diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index 83288fa541e44..3ef1155b91688 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -9,8 +9,9 @@ use rustc_session::lint::Level;
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::Span;
 
-struct UnsafetyVisitor<'tcx> {
+struct UnsafetyVisitor<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
+    thir: &'a Thir<'tcx>,
     /// The `HirId` of the current scope, which would be the `HirId`
     /// of the current HIR node, modulo adjustments. Used for lint levels.
     hir_context: hir::HirId,
@@ -20,7 +21,7 @@ struct UnsafetyVisitor<'tcx> {
     body_unsafety: BodyUnsafety,
 }
 
-impl<'tcx> UnsafetyVisitor<'tcx> {
+impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
     fn in_safety_context<R>(
         &mut self,
         safety_context: SafetyContext,
@@ -127,8 +128,12 @@ impl<'tcx> UnsafetyVisitor<'tcx> {
     }
 }
 
-impl<'thir, 'tcx> Visitor<'thir, 'tcx> for UnsafetyVisitor<'tcx> {
-    fn visit_block(&mut self, block: &Block<'thir, 'tcx>) {
+impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
+    fn thir(&self) -> &'a Thir<'tcx> {
+        &self.thir
+    }
+
+    fn visit_block(&mut self, block: &Block) {
         if let BlockSafety::ExplicitUnsafe(hir_id) = block.safety_mode {
             self.in_safety_context(
                 SafetyContext::UnsafeBlock { span: block.span, hir_id, used: false },
@@ -139,17 +144,17 @@ impl<'thir, 'tcx> Visitor<'thir, 'tcx> for UnsafetyVisitor<'tcx> {
         }
     }
 
-    fn visit_expr(&mut self, expr: &'thir Expr<'thir, 'tcx>) {
+    fn visit_expr(&mut self, expr: &Expr<'tcx>) {
         match expr.kind {
             ExprKind::Scope { value, lint_level: LintLevel::Explicit(hir_id), region_scope: _ } => {
                 let prev_id = self.hir_context;
                 self.hir_context = hir_id;
-                self.visit_expr(value);
+                self.visit_expr(&self.thir[value]);
                 self.hir_context = prev_id;
                 return;
             }
             ExprKind::Call { fun, ty: _, args: _, from_hir_call: _, fn_span: _ } => {
-                if fun.ty.fn_sig(self.tcx).unsafety() == hir::Unsafety::Unsafe {
+                if self.thir[fun].ty.fn_sig(self.tcx).unsafety() == hir::Unsafety::Unsafe {
                     self.requires_unsafe(expr.span, CallToUnsafeFunction);
                 }
             }
@@ -293,7 +298,12 @@ impl UnsafeOpKind {
 
 // FIXME: checking unsafety for closures should be handled by their parent body,
 // as they inherit their "safety context" from their declaration site.
-pub fn check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, thir: &Expr<'_, 'tcx>, hir_id: hir::HirId) {
+pub fn check_unsafety<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    thir: &Thir<'tcx>,
+    expr: ExprId,
+    hir_id: hir::HirId,
+) {
     let body_unsafety = tcx.hir().fn_sig_by_hir_id(hir_id).map_or(BodyUnsafety::Safe, |fn_sig| {
         if fn_sig.header.unsafety == hir::Unsafety::Unsafe {
             BodyUnsafety::Unsafe(fn_sig.span)
@@ -303,8 +313,9 @@ pub fn check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, thir: &Expr<'_, 'tcx>, hir_id: hi
     });
     let safety_context =
         if body_unsafety.is_unsafe() { SafetyContext::UnsafeFn } else { SafetyContext::Safe };
-    let mut visitor = UnsafetyVisitor { tcx, safety_context, hir_context: hir_id, body_unsafety };
-    visitor.visit_expr(thir);
+    let mut visitor =
+        UnsafetyVisitor { tcx, thir, safety_context, hir_context: hir_id, body_unsafety };
+    visitor.visit_expr(&thir[expr]);
 }
 
