diff --git a/src/Init/Data/String/Extra.lean b/src/Init/Data/String/Extra.lean
index bcb125052c..5be0a740b7 100644
--- a/src/Init/Data/String/Extra.lean
+++ b/src/Init/Data/String/Extra.lean
@@ -33,6 +33,10 @@ opaque fromUTF8Unchecked (a : @& ByteArray) : String
 @[extern "lean_string_to_utf8"]
 opaque toUTF8 (a : @& String) : ByteArray
 
+/-- Accesses a byte in the UTF-8 encoding of the `String`. O(1) -/
+@[extern "lean_string_get_byte_fast"]
+opaque getUtf8Byte (s : @& String) (n : Nat) (h : n < s.utf8ByteSize) : UInt8
+
 theorem Iterator.sizeOf_next_lt_of_hasNext (i : String.Iterator) (h : i.hasNext) : sizeOf i.next < sizeOf i := by
   cases i; rename_i s pos; simp [Iterator.next, Iterator.sizeOf_eq]; simp [Iterator.hasNext] at h
   exact Nat.sub_lt_sub_left h (String.lt_next s pos)
diff --git a/src/Lean/Data/Trie.lean b/src/Lean/Data/Trie.lean
index 848284eb22..0756c3269d 100644
--- a/src/Lean/Data/Trie.lean
+++ b/src/Lean/Data/Trie.lean
@@ -1,110 +1,202 @@
 /-
 Copyright (c) 2018 Microsoft Corporation. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Author: Sebastian Ullrich, Leonardo de Moura
+Author: Sebastian Ullrich, Leonardo de Moura, Joachim Breitner
 
-Trie for tokenizing the Lean language
+A string trie data structure, used for tokenizing the Lean language
 -/
 import Lean.Data.Format
 
 namespace Lean
-namespace Parser
+namespace Data
 
+/-
+## Implementation notes
+
+Tries have typically many nodes with small degree, where a linear scan
+through the (compact) `ByteArray` is faster than using binary search or
+search trees like `RBTree`.
+
+Moreover, many nodes have degree 1, which justifies the special case `Node1`
+constructor.
+
+The code would be a bit less repetitive if we used something like the following
+```
+mutual
+def Trie α := Option α × ByteAssoc α
+
+inductive ByteAssoc α where
+  | leaf : Trie α
+  | node1 : UInt8 → Trie α → Trie α
+  | node : ByteArray → Array (Trie α) → Trie α
+end
+```
+but that would come at the cost of extra indirections.
+-/
+
+/-- A Trie is a key-value store where the keys are of type `String`,
+and the internal structure is a tree that branches on the bytes of the string.  -/
 inductive Trie (α : Type) where
-  | Node : Option α → RBNode Char (fun _ => Trie α) → Trie α
+  | leaf : Option α → Trie α
+  | node1 : Option α → UInt8 → Trie α → Trie α
+  | node : Option α → ByteArray → Array (Trie α) → Trie α
 
 namespace Trie
 variable {α : Type}
 
-def empty : Trie α :=
-  ⟨none, RBNode.leaf⟩
+/-- The empty `Trie` -/
+def empty : Trie α := leaf none
 
 instance : EmptyCollection (Trie α) :=
   ⟨empty⟩
 
 instance : Inhabited (Trie α) where
-  default := Node none RBNode.leaf
+  default := empty
 
