Remove all unstable placement features

Closes #22181, #27779

src/libsyntax/ast.rs

@@ -1011,7 +1011,6 @@ impl Expr {
     pub fn precedence(&self) -> ExprPrecedence {
         match self.node {
             ExprKind::Box(_) => ExprPrecedence::Box,
-            ExprKind::InPlace(..) => ExprPrecedence::InPlace,
             ExprKind::Array(_) => ExprPrecedence::Array,
             ExprKind::Call(..) => ExprPrecedence::Call,
             ExprKind::MethodCall(..) => ExprPrecedence::MethodCall,
@@ -1071,8 +1070,6 @@ pub enum RangeLimits {
 pub enum ExprKind {
     /// A `box x` expression.
     Box(P<Expr>),
-    /// First expr is the place; second expr is the value.
-    InPlace(P<Expr>, P<Expr>),
     /// An array (`[a, b, c, d]`)
     Array(Vec<P<Expr>>),
     /// A function call