 crate fn thir_check_unsafety_inner<'tcx>(
@@ -314,10 +325,8 @@ crate fn thir_check_unsafety_inner<'tcx>(
     let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
     let body_id = tcx.hir().body_owned_by(hir_id);
     let body = tcx.hir().body(body_id);
-
-    let arena = Arena::default();
-    let thir = cx::build_thir(tcx, def, &arena, &body.value);
-    check_unsafety(tcx, thir, hir_id);
+    let (thir, expr) = cx::build_thir(tcx, def, &body.value);
+    check_unsafety(tcx, &thir, expr, hir_id);
 }
 
 crate fn thir_check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) {
diff --git a/compiler/rustc_mir_build/src/thir/visit.rs b/compiler/rustc_mir_build/src/thir/visit.rs
index 9c5b07e2b2ada..671d1fe9b0305 100644
--- a/compiler/rustc_mir_build/src/thir/visit.rs
+++ b/compiler/rustc_mir_build/src/thir/visit.rs
@@ -1,123 +1,124 @@
 use crate::thir::*;
 
-pub trait Visitor<'thir, 'tcx>: Sized {
-    fn visit_expr(&mut self, expr: &'thir Expr<'thir, 'tcx>) {
+pub trait Visitor<'a, 'tcx: 'a>: Sized {
+    fn thir(&self) -> &'a Thir<'tcx>;
+
+    fn visit_expr(&mut self, expr: &Expr<'tcx>) {
         walk_expr(self, expr);
     }
 
-    fn visit_stmt(&mut self, stmt: &'thir Stmt<'thir, 'tcx>) {
+    fn visit_stmt(&mut self, stmt: &Stmt<'tcx>) {
         walk_stmt(self, stmt);
     }
 
-    fn visit_block(&mut self, block: &Block<'thir, 'tcx>) {
+    fn visit_block(&mut self, block: &Block) {
         walk_block(self, block);
     }
 
-    fn visit_arm(&mut self, arm: &'thir Arm<'thir, 'tcx>) {
+    fn visit_arm(&mut self, arm: &Arm<'tcx>) {
         walk_arm(self, arm);
     }
 
