc_mir_build/src/build/block.rs
@@ -11,7 +11,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         destination: Place<'tcx>,
         block: BasicBlock,
-        ast_block: &Block,
+        ast_block: BlockId,
         source_info: SourceInfo,
     ) -> BlockAnd<()> {
         let Block {
@@ -22,7 +22,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             expr,
             targeted_by_break,
             safety_mode,
-        } = *ast_block;
+        } = self.thir[ast_block];
         let expr = expr.map(|expr| &self.thir[expr]);
         self.in_opt_scope(opt_destruction_scope.map(|de| (de, source_info)), move |this| {
             this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
@@ -146,7 +146,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                                 block,
                                                 init,
                                                 initializer_span,
-                                                else_block,
+                                                *else_block,
                                                 visibility_scope,
                                                 last_remainder_scope,
                                                 remainder_span,
diff --git a/compiler/rustc_mir_build/src/build/expr/as_constant.rs b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
index 19d420f154d..b316a6eeac1 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_constant.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
@@ -41,11 +41,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 Constant { span, user_ty: None, literal }
             }
-            ExprKind::NonHirLiteral { lit, user_ty } => {
-                let user_ty = user_ty.map(|user_ty| {
+            ExprKind::NonHirLiteral { lit, ref user_ty } => {
+                let user_ty = user_ty.as_ref().map(|box user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,
-                        user_ty,
+                        user_ty: *user_ty,
                         inferred_ty: ty,
                     })
                 });
@@ -53,11 +53,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 Constant { span, user_ty: user_ty, literal }
             }
-            ExprKind::ZstLiteral { user_ty } => {
-                let user_ty = user_ty.map(|user_ty| {
+            ExprKind::ZstLiteral { ref user_ty } => {
+                let user_ty = user_ty.as_ref().map(|box user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,
-                        user_ty,
+                        user_ty: *user_ty,
                         inferred_ty: ty,
                     })
                 });
@@ -65,11 +65,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 Constant { span, user_ty: user_ty, literal }
             }
-            ExprKind::NamedConst { def_id, substs, user_ty } => {
-                let user_ty = user_ty.map(|user_ty| {
+            ExprKind::NamedConst { def_id, substs, ref user_ty } => {
+                let user_ty = user_ty.as_ref().map(|box user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,
-                        user_ty,
+                        user_ty: *user_ty,
                         inferred_ty: ty,
                     })
                 });
diff --git a/compiler/rustc_mir_build/src/build/expr/as_place.rs b/compiler/rustc_mir_build/src/build/expr/as_place.rs
index 0c06aad4e44..b8277f28cdc 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_place.rs
@@ -513,7 +513,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(place_builder)
             }
 
-            ExprKind::PlaceTypeAscription { source, user_ty } => {
+            ExprKind::PlaceTypeAscription { source, ref user_ty } => {
                 let place_builder = unpack!(
                     block = this.expr_as_place(
                         block,
@@ -522,11 +522,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         fake_borrow_temps,
                     )
                 );
-                if let Some(user_ty) = user_ty {
+                if let Some(box user_ty) = user_ty {
                     let annotation_index =
                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                             span: source_info.span,
-                            user_ty,
+                            user_ty: *user_ty,
                             inferred_ty: expr.ty,
                         });
 
