This patch revert 4b989009e3839cfc6f021d1552a46561cee6cde2 (CONST LOOPS ARE HERE).

diff --git a/crates/parser/src/grammar/expressions.rs b/crates/parser/src/grammar/expressions.rs
index 5f885edfd..e72929f8c 100644
--- a/crates/parser/src/grammar/expressions.rs
+++ b/crates/parser/src/grammar/expressions.rs
@@ -316,7 +316,7 @@ fn expr_bp(p: &mut Parser, mut r: Restrictions, bp: u8) -> (Option<CompletedMark
 }

 const LHS_FIRST: TokenSet =
-    atom::ATOM_EXPR_FIRST.union(TokenSet::new(&[T![&], T![*], T![!], T![.], T![-]]));
+    atom::ATOM_EXPR_FIRST.union(token_set![T![&], T![*], T![!], T![.], T![-]]);

 fn lhs(p: &mut Parser, r: Restrictions) -> Option<(CompletedMarker, BlockLike)> {
     let m;
diff --git a/crates/parser/src/grammar/expressions/atom.rs b/crates/parser/src/grammar/expressions/atom.rs
index 66a92a4e1..ba6dd2fbc 100644
--- a/crates/parser/src/grammar/expressions/atom.rs
+++ b/crates/parser/src/grammar/expressions/atom.rs
@@ -15,7 +15,7 @@ use super::*;
 //     let _ = b"e";
 //     let _ = br"f";
 // }
-pub(crate) const LITERAL_FIRST: TokenSet = TokenSet::new(&[
+pub(crate) const LITERAL_FIRST: TokenSet = token_set![
     TRUE_KW,
     FALSE_KW,
     INT_NUMBER,
@@ -25,8 +25,8 @@ pub(crate) const LITERAL_FIRST: TokenSet = TokenSet::new(&[
     STRING,
     RAW_STRING,
     BYTE_STRING,
-    RAW_BYTE_STRING,
-]);
+    RAW_BYTE_STRING
+];

 pub(crate) fn literal(p: &mut Parser) -> Option<CompletedMarker> {
     if !p.at_ts(LITERAL_FIRST) {
@@ -39,7 +39,7 @@ pub(crate) fn literal(p: &mut Parser) -> Option<CompletedMarker> {

 // E.g. for after the break in `if break {}`, this should not match
 pub(super) const ATOM_EXPR_FIRST: TokenSet =
-    LITERAL_FIRST.union(paths::PATH_FIRST).union(TokenSet::new(&[
+    LITERAL_FIRST.union(paths::PATH_FIRST).union(token_set![
         T!['('],
         T!['{'],
         T!['['],
@@ -59,9 +59,9 @@ pub(super) const ATOM_EXPR_FIRST: TokenSet =
         T![loop],
         T![for],
         LIFETIME,
-    ]));
+    ]);

-const EXPR_RECOVERY_SET: TokenSet = TokenSet::new(&[LET_KW, R_DOLLAR]);
+const EXPR_RECOVERY_SET: TokenSet = token_set![LET_KW, R_DOLLAR];

 pub(super) fn atom_expr(p: &mut Parser, r: Restrictions) -> Option<(CompletedMarker, BlockLike)> {
     if let Some(m) = literal(p) {
diff --git a/crates/parser/src/grammar/items.rs b/crates/parser/src/grammar/items.rs
index 22810e6fb..8fd8f3b80 100644
--- a/crates/parser/src/grammar/items.rs
+++ b/crates/parser/src/grammar/items.rs
@@ -26,7 +26,7 @@ pub(super) fn mod_contents(p: &mut Parser, stop_on_r_curly: bool) {
     }
 }

-pub(super) const ITEM_RECOVERY_SET: TokenSet = TokenSet::new(&[
+pub(super) const ITEM_RECOVERY_SET: TokenSet = token_set![
     FN_KW,
     STRUCT_KW,
     ENUM_KW,
@@ -41,7 +41,7 @@ pub(super) const ITEM_RECOVERY_SET: TokenSet = TokenSet::new(&[
     USE_KW,
     MACRO_KW,
     T![;],
-]);
+];

 pub(super) fn item_or_macro(p: &mut Parser, stop_on_r_curly: bool) {
     let m = p.start();
diff --git a/crates/parser/src/grammar/paths.rs b/crates/parser/src/grammar/paths.rs
index 5d297e2d6..52562afa4 100644
--- a/crates/parser/src/grammar/paths.rs
+++ b/crates/parser/src/grammar/paths.rs
@@ -3,7 +3,7 @@
 use super::*;

 pub(super) const PATH_FIRST: TokenSet =
-    TokenSet::new(&[IDENT, T![self], T![super], T![crate], T![:], T![<]]);
+    token_set![IDENT, T![self], T![super], T![crate], T![:], T![<]];

 pub(super) fn is_path_start(p: &Parser) -> bool {
     is_use_path_start(p) || p.at(T![<])
diff --git a/crates/parser/src/grammar/patterns.rs b/crates/parser/src/grammar/patterns.rs
index 796f206e1..07b1d6dd5 100644
--- a/crates/parser/src/grammar/patterns.rs
+++ b/crates/parser/src/grammar/patterns.rs
@@ -2,18 +2,9 @@

 use super::*;