-partial def insert (t : Trie α) (s : String) (val : α) : Trie α :=
-  let rec insertEmpty (i : String.Pos) : Trie α :=
-    match s.atEnd i with
-    | true => Trie.Node (some val) RBNode.leaf
-    | false =>
-      let c := s.get i
-      let t := insertEmpty (s.next i)
-      Trie.Node none (RBNode.singleton c t)
+/-- Insert or update the value at a the given key `s`.  -/
+partial def upsert (t : Trie α) (s : String) (f : Option α → α) : Trie α :=
+  let rec insertEmpty (i : Nat) : Trie α :=
+    if h : i < s.utf8ByteSize then
+      let c := s.getUtf8Byte i h
+      let t := insertEmpty (i + 1)
+      node1 none c t
+    else
+      leaf (f .none)
   let rec loop
-    | Trie.Node v m, i =>
-      match s.atEnd i with
-      | true  => Trie.Node (some val) m -- overrides old value
-      | false =>
-        let c := s.get i
-        let i := s.next i
-        let t := match RBNode.find compare m c with
-          | none   => insertEmpty i
-          | some t => loop t i
-        Trie.Node v (RBNode.insert compare m c t)
-  loop t 0
+    | i, leaf v =>
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        let t := insertEmpty (i + 1)
+        node1 v c t
+      else
+        leaf (f v)
+    | i, node1 v c' t' =>
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        if c == c'
+        then node1 v c' (loop (i + 1) t')
+        else 
+          let t := insertEmpty (i + 1)
+          node v (.mk #[c, c']) #[t, t']
+      else
+        node1 (f v) c' t'
+    | i, node v cs ts =>
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        match cs.findIdx? (· == c) with
+          | none   =>
+            let t := insertEmpty (i + 1)
+            node v (cs.push c) (ts.push t)
+          | some idx =>
+            node v cs (ts.modify idx (loop (i + 1)))
+      else
+        node (f v) cs ts
+  loop 0 t
 
+/-- Inserts a value at a the given key `s`, overriding an existing value if present. -/
+partial def insert (t : Trie α) (s : String) (val : α) : Trie α :=
+  upsert t s (fun _ => val)
+
+/-- Looks up a value at the given key `s`.  -/
 partial def find? (t : Trie α) (s : String) : Option α :=
   let rec loop
-    | Trie.Node val m, i =>
-      match s.atEnd i with
-      | true  => val
-      | false =>
-        let c := s.get i
-        let i := s.next i
-        match RBNode.find compare m c with
+    | i, leaf val =>
+      if i < s.utf8ByteSize then
+        none
+      else
+        val
+    | i, node1 val c' t' =>
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        if c == c'
+        then loop (i + 1) t'
+        else none
+      else
+        val
+    | i, node val cs ts =>
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        match cs.findIdx? (· == c) with
         | none   => none
-        | some t => loop t i
-  loop t 0
+        | some idx => loop (i + 1) (ts.get! idx)
+      else
+        val
+  loop 0 t
 
-/-- Return values that match the given `prefix` -/
-partial def findPrefix (t : Trie α) (pre : String) : Array α :=
-  go t 0 |>.run #[] |>.2
-where
-  go (t : Trie α) (i : String.Pos) : StateM (Array α) Unit :=
-    if pre.atEnd i then
-      collect t
-    else
-      let k := pre.get i
-      let i := pre.next i
-      let ⟨_, cs⟩ := t
-      cs.forM fun k' c => do
-        if k == k' then go c i
+/-- Returns an `Array` of all values in the trie, in no particular order. -/
+partial def values (t : Trie α) : Array α := go t |>.run #[] |>.2
+  where
+    go : Trie α → StateM (Array α) Unit
+      | leaf a? => do
+        if let some a := a? then
+          modify (·.push a)
+      | node1 a? _ t' => do
+        if let some a := a? then
+          modify (·.push a)
+        go t'
+      | node a? _ ts => do
+        if let some a := a? then
+          modify (·.push a)
+        ts.forM fun t' => go t'
 
-  collect (t : Trie α) : StateM (Array α) Unit := do
-    let ⟨a?, cs⟩ := t
-    if let some a := a? then
-      modify (·.push a)
-    cs.forM fun _ c => collect c
+/-- Returns all values whose key have the given string `pre` as a prefix, in no particular order. -/
+partial def findPrefix (t : Trie α) (pre : String) : Array α := go t 0
+  where
+    go (t : Trie α) (i : Nat) : Array α :=
+      if h : i < pre.utf8ByteSize then
+        let c := pre.getUtf8Byte i h
+        match t with
+        | leaf _val => .empty
+        | node1 _val c' t' =>
+          if c == c'
+          then go t' (i + 1)
+          else .empty
+        | node _val cs ts =>
+          match cs.findIdx? (· == c) with
+          | none   => .empty
+          | some idx => go (ts.get! idx) (i + 1)
+      else
+        t.values
 