@@ -547,15 +547,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 }
                 block.and(place_builder)
             }
-            ExprKind::ValueTypeAscription { source, user_ty } => {
+            ExprKind::ValueTypeAscription { source, ref user_ty } => {
                 let source = &this.thir[source];
                 let temp =
                     unpack!(block = this.as_temp(block, source.temp_lifetime, source, mutability));
-                if let Some(user_ty) = user_ty {
+                if let Some(box user_ty) = user_ty {
                     let annotation_index =
                         this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                             span: source_info.span,
-                            user_ty,
+                            user_ty: *user_ty,
                             inferred_ty: expr.ty,
                         });
                     this.cfg.push(
diff --git a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs b/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
index 15f2d17c4e0..4b232a1b515 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
@@ -302,7 +302,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 block.and(Rvalue::Aggregate(Box::new(AggregateKind::Tuple), fields))
             }
-            ExprKind::Closure { closure_id, substs, ref upvars, movability, ref fake_reads } => {
+            ExprKind::Closure(box ClosureExpr {
+                closure_id,
+                substs,
+                ref upvars,
+                movability,
+                ref fake_reads,
+            }) => {
                 // Convert the closure fake reads, if any, from `ExprRef` to mir `Place`
                 // and push the fake reads.
                 // This must come before creating the operands. This is required in case
diff --git a/compiler/rustc_mir_build/src/build/expr/as_temp.rs b/compiler/rustc_mir_build/src/build/expr/as_temp.rs
index 724b72f8769..e5dafb820bf 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_temp.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_temp.rs
@@ -83,8 +83,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             // Don't bother with StorageLive and Dead for these temporaries,
             // they are never assigned.
             ExprKind::Break { .. } | ExprKind::Continue { .. } | ExprKind::Return { .. } => (),
-            ExprKind::Block { body: Block { expr: None, targeted_by_break: false, .. } }
-                if expr_ty.is_never() => {}
+            ExprKind::Block { block }
+                if let Block { expr: None, targeted_by_break: false, .. } = this.thir[block]
+                    && expr_ty.is_never() => {}
             _ => {
                 this.cfg
                     .push(block, Statement { source_info, kind: StatementKind::StorageLive(temp) });
diff --git a/compiler/rustc_mir_build/src/build/expr/into.rs b/compiler/rustc_mir_build/src/build/expr/into.rs
index 017d43d10a9..48ec7a06724 100644
--- a/compiler/rustc_mir_build/src/build/expr/into.rs
+++ b/compiler/rustc_mir_build/src/build/expr/into.rs
@@ -46,7 +46,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     })
                 })
             }
-            ExprKind::Block { body: ref ast_block } => {
+            ExprKind::Block { block: ast_block } => {
                 this.ast_block(destination, block, ast_block, source_info)
             }
             ExprKind::Match { scrutinee, ref arms } => {
@@ -314,11 +314,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 this.cfg.push_assign(block, source_info, destination, address_of);
                 block.unit()
             }
-            ExprKind::Adt(box Adt {
+            ExprKind::Adt(box AdtExpr {
                 adt_def,
                 variant_index,
                 substs,
-                user_ty,
+                ref user_ty,
                 ref fields,
                 ref base,
             }) => {
@@ -378,10 +378,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 };
 