     fn visit_const(&mut self, _cnst: &'tcx Const<'tcx>) {}
 }
 
-pub fn walk_expr<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
-    visitor: &mut V,
-    expr: &'thir Expr<'thir, 'tcx>,
-) {
+pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Expr<'tcx>) {
     use ExprKind::*;
     match expr.kind {
-        Scope { value, region_scope: _, lint_level: _ } => visitor.visit_expr(value),
-        Box { value } => visitor.visit_expr(value),
+        Scope { value, region_scope: _, lint_level: _ } => {
+            visitor.visit_expr(&visitor.thir()[value])
+        }
+        Box { value } => visitor.visit_expr(&visitor.thir()[value]),
         If { cond, then, else_opt } => {
-            visitor.visit_expr(cond);
-            visitor.visit_expr(then);
+            visitor.visit_expr(&visitor.thir()[cond]);
+            visitor.visit_expr(&visitor.thir()[then]);
             if let Some(else_expr) = else_opt {
-                visitor.visit_expr(else_expr);
+                visitor.visit_expr(&visitor.thir()[else_expr]);
             }
         }
-        Call { fun, args, ty: _, from_hir_call: _, fn_span: _ } => {
-            visitor.visit_expr(fun);
-            for arg in args {
-                visitor.visit_expr(arg);
+        Call { fun, ref args, ty: _, from_hir_call: _, fn_span: _ } => {
+            visitor.visit_expr(&visitor.thir()[fun]);
+            for &arg in &**args {
+                visitor.visit_expr(&visitor.thir()[arg]);
             }
         }
-        Deref { arg } => visitor.visit_expr(arg),
+        Deref { arg } => visitor.visit_expr(&visitor.thir()[arg]),
         Binary { lhs, rhs, op: _ } | LogicalOp { lhs, rhs, op: _ } => {
-            visitor.visit_expr(lhs);
-            visitor.visit_expr(rhs);
-        }
-        Unary { arg, op: _ } => visitor.visit_expr(arg),
-        Cast { source } => visitor.visit_expr(source),
-        Use { source } => visitor.visit_expr(source),
-        NeverToAny { source } => visitor.visit_expr(source),
-        Pointer { source, cast: _ } => visitor.visit_expr(source),
-        Loop { body } => visitor.visit_expr(body),
-        Match { scrutinee, arms } => {
-            visitor.visit_expr(scrutinee);
-            for arm in arms {
-                visitor.visit_arm(arm);
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[rhs]);
+        }
+        Unary { arg, op: _ } => visitor.visit_expr(&visitor.thir()[arg]),
+        Cast { source } => visitor.visit_expr(&visitor.thir()[source]),
+        Use { source } => visitor.visit_expr(&visitor.thir()[source]),
+        NeverToAny { source } => visitor.visit_expr(&visitor.thir()[source]),
+        Pointer { source, cast: _ } => visitor.visit_expr(&visitor.thir()[source]),
+        Loop { body } => visitor.visit_expr(&visitor.thir()[body]),
+        Match { scrutinee, ref arms } => {
+            visitor.visit_expr(&visitor.thir()[scrutinee]);
+            for &arm in &**arms {
+                visitor.visit_arm(&visitor.thir()[arm]);
             }
         }
         Block { ref body } => visitor.visit_block(body),
         Assign { lhs, rhs } | AssignOp { lhs, rhs, op: _ } => {
-            visitor.visit_expr(lhs);
-            visitor.visit_expr(rhs);
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[rhs]);
         }
-        Field { lhs, name: _ } => visitor.visit_expr(lhs),
+        Field { lhs, name: _ } => visitor.visit_expr(&visitor.thir()[lhs]),
         Index { lhs, index } => {
-            visitor.visit_expr(lhs);
-            visitor.visit_expr(index);
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[index]);
         }
         VarRef { id: _ } | UpvarRef { closure_def_id: _, var_hir_id: _ } => {}
-        Borrow { arg, borrow_kind: _ } => visitor.visit_expr(arg),
-        AddressOf { arg, mutability: _ } => visitor.visit_expr(arg),
+        Borrow { arg, borrow_kind: _ } => visitor.visit_expr(&visitor.thir()[arg]),
+        AddressOf { arg, mutability: _ } => visitor.visit_expr(&visitor.thir()[arg]),
         Break { value, label: _ } => {
             if let Some(value) = value {
-                visitor.visit_expr(value)
+                visitor.visit_expr(&visitor.thir()[value])
             }
         }
         Continue { label: _ } => {}
         Return { value } => {
             if let Some(value) = value {
-                visitor.visit_expr(value)
+                visitor.visit_expr(&visitor.thir()[value])
             }
         }
         ConstBlock { value } => visitor.visit_const(value),
         Repeat { value, count } => {
-            visitor.visit_expr(value);
+            visitor.visit_expr(&visitor.thir()[value]);
             visitor.visit_const(count);
         }
-        Array { fields } | Tuple { fields } => {
-            for field in fields {
-                visitor.visit_expr(field);
+        Array { ref fields } | Tuple { ref fields } => {
+            for &field in &**fields {
+                visitor.visit_expr(&visitor.thir()[field]);
             }
         }
-        Adt { fields, ref base, adt_def: _, variant_index: _, substs: _, user_ty: _ } => {
-            for field in fields {
-                visitor.visit_expr(field.expr);
+        Adt { ref fields, ref base, adt_def: _, variant_index: _, substs: _, user_ty: _ } => {
+            for field in &**fields {
+                visitor.visit_expr(&visitor.thir()[field.expr]);
             }
             if let Some(base) = base {
-                visitor.visit_expr(base.base);
+                visitor.visit_expr(&visitor.thir()[base.base]);
             }
         }
         PlaceTypeAscription { source, user_ty: _ } | ValueTypeAscription { source, user_ty: _ } => {
-            visitor.visit_expr(source)
+            visitor.visit_expr(&visitor.thir()[source])
         }
         Closure { closure_id: _, substs: _, upvars: _, movability: _, fake_reads: _ } => {}
         Literal { literal, user_ty: _, const_id: _ } => visitor.visit_const(literal),
         StaticRef { literal, def_id: _ } => visitor.visit_const(literal),
-        InlineAsm { operands, template: _, options: _, line_spans: _ } => {
-            for op in operands {
+        InlineAsm { ref operands, template: _, options: _, line_spans: _ } => {
+            for op in &**operands {
                 use InlineAsmOperand::*;
                 match op {
                     In { expr, reg: _ }
                     | Out { expr: Some(expr), reg: _, late: _ }
                     | InOut { expr, reg: _, late: _ }
-                    | SymFn { expr } => visitor.visit_expr(expr),
+                    | SymFn { expr } => visitor.visit_expr(&visitor.thir()[*expr]),
                     SplitInOut { in_expr, out_expr, reg: _, late: _ } => {
-                        visitor.visit_expr(in_expr);
+                        visitor.visit_expr(&visitor.thir()[*in_expr]);
                         if let Some(out_expr) = out_expr {
-                            visitor.visit_expr(out_expr);
+                            visitor.visit_expr(&visitor.thir()[*out_expr]);
                         }
                     }
                     Out { expr: None, reg: _, late: _ }
@@ -127,24 +128,21 @@ pub fn walk_expr<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
             }
         }
         ThreadLocalRef(_) => {}
-        LlvmInlineAsm { outputs, inputs, asm: _ } => {
-            for out_expr in outputs {
-                visitor.visit_expr(out_expr);
+        LlvmInlineAsm { ref outputs, ref inputs, asm: _ } => {
+            for &out_expr in &**outputs {
+                visitor.visit_expr(&visitor.thir()[out_expr]);
             }
-            for in_expr in inputs {
-                visitor.visit_expr(in_expr);
+            for &in_expr in &**inputs {
+                visitor.visit_expr(&visitor.thir()[in_expr]);
             }
         }
-        Yield { value } => visitor.visit_expr(value),
+        Yield { value } => visitor.visit_expr(&visitor.thir()[value]),
     }
 }
 
