syntax: Add feature gate.

This commit adds a `const_in_array_repeat_expressions` feature gate and
only create `Candidate::Repeat` if it is enabled.

Author: davidtwco

src/doc/unstable-book/src/language-features/const-in-array-repeat-expressions.md

@@ -0,0 +1,11 @@
+# `const_in_array_repeat_expressions`
+
+The tracking issue for this feature is: [#49147]
+
+[#44109]: https://github.com/rust-lang/rust/issues/49147
+
+------------------------
+
+Relaxes the rules for repeat expressions, `[x; N]` such that `x` may also be `const` (strictly
+speaking rvalue promotable), in addition to `typeof(x): Copy`. The result of `[x; N]` where `x` is
+`const` is itself also `const`.

src/librustc_mir/transform/qualify_consts.rs

@@ -811,7 +811,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
                 let not_promotable = IsNotImplicitlyPromotable::in_operand(self, operand) ||
                                      IsNotPromotable::in_operand(self, operand);
                 debug!("assign: self.def_id={:?} operand={:?}", self.def_id, operand);
-                if !not_promotable {
+                if !not_promotable && self.tcx.features().const_in_array_repeat_expressions {
                     debug!("assign: candidate={:?}", candidate);
                     self.promotion_candidates.push(candidate);
                 }

src/libsyntax/feature_gate.rs

@@ -562,10 +562,10 @@ declare_features! (
     // Allows `if/while p && let q = r && ...` chains.
     (active, let_chains, "1.37.0", Some(53667), None),
 
-    // #[repr(transparent)] on enums.
+    // Allows #[repr(transparent)] on enums (RFC 2645).
     (active, transparent_enums, "1.37.0", Some(60405), None),
 
-    // #[repr(transparent)] on unions.
+    // Allows #[repr(transparent)] on unions (RFC 2645).
     (active, transparent_unions, "1.37.0", Some(60405), None),
 
     // Allows explicit discriminants on non-unit enum variants.
@@ -580,6 +580,9 @@ declare_features! (
     // Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests
     (active, cfg_doctest, "1.37.0", Some(62210), None),
 
+    // Allows `[x; N]` where `x` is a constant (RFC 2203).
+    (active, const_in_array_repeat_expressions, "1.37.0", Some(49147), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/libsyntax_pos/symbol.rs

@@ -199,6 +199,7 @@ symbols! {
         const_fn_union,
         const_generics,
         const_indexing,
+        const_in_array_repeat_expressions,
         const_let,
         const_panic,
         const_raw_ptr_deref,

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/migrate-borrowck.rs

@@ -1,5 +1,6 @@
 // ignore-compile-mode-nll
 // compile-flags: -Z borrowck=migrate
+#![feature(constants_in_array_repeat_expressions)]
 #![allow(warnings)]
 
 // Some type that is not copyable.

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/migrate-borrowck.stderr

@@ -1,13 +1,13 @@
 error: repeated expression does not implement `std::marker::Copy`
-  --> $DIR/migrate-borrowck.rs:87:37
+  --> $DIR/migrate-borrowck.rs:88:37
    |
 LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |                                     ^^^^^^ the trait `std::marker::Copy` is not implemented for `std::option::Option<Bar>`
    |
    = note: the `std::marker::Copy` trait is required because the repeated element will be copied
 
 error: repeated expression does not implement `std::marker::Copy`
-  --> $DIR/migrate-borrowck.rs:103:37
+  --> $DIR/migrate-borrowck.rs:104:37
    |
 LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |                                     ^^^^^^ the trait `std::marker::Copy` is not implemented for `std::option::Option<Bar>`

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/nll-borrowck.rs

@@ -1,6 +1,6 @@
 // ignore-compile-mode-nll
 #![allow(warnings)]
-#![feature(nll)]
+#![feature(constants_in_array_repeat_expressions, nll)]
 
 // Some type that is not copyable.
 struct Bar;

src/test/ui/feature-gates/feature-gate-const_in_array_repeat_expressions.rs

@@ -0,0 +1,11 @@
+// ignore-tidy-linelength
+#![allow(warnings)]
+
+struct Bar;
+
+fn foo() {
+    let arr: [Option<String>; 2] = [None::<String>; 2];
+    //~^ ERROR the trait bound `std::option::Option<std::string::String>: std::marker::Copy` is not satisfied [E0277]
+}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-const_in_array_repeat_expressions.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `std::option::Option<std::string::String>: std::marker::Copy` is not satisfied
+  --> $DIR/feature-gate-const_in_array_repeat_expressions.rs:7:36
+   |
+LL |     let arr: [Option<String>; 2] = [None::<String>; 2];
+   |                                    ^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::option::Option<std::string::String>`
+   |
+   = help: the following implementations were found:
+             <std::option::Option<T> as std::marker::Copy>
+   = note: the `Copy` trait is required because the repeated element will be copied
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.