                 let inferred_ty = expr.ty;
-                let user_ty = user_ty.map(|ty| {
+                let user_ty = user_ty.as_ref().map(|box user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span: source_info.span,
-                        user_ty: ty,
+                        user_ty: *user_ty,
                         inferred_ty,
                     })
                 });
@@ -400,7 +400,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 );
                 block.unit()
             }
-            ExprKind::InlineAsm { template, ref operands, options, line_spans } => {
+            ExprKind::InlineAsm(box InlineAsmExpr {
+                template,
+                ref operands,
+                options,
+                line_spans,
+            }) => {
                 use rustc_middle::{mir, thir};
                 let operands = operands
                     .into_iter()
diff --git a/compiler/rustc_mir_build/src/build/expr/stmt.rs b/compiler/rustc_mir_build/src/build/expr/stmt.rs
index a7e1331aabc..00dbcaeb0c9 100644
--- a/compiler/rustc_mir_build/src/build/expr/stmt.rs
+++ b/compiler/rustc_mir_build/src/build/expr/stmt.rs
@@ -116,14 +116,22 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // it is usually better to focus on `the_value` rather
                 // than the entirety of block(s) surrounding it.
                 let adjusted_span = (|| {
-                    if let ExprKind::Block { body } = &expr.kind && let Some(tail_ex) = body.expr {
+                    if let ExprKind::Block { block } = expr.kind
+                        && let Some(tail_ex) = this.thir[block].expr
+                    {
                         let mut expr = &this.thir[tail_ex];
-                        while let ExprKind::Block {
-                            body: Block { expr: Some(nested_expr), .. },
-                        }
-                        | ExprKind::Scope { value: nested_expr, .. } = expr.kind
-                        {
-                            expr = &this.thir[nested_expr];
+                        loop {
+                            match expr.kind {
+                                ExprKind::Block { block }
+                                    if let Some(nested_expr) = this.thir[block].expr =>
+                                {
+                                    expr = &this.thir[nested_expr];
+                                }
+                                ExprKind::Scope { value: nested_expr, .. } => {
+                                    expr = &this.thir[nested_expr];
+                                }
+                                _ => break,
+                            }
                         }
                         this.block_context.push(BlockFrame::TailExpr {
                             tail_result_is_ignored: true,
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index ce38283724d..080dab03031 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -2280,15 +2280,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         mut block: BasicBlock,
         init: &Expr<'tcx>,
         initializer_span: Span,
-        else_block: &Block,
+        else_block: BlockId,
         visibility_scope: Option<SourceScope>,
         remainder_scope: region::Scope,
         remainder_span: Span,
         pattern: &Pat<'tcx>,
     ) -> BlockAnd<()> {
+        let else_block_span = self.thir[else_block].span;
         let (matching, failure) = self.in_if_then_scope(remainder_scope, |this| {
             let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
-            let pat = Pat { ty: init.ty, span: else_block.span, kind: Box::new(PatKind::Wild) };
+            let pat = Pat { ty: init.ty, span: else_block_span, kind: Box::new(PatKind::Wild) };
             let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false);
             this.declare_bindings(
                 visibility_scope,
@@ -2318,7 +2319,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             );
             // This block is for the failure case
             let failure = this.bind_pattern(
-                this.source_info(else_block.span),
+                this.source_info(else_block_span),
                 wildcard,
                 None,
                 &fake_borrow_temps,
@@ -2334,19 +2335,19 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // This place is not really used because this destination place
         // should never be used to take values at the end of the failure
         // block.
-        let dummy_place = self.temp(self.tcx.types.never, else_block.span);
+        let dummy_place = self.temp(self.tcx.types.never, else_block_span);
         let failure_block;
         unpack!(
             failure_block = self.ast_block(
                 dummy_place,
                 failure,
                 else_block,
-                self.source_info(else_block.span),
+                self.source_info(else_block_span),
             )
         );
         self.cfg.terminate(
             failure_block,
-            self.source_info(else_block.span),
+            self.source_info(else_block_span),
             TerminatorKind::Unreachable,
         );
         matching.unit()
diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index f0b0456c4b9..bf5a2e7c73f 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -391,7 +391,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             ExprKind::InlineAsm { .. } => {
                 self.requires_unsafe(expr.span, UseOfInlineAssembly);
             }
-            ExprKind::Adt(box Adt {
+            ExprKind::Adt(box AdtExpr {
                 adt_def,
                 variant_index: _,
                 substs: _,
@@ -402,13 +402,13 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 (Bound::Unbounded, Bound::Unbounded) => {}
                 _ => self.requires_unsafe(expr.span, InitializingTypeWith),
             },
-            ExprKind::Closure {
+            ExprKind::Closure(box ClosureExpr {
                 closure_id,
                 substs: _,
                 upvars: _,
                 movability: _,
                 fake_reads: _,
-            } => {
+            }) => {
                 let closure_def = if let Some((did, const_param_id)) =
                     ty::WithOptConstParam::try_lookup(closure_id, self.tcx)
                 {
diff --git a/compiler/rustc_mir_build/src/thir/cx/block.rs b/compiler/rustc_mir_build/src/thir/cx/block.rs
index dccaa61ed89..54c4b9eda70 100644
--- a/compiler/rustc_mir_build/src/thir/cx/block.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/block.rs
@@ -9,13 +9,13 @@ use rustc_index::vec::Idx;
 use rustc_middle::ty::CanonicalUserTypeAnnotation;
 
 impl<'tcx> Cx<'tcx> {
-    pub(crate) fn mirror_block(&mut self, block: &'tcx hir::Block<'tcx>) -> Block {
+    pub(crate) fn mirror_block(&mut self, block: &'tcx hir::Block<'tcx>) -> BlockId {
         // We have to eagerly lower the "spine" of the statements
         // in order to get the lexical scoping correctly.
         let stmts = self.mirror_stmts(block.hir_id.local_id, block.stmts);
         let opt_destruction_scope =
             self.region_scope_tree.opt_destruction_scope(block.hir_id.local_id);
-        Block {
+        let block = Block {
             targeted_by_break: block.targeted_by_break,
             region_scope: region::Scope {
                 id: block.hir_id.local_id,
@@ -34,7 +34,9 @@ impl<'tcx> Cx<'tcx> {
                     BlockSafety::ExplicitUnsafe(block.hir_id)
                 }
             },
-        }
+        };
+
+        self.thir.blocks.push(block)
     }
 
