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Move unsized_tuple_coercion behind a feature gate.
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# `unsized_tuple_coercion`
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The tracking issue for this feature is: [#XXXXX]
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[#XXXXX]: https://github.com/rust-lang/rust/issues/XXXXX
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------------------------
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This is a part of [RFC0401]. According to the RFC, there should be an implementation like this:
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```rust
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impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}
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```
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This implementation is currently gated behind `#[feature(unsized_tuple_coercion)]` to avoid insta-stability. Therefore you can use it like this:
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```rust
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#![feature(unsized_tuple_coercion)]
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fn main() {
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let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
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let y : &([i32; 3], [i32]) = &x;
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assert_eq!(y.1[0], 4);
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}
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```
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[RFC0401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
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@ -76,6 +76,7 @@ use rustc::ty::relate::RelateResult;
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use rustc::ty::subst::Subst;
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use errors::DiagnosticBuilder;
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use syntax::abi;
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use syntax::feature_gate;
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use syntax::ptr::P;
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use syntax_pos;
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@ -520,6 +521,8 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
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coerce_source,
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&[coerce_target]));
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let mut has_unsized_tuple_coercion = false;
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// Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
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// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
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// inference might unify those two inner type variables later.
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@ -527,7 +530,15 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
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while let Some(obligation) = queue.pop_front() {
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debug!("coerce_unsized resolve step: {:?}", obligation);
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let trait_ref = match obligation.predicate {
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ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => tr.clone(),
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ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
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if unsize_did == tr.def_id() {
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if let ty::TyTuple(..) = tr.0.input_types().nth(1).unwrap().sty {
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debug!("coerce_unsized: found unsized tuple coercion");
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has_unsized_tuple_coercion = true;
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}
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}
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tr.clone()
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}
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_ => {
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coercion.obligations.push(obligation);
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continue;
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@ -557,6 +568,14 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
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}
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}
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if has_unsized_tuple_coercion && !self.tcx.sess.features.borrow().unsized_tuple_coercion {
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feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
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"unsized_tuple_coercion",
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self.cause.span,
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feature_gate::GateIssue::Language,
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feature_gate::EXPLAIN_UNSIZED_TUPLE_COERCION);
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}
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Ok(coercion)
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}
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@ -357,6 +357,9 @@ declare_features! (
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// Allows a test to fail without failing the whole suite
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(active, allow_fail, "1.19.0", Some(42219)),
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// Allows unsized tuple coercion.
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(active, unsized_tuple_coercion, "1.20.0", None),
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);
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declare_features! (
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@ -1041,6 +1044,9 @@ pub const EXPLAIN_VIS_MATCHER: &'static str =
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pub const EXPLAIN_PLACEMENT_IN: &'static str =
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"placement-in expression syntax is experimental and subject to change.";
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pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &'static str =
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"Unsized tuple coercion is not stable enough for use and is subject to change";
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struct PostExpansionVisitor<'a> {
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context: &'a Context<'a>,
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}
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@ -10,6 +10,8 @@
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// Forbid assignment into a dynamically sized type.
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#![feature(unsized_tuple_coercion)]
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type Fat<T: ?Sized> = (isize, &'static str, T);
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//~^ WARNING trait bounds are not (yet) enforced
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@ -10,6 +10,8 @@
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// Attempt to change the type as well as unsizing.
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#![feature(unsized_tuple_coercion)]
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struct Fat<T: ?Sized> {
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ptr: T
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}
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@ -10,6 +10,8 @@
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// Attempt to extend the lifetime as well as unsizing.
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#![feature(unsized_tuple_coercion)]
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struct Fat<T: ?Sized> {
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ptr: T
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}
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@ -10,6 +10,8 @@
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// Attempt to coerce from unsized to sized.
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#![feature(unsized_tuple_coercion)]
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struct Fat<T: ?Sized> {
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ptr: T
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}
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@ -13,6 +13,8 @@
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// because it would require stack allocation of an unsized temporary (*g in the
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// test).
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#![feature(unsized_tuple_coercion)]
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pub fn main() {
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let f: ([isize; 3],) = ([5, 6, 7],);
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let g: &([isize],) = &f;
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14
src/test/compile-fail/feature-gate-unsized_tuple_coercion.rs
Normal file
14
src/test/compile-fail/feature-gate-unsized_tuple_coercion.rs
Normal file
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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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fn main() {
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let _ : &(Send,) = &((),);
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//~^ ERROR Unsized tuple coercion is not stable enough
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}
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@ -10,6 +10,8 @@
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// no-prefer-dynamic
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#![feature(unsized_tuple_coercion)]
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static mut DROP_RAN: bool = false;
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struct Foo;
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// no-prefer-dynamic
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#![feature(unsized_tuple_coercion)]
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static mut DROP_RAN: isize = 0;
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struct Foo;
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@ -8,6 +8,8 @@
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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#![feature(unsized_tuple_coercion)]
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struct Test<T: ?Sized>(T);
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fn main() {
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// Test DST raw pointers
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#![feature(unsized_tuple_coercion)]
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trait Trait {
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fn foo(&self) -> isize;
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}
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#![allow(unused_features)]
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#![feature(box_syntax)]
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#![feature(unsized_tuple_coercion)]
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type Fat<T: ?Sized> = (isize, &'static str, T);
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// As dst-tuple.rs, but the unsized field is the only field in the tuple.
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#![feature(unsized_tuple_coercion)]
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type Fat<T: ?Sized> = (T,);
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// x is a fat pointer
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#![allow(unknown_features)]
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#![feature(box_syntax)]
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#![feature(unsized_tuple_coercion)]
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type Fat<T: ?Sized> = (isize, &'static str, T);
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