Auto merge of #48333 - aidanhs:aphs-no-place-for-placement, r=nikomatsakis

Remove all unstable placement features

Closes #22181, #27779. Effectively makes the assortment of placement RFCs (rust-lang/rfcs#470, rust-lang/rfcs#809, rust-lang/rfcs#1228) 'unaccepted'. It leaves `box_syntax` and keeps the `<-` token as recognised by libsyntax.

------------------------

I don't know the correct process for unaccepting an unstable feature that was accepted as an RFC so...here's a PR.

Let me preface this by saying I'm not particularly happy about doing this (I know it'll be unpopular), but I think it's the most honest expression of how things stand today. I've been motivated by a [post on reddit](https://www.reddit.com/r/rust/comments/7wrqk2/when_will_box_and_placementin_syntax_be_stable/) which asks when these features will be stable - the features have received little RFC-style design work since the end of 2015 (~2 years ago) and leaving them in limbo confuses people who want to know where they're up to. Without additional design work that needs to happen (see the collection of unresolved questions later in this post) they can't really get stabilised, and I think that design work would be most suited to an RFC rather than (currently mostly unused) experimental features in Rust nightly.

I have my own motivations - it's very simple to 'defeat' placement in debug mode today and I don't want a placement in Rust that a) has no guarantees to work and b) has no plan for in-place serde deserialisation.

There's a quote in [1]: "Ordinarily these uncertainties might lead to the RFC being postponed. [The RFC seems like a promising direction hence we will accept since it] will thus give us immediate experience with the design and help in determining the best final solution.". I propose that there have been enough additional uncertainties raised since then that the original direction is less promising and we should be think about the problem anew.

(a historical note: the first mention of placement (under that name - uninit pointers were earlier) in an RFC AFAIK is [0] in late 2014 (pre-1.0). RFCs since then have built on this base - [1] is a comment in Feb 2015 accepting a more conservative design of the Place* traits - this is back when serde still required aster and seemed to break every other nightly! A lot has changed since then, perhaps placement should too)

------------------------

Concrete unresolved questions include:

 - making placement work in debug mode [7]
 - making placement work for serde/with fallible creation [5], [irlo2], [8]
 - trait design:
   - opting into not consuming the placer in `Placer::make_place` - [2]
   - trait proliferation - [4] (+ others in that thread)
   - fallible allocation - [3], [4] (+ others in that thread)
 - support for DSTs/unsized structs (if at all) - [1], [6]

More speculative unresolved questions include:

 - better trait design with in the context of future language features [irlo1] (Q11), [irlo3]
 - interaction between custom allocators and placement [irlo3]

[0] https://github.com/rust-lang/rfcs/pull/470
[1] https://github.com/rust-lang/rfcs/pull/809#issuecomment-73910414
[2] https://github.com/rust-lang/rfcs/issues/1286
[3] https://github.com/rust-lang/rfcs/issues/1315
[4] https://github.com/rust-lang/rust/issues/27779#issuecomment-146711893
[5] https://github.com/rust-lang/rust/issues/27779#issuecomment-285562402
[6] https://github.com/rust-lang/rust/issues/27779#issuecomment-354464938
[7] https://github.com/rust-lang/rust/issues/27779#issuecomment-358025344
[8] https://github.com/rust-lang/rfcs/pull/1228#issuecomment-190825370
[irlo1] https://internals.rust-lang.org/t/placement-nwbi-faq-new-box-in-left-arrow/2789
[irlo2] https://internals.rust-lang.org/t/placement-nwbi-faq-new-box-in-left-arrow/2789/19
[irlo3] https://internals.rust-lang.org/t/lang-team-minutes-feature-status-report-placement-in-and-box/4646