     fn mirror_stmts(
diff --git a/compiler/rustc_mir_build/src/thir/cx/expr.rs b/compiler/rustc_mir_build/src/thir/cx/expr.rs
index 985601712c4..0c2b117453f 100644
--- a/compiler/rustc_mir_build/src/thir/cx/expr.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/expr.rs
@@ -108,8 +108,8 @@ impl<'tcx> Cx<'tcx> {
         //   // ^ error message points at this expression.
         // }
         let mut adjust_span = |expr: &mut Expr<'tcx>| {
-            if let ExprKind::Block { body } = &expr.kind {
-                if let Some(last_expr) = body.expr {
+            if let ExprKind::Block { block } = expr.kind {
+                if let Some(last_expr) = self.thir[block].expr {
                     span = self.thir[last_expr].span;
                     expr.span = span;
                 }
@@ -329,7 +329,7 @@ impl<'tcx> Cx<'tcx> {
                                 if let UserType::TypeOf(ref mut did, _) = &mut u_ty.value {
                                     *did = adt_def.did();
                                 }
-                                u_ty
+                                Box::new(u_ty)
                             });
                         debug!("make_mirror_unadjusted: (call) user_ty={:?}", user_ty);
 
@@ -341,7 +341,7 @@ impl<'tcx> Cx<'tcx> {
                                 expr: self.mirror_expr(e),
                             })
                             .collect();
-                        ExprKind::Adt(Box::new(Adt {
+                        ExprKind::Adt(Box::new(AdtExpr {
                             adt_def,
                             substs,
                             variant_index: index,
@@ -369,7 +369,7 @@ impl<'tcx> Cx<'tcx> {
                 ExprKind::AddressOf { mutability, arg: self.mirror_expr(arg) }
             }
 
-            hir::ExprKind::Block(ref blk, _) => ExprKind::Block { body: self.mirror_block(blk) },
+            hir::ExprKind::Block(ref blk, _) => ExprKind::Block { block: self.mirror_block(blk) },
 
             hir::ExprKind::Assign(ref lhs, ref rhs, _) => {
                 ExprKind::Assign { lhs: self.mirror_expr(lhs), rhs: self.mirror_expr(rhs) }
@@ -464,9 +464,9 @@ impl<'tcx> Cx<'tcx> {
                 ty::Adt(adt, substs) => match adt.adt_kind() {
                     AdtKind::Struct | AdtKind::Union => {
                         let user_provided_types = self.typeck_results().user_provided_types();
-                        let user_ty = user_provided_types.get(expr.hir_id).copied();
+                        let user_ty = user_provided_types.get(expr.hir_id).copied().map(Box::new);
                         debug!("make_mirror_unadjusted: (struct/union) user_ty={:?}", user_ty);
-                        ExprKind::Adt(Box::new(Adt {
+                        ExprKind::Adt(Box::new(AdtExpr {
                             adt_def: *adt,
                             variant_index: VariantIdx::new(0),
                             substs,
@@ -490,9 +490,10 @@ impl<'tcx> Cx<'tcx> {
                                 let index = adt.variant_index_with_id(variant_id);
                                 let user_provided_types =
                                     self.typeck_results().user_provided_types();
-                                let user_ty = user_provided_types.get(expr.hir_id).copied();
+                                let user_ty =
+                                    user_provided_types.get(expr.hir_id).copied().map(Box::new);
                                 debug!("make_mirror_unadjusted: (variant) user_ty={:?}", user_ty);
-                                ExprKind::Adt(Box::new(Adt {
+                                ExprKind::Adt(Box::new(AdtExpr {
                                     adt_def: *adt,
                                     variant_index: index,
                                     substs,
@@ -547,7 +548,13 @@ impl<'tcx> Cx<'tcx> {
                     None => Vec::new(),
                 };
 