-pub(super) const PATTERN_FIRST: TokenSet =
-    expressions::LITERAL_FIRST.union(paths::PATH_FIRST).union(TokenSet::new(&[
-        T![box],
-        T![ref],
-        T![mut],
-        T!['('],
-        T!['['],
-        T![&],
-        T![_],
-        T![-],
-        T![.],
-    ]));
+pub(super) const PATTERN_FIRST: TokenSet = expressions::LITERAL_FIRST
+    .union(paths::PATH_FIRST)
+    .union(token_set![T![box], T![ref], T![mut], T!['('], T!['['], T![&], T![_], T![-], T![.]]);

 pub(crate) fn pattern(p: &mut Parser) {
     pattern_r(p, PAT_RECOVERY_SET);
@@ -83,7 +74,7 @@ fn pattern_single_r(p: &mut Parser, recovery_set: TokenSet) {
 }

 const PAT_RECOVERY_SET: TokenSet =
-    TokenSet::new(&[LET_KW, IF_KW, WHILE_KW, LOOP_KW, MATCH_KW, R_PAREN, COMMA]);
+    token_set![LET_KW, IF_KW, WHILE_KW, LOOP_KW, MATCH_KW, R_PAREN, COMMA];

 fn atom_pat(p: &mut Parser, recovery_set: TokenSet) -> Option<CompletedMarker> {
     let m = match p.nth(0) {
diff --git a/crates/parser/src/grammar/types.rs b/crates/parser/src/grammar/types.rs
index 1ea130ac5..9d00eb9b9 100644
--- a/crates/parser/src/grammar/types.rs
+++ b/crates/parser/src/grammar/types.rs
@@ -2,7 +2,7 @@

 use super::*;

-pub(super) const TYPE_FIRST: TokenSet = paths::PATH_FIRST.union(TokenSet::new(&[
+pub(super) const TYPE_FIRST: TokenSet = paths::PATH_FIRST.union(token_set![
     T!['('],
     T!['['],
     T![<],
@@ -16,16 +16,16 @@ pub(super) const TYPE_FIRST: TokenSet = paths::PATH_FIRST.union(TokenSet::new(&[
     T![for],
     T![impl],
     T![dyn],
-]));
+]);

-const TYPE_RECOVERY_SET: TokenSet = TokenSet::new(&[
+const TYPE_RECOVERY_SET: TokenSet = token_set![
     T![')'],
     T![,],
     L_DOLLAR,
     // test_err struct_field_recover
     // struct S { f pub g: () }
     T![pub],
-]);
+];

 pub(crate) fn type_(p: &mut Parser) {
     type_with_bounds_cond(p, true);
diff --git a/crates/parser/src/token_set.rs b/crates/parser/src/token_set.rs
index a68f0144e..994017acf 100644
--- a/crates/parser/src/token_set.rs
+++ b/crates/parser/src/token_set.rs
@@ -9,21 +9,15 @@ pub(crate) struct TokenSet(u128);
 impl TokenSet {
     pub(crate) const EMPTY: TokenSet = TokenSet(0);

-    pub(crate) const fn new(kinds: &[SyntaxKind]) -> TokenSet {
-        let mut res = 0u128;
-        let mut i = 0;
-        while i < kinds.len() {
-            res |= mask(kinds[i]);
-            i += 1
-        }
-        TokenSet(res)
+    pub(crate) const fn singleton(kind: SyntaxKind) -> TokenSet {
+        TokenSet(mask(kind))
     }

     pub(crate) const fn union(self, other: TokenSet) -> TokenSet {
         TokenSet(self.0 | other.0)
     }

-    pub(crate) const fn contains(&self, kind: SyntaxKind) -> bool {
+    pub(crate) fn contains(&self, kind: SyntaxKind) -> bool {
         self.0 & mask(kind) != 0
     }
 }
@@ -32,10 +26,16 @@ const fn mask(kind: SyntaxKind) -> u128 {
     1u128 << (kind as usize)
 }

+#[macro_export]
+macro_rules! token_set {
+    ($($t:expr),*) => { TokenSet::EMPTY$(.union(TokenSet::singleton($t)))* };
+    ($($t:expr),* ,) => { token_set!($($t),*) };
+}
+
 #[test]
 fn token_set_works_for_tokens() {
     use crate::SyntaxKind::*;
-    let ts = TokenSet::new(&[EOF, SHEBANG]);
+    let ts = token_set![EOF, SHEBANG];
     assert!(ts.contains(EOF));
     assert!(ts.contains(SHEBANG));
     assert!(!ts.contains(PLUS));
diff --git a/xtask/src/install.rs b/xtask/src/install.rs
index d829790d7..b25a6e301 100644
--- a/xtask/src/install.rs
+++ b/xtask/src/install.rs
@@ -7,7 +7,7 @@ use anyhow::{bail, format_err, Context, Result};
 use crate::not_bash::{pushd, run};

 // Latest stable, feel free to send a PR if this lags behind.
-const REQUIRED_RUST_VERSION: u32 = 46;
+const REQUIRED_RUST_VERSION: u32 = 43;

 pub struct InstallCmd {
     pub client: Option<ClientOpt>,