7 files changed, 4 insertions, 53 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index c90b0aecfc0..31bb1c88b87 100644
--- a/src/libsyntax/ast.rs
+++ b/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
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 463e76e1461..e734a4e3735 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -146,7 +146,6 @@ declare_features! (
     (active, rustc_diagnostic_macros, "1.0.0", None, None),
     (active, rustc_const_unstable, "1.0.0", None, None),
     (active, box_syntax, "1.0.0", Some(27779), None),
-    (active, placement_in_syntax, "1.0.0", Some(27779), None),
     (active, unboxed_closures, "1.0.0", Some(29625), None),
 
     (active, fundamental, "1.0.0", Some(29635), None),
@@ -1287,9 +1286,6 @@ pub const EXPLAIN_VIS_MATCHER: &'static str =
 pub const EXPLAIN_LIFETIME_MATCHER: &'static str =
     ":lifetime fragment specifier is experimental and subject to change";
 
-pub const EXPLAIN_PLACEMENT_IN: &'static str =
-    "placement-in expression syntax is experimental and subject to change.";
-
 pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &'static str =
     "Unsized tuple coercion is not stable enough for use and is subject to change";
 
@@ -1636,9 +1632,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 gate_feature_post!(&self, type_ascription, e.span,
                                   "type ascription is experimental");
             }
-            ast::ExprKind::InPlace(..) => {
-                gate_feature_post!(&self, placement_in_syntax, e.span, EXPLAIN_PLACEMENT_IN);
-            }
             ast::ExprKind::Yield(..) => {
                 gate_feature_post!(&self, generators,
                                   e.span,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 05a3150c139..e702bf56e7f 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -1167,9 +1167,6 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             ExprKind::Box(e) => {
                 ExprKind::Box(folder.fold_expr(e))
             }
-            ExprKind::InPlace(p, e) => {
-                ExprKind::InPlace(folder.fold_expr(p), folder.fold_expr(e))
-            }
             ExprKind::Array(exprs) => {
                 ExprKind::Array(folder.fold_exprs(exprs))
             }
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index f7cdd4ba2b4..f5ab023b30e 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2850,17 +2850,6 @@ impl<'a> Parser<'a> {
                 let (span, e) = self.interpolated_or_expr_span(e)?;
                 (lo.to(span), ExprKind::AddrOf(m, e))
             }
-            token::Ident(..) if self.token.is_keyword(keywords::In) => {
-                self.bump();
-                let place = self.parse_expr_res(
-                    Restrictions::NO_STRUCT_LITERAL,
-                    None,
-                )?;
-                let blk = self.parse_block()?;
-                let span = blk.span;
-                let blk_expr = self.mk_expr(span, ExprKind::Block(blk), ThinVec::new());
-                (lo.to(span), ExprKind::InPlace(place, blk_expr))
-            }
             token::Ident(..) if self.token.is_keyword(keywords::Box) => {
                 self.bump();
                 let e = self.parse_prefix_expr(None);
@@ -3023,8 +3012,6 @@ impl<'a> Parser<'a> {
                 }
                 AssocOp::Assign =>
                     self.mk_expr(span, ExprKind::Assign(lhs, rhs), ThinVec::new()),
-                AssocOp::Inplace =>
-                    self.mk_expr(span, ExprKind::InPlace(lhs, rhs), ThinVec::new()),
                 AssocOp::AssignOp(k) => {
                     let aop = match k {
                         token::Plus =>    BinOpKind::Add,
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index ae045fc095a..c3785c10f69 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1877,16 +1877,6 @@ impl<'a> State<'a> {
         Ok(())
     }
 