-pub fn walk_stmt<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
-    visitor: &mut V,
-    stmt: &'thir Stmt<'thir, 'tcx>,
-) {
+pub fn walk_stmt<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, stmt: &Stmt<'tcx>) {
     match stmt.kind {
-        StmtKind::Expr { expr, scope: _ } => visitor.visit_expr(expr),
+        StmtKind::Expr { expr, scope: _ } => visitor.visit_expr(&visitor.thir()[expr]),
         StmtKind::Let {
             initializer,
             remainder_scope: _,
@@ -153,34 +151,28 @@ pub fn walk_stmt<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
             lint_level: _,
         } => {
             if let Some(init) = initializer {
-                visitor.visit_expr(init);
+                visitor.visit_expr(&visitor.thir()[init]);
             }
         }
     }
 }
 
-pub fn walk_block<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
-    visitor: &mut V,
-    block: &Block<'thir, 'tcx>,
-) {
-    for stmt in block.stmts {
-        visitor.visit_stmt(stmt);
+pub fn walk_block<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, block: &Block) {
+    for &stmt in &*block.stmts {
+        visitor.visit_stmt(&visitor.thir()[stmt]);
     }
     if let Some(expr) = block.expr {
-        visitor.visit_expr(expr);
+        visitor.visit_expr(&visitor.thir()[expr]);
     }
 }
 
-pub fn walk_arm<'thir, 'tcx, V: Visitor<'thir, 'tcx>>(
-    visitor: &mut V,
-    arm: &'thir Arm<'thir, 'tcx>,
-) {
+pub fn walk_arm<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, arm: &Arm<'tcx>) {
     match arm.guard {
-        Some(Guard::If(expr)) => visitor.visit_expr(expr),
+        Some(Guard::If(expr)) => visitor.visit_expr(&visitor.thir()[expr]),
         Some(Guard::IfLet(ref _pat, expr)) => {
-            visitor.visit_expr(expr);
+            visitor.visit_expr(&visitor.thir()[expr]);
         }
         None => {}
     }
-    visitor.visit_expr(arm.body);
+    visitor.visit_expr(&visitor.thir()[arm.body]);
 }