Reestablish feature gate for RangeFrom in slices

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -540,6 +540,22 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
         match &pattern.kind {
+            PatKind::Slice(pats) => {
+                for pat in pats {
+                    let inner_pat = match &pat.kind {
+                        PatKind::Ident(.., Some(pat)) => pat,
+                        _ => pat,
+                    };
+                    if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
+                        gate_feature_post!(
+                            &self,
+                            half_open_range_patterns,
+                            pat.span,
+                            "`X..` patterns in slices are experimental"
+                        );
+                    }
+                }
+            }
             PatKind::Box(..) => {
                 gate_feature_post!(
                     &self,

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem0.rs

@@ -0,0 +1,16 @@
+#![feature(half_open_range_patterns)]
+#![feature(exclusive_range_pattern)]
+
+fn main() {
+    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
+    let [a, b, c, rest @ ..] = xs;
+    // Consider the following example:
+    assert!(a == 13 && b == 1 && c == 5 && rest.len() == 5);
+
+    // What if we wanted to pull this apart without individually binding a, b, and c?
+    let [first_three @ ..3, rest @ 2..] = xs;
+    //~^ pattern requires 2 elements but array has 8
+    // This is somewhat unintuitive and makes slice patterns exceedingly verbose.
+    // We want to stabilize half-open RangeFrom (`X..`) patterns
+    // but without banning us from using them for a more efficient slice pattern syntax.
+}

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem0.stderr

@@ -0,0 +1,9 @@
+error[E0527]: pattern requires 2 elements but array has 8
+  --> $DIR/slice_pattern_syntax_problem0.rs:11:9
+   |
+LL |     let [first_three @ ..3, rest @ 2..] = xs;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 8 elements
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0527`.

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem1.rs

@@ -0,0 +1,9 @@
+// Instead of allowing the previous case, maintain the feature gate for slice patterns for now.
+fn main() {
+    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
+    let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+    //~^ `X..` patterns in slices are experimental
+    //~| half-open range patterns are unstable
+    //~| exclusive range pattern syntax is experimental
+    //~| exclusive range pattern syntax is experimental
+}

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem1.stderr

@@ -0,0 +1,39 @@
+error[E0658]: half-open range patterns are unstable
+  --> $DIR/slice_pattern_syntax_problem1.rs:4:23
+   |
+LL |     let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+   |                       ^^^
+   |
+   = note: see issue #67264 <https://github.com/rust-lang/rust/issues/67264> for more information
+   = help: add `#![feature(half_open_range_patterns)]` to the crate attributes to enable
+
+error[E0658]: `X..` patterns in slices are experimental
+  --> $DIR/slice_pattern_syntax_problem1.rs:4:10
+   |
+LL |     let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+   |          ^^^^^^^
+   |
+   = note: see issue #67264 <https://github.com/rust-lang/rust/issues/67264> for more information
+   = help: add `#![feature(half_open_range_patterns)]` to the crate attributes to enable
+
+error[E0658]: exclusive range pattern syntax is experimental
+  --> $DIR/slice_pattern_syntax_problem1.rs:4:23
+   |
+LL |     let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+   |                       ^^^
+   |
+   = note: see issue #37854 <https://github.com/rust-lang/rust/issues/37854> for more information
+   = help: add `#![feature(exclusive_range_pattern)]` to the crate attributes to enable
+
+error[E0658]: exclusive range pattern syntax is experimental
+  --> $DIR/slice_pattern_syntax_problem1.rs:4:32
+   |
+LL |     let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs;
+   |                                ^^^^
+   |
+   = note: see issue #37854 <https://github.com/rust-lang/rust/issues/37854> for more information
+   = help: add `#![feature(exclusive_range_pattern)]` to the crate attributes to enable
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/half-open-range-patterns/slice_pattern_syntax_problem2.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+fn main() {
+    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
+    if let [3..=14, ..] = xs {
+        /* this variant must pass for now, unfortunately.
+         * This test is included here to help inform a future plan for these.
+         */
+    };
+}