-    fn print_expr_in_place(&mut self,
-                           place: &ast::Expr,
-                           expr: &ast::Expr) -> io::Result<()> {
-        let prec = AssocOp::Inplace.precedence() as i8;
-        self.print_expr_maybe_paren(place, prec + 1)?;
-        self.s.space()?;
-        self.word_space("<-")?;
-        self.print_expr_maybe_paren(expr, prec)
-    }
-
     fn print_expr_vec(&mut self, exprs: &[P<ast::Expr>],
                       attrs: &[Attribute]) -> io::Result<()> {
         self.ibox(INDENT_UNIT)?;
@@ -2056,9 +2046,6 @@ impl<'a> State<'a> {
                 self.word_space("box")?;
                 self.print_expr_maybe_paren(expr, parser::PREC_PREFIX)?;
             }
-            ast::ExprKind::InPlace(ref place, ref expr) => {
-                self.print_expr_in_place(place, expr)?;
-            }
             ast::ExprKind::Array(ref exprs) => {
                 self.print_expr_vec(&exprs[..], attrs)?;
             }
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 86963c4000b..4770273e8c4 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -56,8 +56,6 @@ pub enum AssocOp {
     GreaterEqual,
     /// `=`
     Assign,
-    /// `<-`
-    Inplace,
     /// `?=` where ? is one of the BinOpToken
     AssignOp(BinOpToken),
     /// `as`
@@ -86,7 +84,6 @@ impl AssocOp {
         use self::AssocOp::*;
         match *t {
             Token::BinOpEq(k) => Some(AssignOp(k)),
-            Token::LArrow => Some(Inplace),
             Token::Eq => Some(Assign),
             Token::BinOp(BinOpToken::Star) => Some(Multiply),
             Token::BinOp(BinOpToken::Slash) => Some(Divide),
@@ -156,7 +153,6 @@ impl AssocOp {
             LAnd => 6,
             LOr => 5,
             DotDot | DotDotEq => 4,
-            Inplace => 3,
             Assign | AssignOp(_) => 2,
         }
     }
@@ -166,7 +162,7 @@ impl AssocOp {
         use self::AssocOp::*;
         // NOTE: it is a bug to have an operators that has same precedence but different fixities!
         match *self {
-            Inplace | Assign | AssignOp(_) => Fixity::Right,
+            Assign | AssignOp(_) => Fixity::Right,
             As | Multiply | Divide | Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd |
             BitXor | BitOr | Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual |
             LAnd | LOr | Colon => Fixity::Left,
@@ -178,7 +174,7 @@ impl AssocOp {
         use self::AssocOp::*;
         match *self {
             Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => true,
-            Inplace | Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add | Subtract |
+            Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add | Subtract |
             ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr |
             DotDot | DotDotEq | Colon => false
         }
@@ -187,7 +183,7 @@ impl AssocOp {
     pub fn is_assign_like(&self) -> bool {
         use self::AssocOp::*;
         match *self {
-            Assign | AssignOp(_) | Inplace => true,
+            Assign | AssignOp(_) => true,
             Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual | As | Multiply | Divide |
             Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd |
             LOr | DotDot | DotDotEq | Colon => false
@@ -215,7 +211,7 @@ impl AssocOp {
             BitOr => Some(BinOpKind::BitOr),
             LAnd => Some(BinOpKind::And),
             LOr => Some(BinOpKind::Or),
-            Inplace | Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
+            Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
         }
     }
 }
@@ -242,7 +238,6 @@ pub enum ExprPrecedence {
 
     Binary(BinOpKind),
 
-    InPlace,
     Cast,
     Type,
 
@@ -310,7 +305,6 @@ impl ExprPrecedence {
 
             // Binop-like expr kinds, handled by `AssocOp`.
             ExprPrecedence::Binary(op) => AssocOp::from_ast_binop(op).precedence() as i8,
-            ExprPrecedence::InPlace => AssocOp::Inplace.precedence() as i8,
             ExprPrecedence::Cast => AssocOp::As.precedence() as i8,
             ExprPrecedence::Type => AssocOp::Colon.precedence() as i8,
 
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index bbf1fe124f1..d8de78054ab 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -654,10 +654,6 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Box(ref subexpression) => {
             visitor.visit_expr(subexpression)
         }
-        ExprKind::InPlace(ref place, ref subexpression) => {
-            visitor.visit_expr(place);
-            visitor.visit_expr(subexpression)
-        }
         ExprKind::Array(ref subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);
         }