-private def updtAcc (v : Option α) (i : String.Pos) (acc : String.Pos × Option α) : String.Pos × Option α :=
-  match v, acc with
-  | some v, (_, _) => (i, some v)  -- we pattern match on `acc` to enable memory reuse
-  | none,   acc    => acc
-
-partial def matchPrefix (s : String) (t : Trie α) (i : String.Pos) : String.Pos × Option α :=
+/-- Find the longest _key_ in the trie that is contained in the given string `s` at position `i`,
+and return the associated value. -/
+partial def matchPrefix (s : String) (t : Trie α) (i : String.Pos) : Option α :=
   let rec loop
-    | Trie.Node v m, i, acc =>
-      match s.atEnd i with
-      | true  => updtAcc v i acc
-      | false =>
-        let acc := updtAcc v i acc
-        let c   := s.get i
-        let i   := s.next i
-        match RBNode.find compare m c with
-        | some t => loop t i acc
-        | none   => acc
-  loop t i (i, none)
+    | leaf v, _, res =>
+      if v.isSome then v else res
+    | node1 v c' t', i, res =>
+      let res := if v.isSome then v else res
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        if c == c'
+        then loop t' (i + 1) res
+        else res
+      else
+        res
+    | node v cs ts, i, res =>
+      let res := if v.isSome then v else res
+      if h : i < s.utf8ByteSize then
+        let c := s.getUtf8Byte i h
+        match cs.findIdx? (· == c) with
+        | none => res
+        | some idx => loop (ts.get! idx) (i + 1) res
+      else
+        res
+  loop t i.byteIdx none
 
 private partial def toStringAux {α : Type} : Trie α → List Format
-  | Trie.Node _ map => map.fold (fun Fs c t =>
-   format (repr c) :: (Format.group $ Format.nest 2 $ flip Format.joinSep Format.line $ toStringAux t) :: Fs) []
+  | leaf _ => []
+  | node1 _ c t =>
+    [ format (repr c), Format.group $ Format.nest 4 $ flip Format.joinSep Format.line $ toStringAux t ]
+  | node _ cs ts =>
+    List.join $ List.zipWith (fun c t =>
+      [ format (repr c), (Format.group $ Format.nest 4 $ flip Format.joinSep Format.line $ toStringAux t) ]
+    ) cs.toList ts.toList
 
 instance {α : Type} : ToString (Trie α) :=
   ⟨fun t => (flip Format.joinSep Format.line $ toStringAux t).pretty⟩
+
 end Trie
 
-end Parser
+end Data
 end Lean
diff --git a/src/Lean/Parser/Basic.lean b/src/Lean/Parser/Basic.lean
index 160bf8e1d7..02c950885c 100644
--- a/src/Lean/Parser/Basic.lean
+++ b/src/Lean/Parser/Basic.lean
@@ -821,7 +821,7 @@ private def tokenFnAux : ParserFn := fun c s =>
   else if curr == '`' && isIdFirstOrBeginEscape (getNext input i) then
     nameLitAux i c s
   else
-    let (_, tk) := c.tokens.matchPrefix input i
+    let tk := c.tokens.matchPrefix input i
     identFnAux i tk .anonymous c s
 
 private def updateTokenCache (startPos : String.Pos) (s : ParserState) : ParserState :=
diff --git a/src/Lean/Parser/Types.lean b/src/Lean/Parser/Types.lean
index bba0735a41..d2663a016a 100644
--- a/src/Lean/Parser/Types.lean
+++ b/src/Lean/Parser/Types.lean
@@ -31,7 +31,7 @@ def minPrec  : Nat := eval_prec min
 