-                ExprKind::Closure { closure_id: def_id, substs, upvars, movability, fake_reads }
+                ExprKind::Closure(Box::new(ClosureExpr {
+                    closure_id: def_id,
+                    substs,
+                    upvars,
+                    movability,
+                    fake_reads,
+                }))
             }
 
             hir::ExprKind::Path(ref qpath) => {
@@ -555,7 +562,7 @@ impl<'tcx> Cx<'tcx> {
                 self.convert_path_expr(expr, res)
             }
 
-            hir::ExprKind::InlineAsm(ref asm) => ExprKind::InlineAsm {
+            hir::ExprKind::InlineAsm(ref asm) => ExprKind::InlineAsm(Box::new(InlineAsmExpr {
                 template: asm.template,
                 operands: asm
                     .operands
@@ -614,7 +621,7 @@ impl<'tcx> Cx<'tcx> {
                     .collect(),
                 options: asm.options,
                 line_spans: asm.line_spans,
-            },
+            })),
 
             hir::ExprKind::ConstBlock(ref anon_const) => {
                 let ty = self.typeck_results().node_type(anon_const.hir_id);
@@ -679,8 +686,8 @@ impl<'tcx> Cx<'tcx> {
                 let body = self.thir.exprs.push(Expr {
                     ty: block_ty,
                     temp_lifetime,
-                    span: block.span,
-                    kind: ExprKind::Block { body: block },
+                    span: self.thir[block].span,
+                    kind: ExprKind::Block { block },
                 });
                 ExprKind::Loop { body }
             }
@@ -712,14 +719,17 @@ impl<'tcx> Cx<'tcx> {
                     });
                     debug!("make_mirror_unadjusted: (cast) user_ty={:?}", user_ty);
 
-                    ExprKind::ValueTypeAscription { source: cast_expr, user_ty: Some(*user_ty) }
+                    ExprKind::ValueTypeAscription {
+                        source: cast_expr,
+                        user_ty: Some(Box::new(*user_ty)),
+                    }
                 } else {
                     cast
                 }
             }
             hir::ExprKind::Type(ref source, ref ty) => {
                 let user_provided_types = self.typeck_results.user_provided_types();
-                let user_ty = user_provided_types.get(ty.hir_id).copied();
+                let user_ty = user_provided_types.get(ty.hir_id).copied().map(Box::new);
                 debug!("make_mirror_unadjusted: (type) user_ty={:?}", user_ty);
                 let mirrored = self.mirror_expr(source);
                 if source.is_syntactic_place_expr() {
@@ -748,7 +758,7 @@ impl<'tcx> Cx<'tcx> {
         &mut self,
         hir_id: hir::HirId,
         res: Res,
-    ) -> Option<ty::CanonicalUserType<'tcx>> {
+    ) -> Option<Box<ty::CanonicalUserType<'tcx>>> {
         debug!("user_substs_applied_to_res: res={:?}", res);
         let user_provided_type = match res {
             // A reference to something callable -- e.g., a fn, method, or
@@ -759,7 +769,7 @@ impl<'tcx> Cx<'tcx> {
             | Res::Def(DefKind::Ctor(_, CtorKind::Fn), _)
             | Res::Def(DefKind::Const, _)
             | Res::Def(DefKind::AssocConst, _) => {
-                self.typeck_results().user_provided_types().get(hir_id).copied()
+                self.typeck_results().user_provided_types().get(hir_id).copied().map(Box::new)
             }
 
             // A unit struct/variant which is used as a value (e.g.,
@@ -767,11 +777,11 @@ impl<'tcx> Cx<'tcx> {
             // this variant -- but with the substitutions given by the
             // user.
             Res::Def(DefKind::Ctor(_, CtorKind::Const), _) => {
-                self.user_substs_applied_to_ty_of_hir_id(hir_id)
+                self.user_substs_applied_to_ty_of_hir_id(hir_id).map(Box::new)
             }
 
             // `Self` is used in expression as a tuple struct constructor or a unit struct constructor
-            Res::SelfCtor(_) => self.user_substs_applied_to_ty_of_hir_id(hir_id),
+            Res::SelfCtor(_) => self.user_substs_applied_to_ty_of_hir_id(hir_id).map(Box::new),
 
