7 files changed, 50 insertions, 4 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 4360fbeb9..a7198fbf8 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -1295,6 +1295,7 @@ impl Expr {
             ExprKind::Yield(..) => ExprPrecedence::Yield,
             ExprKind::Yeet(..) => ExprPrecedence::Yeet,
             ExprKind::FormatArgs(..) => ExprPrecedence::FormatArgs,
+            ExprKind::Become(..) => ExprPrecedence::Become,
             ExprKind::Err => ExprPrecedence::Err,
         }
     }
@@ -1515,6 +1516,11 @@ pub enum ExprKind {
     /// with an optional value to be returned.
     Yeet(Option<P<Expr>>),
 
+    /// A tail call return, with the value to be returned.
+    ///
+    /// While `.0` must be a function call, we check this later, after parsing.
+    Become(P<Expr>),
+
     /// Bytes included via `include_bytes!`
     /// Added for optimization purposes to avoid the need to escape
     /// large binary blobs - should always behave like [`ExprKind::Lit`]
@@ -2646,6 +2652,15 @@ pub struct NormalAttr {
     pub tokens: Option<LazyAttrTokenStream>,
 }
 
+impl NormalAttr {
+    pub fn from_ident(ident: Ident) -> Self {
+        Self {
+            item: AttrItem { path: Path::from_ident(ident), args: AttrArgs::Empty, tokens: None },
+            tokens: None,
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct AttrItem {
     pub path: Path,
diff --git a/compiler/rustc_ast/src/expand/mod.rs b/compiler/rustc_ast/src/expand/mod.rs
index 2ee1bfe0a..942347383 100644
--- a/compiler/rustc_ast/src/expand/mod.rs
+++ b/compiler/rustc_ast/src/expand/mod.rs
@@ -1,3 +1,20 @@
 //! Definitions shared by macros / syntax extensions and e.g. `rustc_middle`.
 
+use rustc_span::{def_id::DefId, symbol::Ident};
+
+use crate::MetaItem;
+
 pub mod allocator;
+
+#[derive(Debug, Clone, Encodable, Decodable, HashStable_Generic)]
+pub struct StrippedCfgItem<ModId = DefId> {
+    pub parent_module: ModId,
+    pub name: Ident,
+    pub cfg: MetaItem,
+}
+
+impl<ModId> StrippedCfgItem<ModId> {
+    pub fn map_mod_id<New>(self, f: impl FnOnce(ModId) -> New) -> StrippedCfgItem<New> {
+        StrippedCfgItem { parent_module: f(self.parent_module), name: self.name, cfg: self.cfg }
+    }
+}
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 66b94d12a..53a9c9a04 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -1457,6 +1457,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
         ExprKind::Yeet(expr) => {
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
+        ExprKind::Become(expr) => vis.visit_expr(expr),
         ExprKind::InlineAsm(asm) => vis.visit_inline_asm(asm),
         ExprKind::FormatArgs(fmt) => vis.visit_format_args(fmt),
         ExprKind::OffsetOf(container, fields) => {
diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 7ef39f802..6646fa944 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -11,6 +11,7 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
 use rustc_macros::HashStable_Generic;
 use rustc_span::symbol::{kw, sym};
+#[cfg_attr(not(bootstrap), allow(hidden_glob_reexports))]
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::{self, edition::Edition, Span, DUMMY_SP};
 use std::borrow::Cow;
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index db296aa44..ca4a739ab 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -25,7 +25,7 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
-use std::{fmt, iter};
+use std::{fmt, iter, mem};
 
 /// When the main Rust parser encounters a syntax-extension invocation, it
 /// parses the arguments to the invocation as a token tree. This is a very
@@ -410,8 +410,17 @@ impl TokenStream {
         t1.next().is_none() && t2.next().is_none()
     }
 
-    pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
-        TokenStream(Lrc::new(self.0.iter().enumerate().map(|(i, tree)| f(i, tree)).collect()))
+    /// Applies the supplied function to each `TokenTree` and its index in `self`, returning a new `TokenStream`
+    ///
+    /// It is equivalent to `TokenStream::new(self.trees().cloned().enumerate().map(|(i, tt)| f(i, tt)).collect())`.
+    pub fn map_enumerated_owned(
+        mut self,
+        mut f: impl FnMut(usize, TokenTree) -> TokenTree,
+    ) -> TokenStream {
+        let owned = Lrc::make_mut(&mut self.0); // clone if necessary
+        // rely on vec's in-place optimizations to avoid another allocation
+        *owned = mem::take(owned).into_iter().enumerate().map(|(i, tree)| f(i, tree)).collect();
+        self
     }
 
     /// Create a token stream containing a single token with alone spacing.
diff --git a/compiler/rustc_ast/src/util/parser.rs b/compiler/rustc_ast/src/util/parser.rs
index 35afd5423..096077e09 100644
--- a/compiler/rustc_ast/src/util/parser.rs
+++ b/compiler/rustc_ast/src/util/parser.rs
@@ -245,6 +245,7 @@ pub enum ExprPrecedence {
     Ret,
     Yield,
     Yeet,
+    Become,
 
     Range,
 
@@ -298,7 +299,8 @@ impl ExprPrecedence {
             | ExprPrecedence::Continue
             | ExprPrecedence::Ret
             | ExprPrecedence::Yield
-            | ExprPrecedence::Yeet => PREC_JUMP,
+            | ExprPrecedence::Yeet
+            | ExprPrecedence::Become => PREC_JUMP,
 
             // `Range` claims to have higher precedence than `Assign`, but `x .. x = x` fails to
             // parse, instead of parsing as `(x .. x) = x`. Giving `Range` a lower precedence
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 275692ad5..d9de5b8e1 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -908,6 +908,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Yeet(optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
+        ExprKind::Become(expr) => visitor.visit_expr(expr),
         ExprKind::MacCall(mac) => visitor.visit_mac_call(mac),
         ExprKind::Paren(subexpression) => visitor.visit_expr(subexpression),
         ExprKind::InlineAsm(asm) => visitor.visit_inline_asm(asm),