 abbrev Token := String
 
-abbrev TokenTable := Trie Token
+abbrev TokenTable := Lean.Data.Trie Token
 
 abbrev SyntaxNodeKindSet := PersistentHashMap SyntaxNodeKind Unit
 
diff --git a/src/include/lean/lean.h b/src/include/lean/lean.h
index 2088ea4144..45a4bd0abd 100644
--- a/src/include/lean/lean.h
+++ b/src/include/lean/lean.h
@@ -1014,6 +1014,11 @@ static inline uint32_t lean_string_utf8_get_fast(b_lean_obj_arg s, b_lean_obj_ar
   if ((c & 0x80) == 0) return c;
   return lean_string_utf8_get_fast_cold(str, idx, lean_string_size(s), c);
 }
+static inline uint8_t lean_string_get_byte_fast(b_lean_obj_arg s, b_lean_obj_arg i) {
+  char const * str = lean_string_cstr(s);
+  size_t idx = lean_unbox(i);
+  return str[idx];
+}
 
 LEAN_SHARED lean_obj_res lean_string_utf8_next(b_lean_obj_arg s, b_lean_obj_arg i);
 LEAN_SHARED lean_obj_res lean_string_utf8_next_fast_cold(size_t i, unsigned char c);
diff --git a/stage0/src/stdlib_flags.h b/stage0/src/stdlib_flags.h
index a574103a7f..296e3f9b3a 100644
--- a/stage0/src/stdlib_flags.h
+++ b/stage0/src/stdlib_flags.h
@@ -6,7 +6,7 @@ options get_default_options() {
     // see https://leanprover.github.io/lean4/doc/dev/bootstrap.html#further-bootstrapping-complications
 #if LEAN_IS_STAGE0 == 1
     // switch to `true` for ABI-breaking changes affecting meta code
-    opts = opts.update({"interpreter", "prefer_native"}, false);
+    opts = opts.update({"interpreter", "prefer_native"}, true);
     // switch to `true` for changing built-in parsers used in quotations
     opts = opts.update({"internal", "parseQuotWithCurrentStage"}, false);
     // toggling `parseQuotWithCurrentStage` may also require toggling the following option if macros/syntax
diff --git a/tests/compiler/trie.lean b/tests/compiler/trie.lean
new file mode 100644
index 0000000000..ea7ee85f51
--- /dev/null
+++ b/tests/compiler/trie.lean
@@ -0,0 +1,88 @@
+import Lean.Data.Trie
+
+/-!
+
+# Tests for the trie data structure
+
+This test tests the `Lean.Parser.Trie` data structure by bisimulation with a simple `Array String`:
+It performs a sequence of trie creation steps, and after each steps checks
+whether the trie is operationally equivalent to the array of strings.
+
+This test does not bother with values that are different than they `String` they are stored under.
+This test does not test `upsert`; since `Trie.insert` goes through it, it should be sufficient
+(and it would make this test approach more complicated.)
+-/
+
+open Lean.Data
+
+/-- These keys used in `T.check` below. Also include keys for negative lookup tests here!  -/
+def keys : Array String :=  #[
+  "",
+  "h",
+  "hello",
+  "helloo",
+  "hellooo",
+  "helloooooo",
+  "hella",
+  "hellx",
+  "hö",
+  "hü",
+  "hä",
+  "💩"
+]
+
+/-- A trie together with a reference value as an array of values -/
+def T := Trie String × Array String
+
+def T.empty : T := (.empty, .empty)
+
+def T.insert : T → String → T := fun (t,a) s =>
+  (t.insert s s, if a.contains s then a else a.push s)
+
+/-- A convenience function for use in this test case -/
+def Array.sorted : Array String → Array String := fun a =>
+  a.qsort (fun s1 s2 => s1 < s2)
+
+/-- The intendend semanics of `Trie.findPrefix` -/
+def Array.findPrefix : Array String → String → Array String := fun a s =>
+  a.filter (fun s' => s.isPrefixOf s')
+
+/-- The intendend semanics of `Trie.matchPrefix`: Longest prefix found in trie -/
+def Array.matchPrefix : Array String → String → Option String := fun a s => Id.run do