             _ => bug!("user_substs_applied_to_res: unexpected res {:?} at {:?}", res, hir_id),
         };
@@ -846,22 +856,22 @@ impl<'tcx> Cx<'tcx> {
 
             Res::Def(DefKind::Const, def_id) | Res::Def(DefKind::AssocConst, def_id) => {
                 let user_ty = self.user_substs_applied_to_res(expr.hir_id, res);
-                ExprKind::NamedConst { def_id, substs, user_ty: user_ty }
+                ExprKind::NamedConst { def_id, substs, user_ty }
             }
 
             Res::Def(DefKind::Ctor(_, CtorKind::Const), def_id) => {
                 let user_provided_types = self.typeck_results.user_provided_types();
-                let user_provided_type = user_provided_types.get(expr.hir_id).copied();
-                debug!("convert_path_expr: user_provided_type={:?}", user_provided_type);
+                let user_ty = user_provided_types.get(expr.hir_id).copied().map(Box::new);
+                debug!("convert_path_expr: user_ty={:?}", user_ty);
                 let ty = self.typeck_results().node_type(expr.hir_id);
                 match ty.kind() {
                     // A unit struct/variant which is used as a value.
                     // We return a completely different ExprKind here to account for this special case.
-                    ty::Adt(adt_def, substs) => ExprKind::Adt(Box::new(Adt {
+                    ty::Adt(adt_def, substs) => ExprKind::Adt(Box::new(AdtExpr {
                         adt_def: *adt_def,
                         variant_index: adt_def.variant_index_with_ctor_id(def_id),
                         substs,
-                        user_ty: user_provided_type,
+                        user_ty,
                         fields: Box::new([]),
                         base: None,
                     })),
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index 7c2f4db94ff..e8ce8e6f23e 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -311,8 +311,15 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
             //     bar::<{ N + 1 }>();
             // }
             // ```
-            ExprKind::Block { body: thir::Block { stmts: box [], expr: Some(e), .. } } => {
-                self.recurse_build(*e)?
+            ExprKind::Block { block } => {
+                if let thir::Block { stmts: box [], expr: Some(e), .. } = &self.body.blocks[*block] {
+                    self.recurse_build(*e)?
+                } else {
+                    self.maybe_supported_error(
+                        node.span,
+                        "blocks are not supported in generic constant",
+                    )?
+                }
             }
             // `ExprKind::Use` happens when a `hir::ExprKind::Cast` is a
             // "coercion cast" i.e. using a coercion or is a no-op.
@@ -349,10 +356,6 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                 node.span,
                 "array construction is not supported in generic constants",
             )?,
-            ExprKind::Block { .. } => self.maybe_supported_error(
-                node.span,
-                "blocks are not supported in generic constant",
-            )?,
             ExprKind::NeverToAny { .. } => self.maybe_supported_error(
                 node.span,
                 "converting nevers to any is not supported in generic constant",
diff --git a/src/test/ui/thir-tree.stdout b/src/test/ui/thir-tree.stdout
index 3c84be8e8f8..960b7f7f4dd 100644
--- a/src/test/ui/thir-tree.stdout
+++ b/src/test/ui/thir-tree.stdout
@@ -1,6 +1,17 @@
 DefId(0:3 ~ thir_tree[8f1d]::main):
 Thir {
     arms: [],
+    blocks: [
+        Block {
+            targeted_by_break: false,
+            region_scope: Node(1),
+            opt_destruction_scope: None,
+            span: $DIR/thir-tree.rs:4:15: 4:17 (#0),
+            stmts: [],
+            expr: None,
+            safety_mode: Safe,
+        },
+    ],
     exprs: [
         Expr {
             ty: (),
@@ -9,15 +20,7 @@ Thir {
             ),
             span: $DIR/thir-tree.rs:4:15: 4:17 (#0),
             kind: Block {
-                body: Block {
-                    targeted_by_break: false,
-                    region_scope: Node(1),
-                    opt_destruction_scope: None,
-                    span: $DIR/thir-tree.rs:4:15: 4:17 (#0),
-                    stmts: [],
-                    expr: None,
-                    safety_mode: Safe,
-                },
+                block: b0,
             },
         },
         Expr {