+  for i in List.reverse (List.range (s.length + 1)) do
+    let pfix := s.take i
+    if let some _ := a.find? (· == pfix) then
+      return some  pfix
+  return none
+
+
+def T.check : T → IO Unit := fun (t,a) => do
+  -- Check lookup equivalence
+  keys.forM fun s => do
+    unless t.find? s = a.find? (· == s) do
+      IO.println s!"find? differs: key = {s}"
+  -- Check findPrefix equivalence
+  keys.forM fun s => do
+    unless (t.findPrefix s).sorted = (a.findPrefix s).sorted do
+      IO.println s!"findPrefix differs: key = {s}"
+  -- Check matchPrefix equivalence
+  keys.forM fun s => do
+    unless t.matchPrefix s 0 = a.matchPrefix s do
+      IO.println s!"matchPrefix differs: key = {s}, got: {t.matchPrefix s 0} exp: {a.matchPrefix s} "
+    let s' := "somePrefix" ++ s
+    unless t.matchPrefix s' ((0 : String.Pos) + "somePrefix") = a.matchPrefix s do
+      IO.println s!"matchPrefix differs (with prefix): key = {s}"
+
+def main : IO Unit := do
+  -- Add tricky insert sequences here:
+  for seq in #[
+    #["hello", "hella", "hellooo", "h", "hö", "hü", "💩", "", "hü"],
+    #["", "helooooo"]
+  ] do
+    IO.println "Resetting trie"
+    let mut t : T := T.empty
+    t.check
+    for s in seq do
+      IO.println s!"Inserting {s}"
+      t := t.insert s
+      t.check
diff --git a/tests/compiler/trie.lean.expected.out b/tests/compiler/trie.lean.expected.out
new file mode 100644
index 0000000000..384d6785cf
--- /dev/null
+++ b/tests/compiler/trie.lean.expected.out
@@ -0,0 +1,13 @@
+Resetting trie
+Inserting hello
+Inserting hella
+Inserting hellooo
+Inserting h
+Inserting hö
+Inserting hü
+Inserting 💩
+Inserting 
+Inserting hü
+Resetting trie
+Inserting 
+Inserting helooooo
diff --git a/tests/playground/flat_parser2.lean b/tests/playground/flat_parser2.lean
index 44708caf52..4de9794453 100644
--- a/tests/playground/flat_parser2.lean
+++ b/tests/playground/flat_parser2.lean
@@ -168,8 +168,8 @@ do tk ← monadLift peekToken,
    | Syntax.atom ⟨_, sym⟩ := do
      cfg ← readCfg,
      (match cfg.tokens.matchPrefix sym.mkIterator with
-      | some ⟨_, tkCfg⟩ := pure tkCfg.lbp
-      | _                := error "currLbp: unreachable")
+      | some tkCfg := pure tkCfg.lbp
+      | _          := error "currLbp: unreachable")
    | Syntax.rawNode {kind := @number, ..}     := pure maxPrec
    | Syntax.rawNode {kind := @stringLit, ..} := pure maxPrec
    | Syntax.ident _                            := pure maxPrec
@@ -182,7 +182,7 @@ do tk ← monadLift peekToken,
    match tk with
    | Syntax.atom ⟨_, sym⟩ := do
      cfg ← read,
-     -- some ⟨_, tkCfg⟩ ← pure (cfg.tokens.matchPrefix sym.mkIterator) | error "currLbp: unreachable",
+     -- some tkCfg ← pure (cfg.tokens.matchPrefix sym.mkIterator) | error "currLbp: unreachable",
      pure 0
    | Syntax.ident _ := pure maxPrec
    | Syntax.rawNode {kind := @number, ..} := pure maxPrec
diff --git a/tests/playground/parser/parser.lean b/tests/playground/parser/parser.lean
index ed89a833a5..555c3756a3 100644
--- a/tests/playground/parser/parser.lean
+++ b/tests/playground/parser/parser.lean
@@ -764,7 +764,7 @@ private def tokenFnAux : BasicParserFn
   else if c.isDigit then
     numberFnAux s d
   else
-    let (_, tk) := cfg.tokens.matchPrefix s i in
+    let tk := cfg.tokens.matchPrefix s i in
     identFnAux i tk Name.anonymous s d
 
 private def updateCache (startPos : Nat) (d : ParserData) : ParserData :=