Add error for pattern-matching on arrays without a fixed size

src/librustc_typeck/check/_match.rs

@@ -400,27 +400,36 @@ pub fn check_pat_walk(
                 let expected_ty = self.structurally_resolved_type(pat.span, expected);
                 let (inner_ty, slice_ty) = match expected_ty.sty {
                     ty::Array(inner_ty, size) => {
-                        let size = size.unwrap_usize(tcx);
-                        let min_len = before.len() as u64 + after.len() as u64;
-                        if slice.is_none() {
-                            if min_len != size {
-                                struct_span_err!(
-                                    tcx.sess, pat.span, E0527,
-                                    "pattern requires {} elements but array has {}",
-                                    min_len, size)
-                                    .span_label(pat.span, format!("expected {} elements", size))
+                        if let Some(size) = size.assert_usize(tcx) {
+                            let min_len = before.len() as u64 + after.len() as u64;
+                            if slice.is_none() {
+                                if min_len != size {
+                                    struct_span_err!(
+                                        tcx.sess, pat.span, E0527,
+                                        "pattern requires {} elements but array has {}",
+                                        min_len, size)
+                                        .span_label(pat.span, format!("expected {} elements", size))
+                                        .emit();
+                                }
+                                (inner_ty, tcx.types.err)
+                            } else if let Some(rest) = size.checked_sub(min_len) {
+                                (inner_ty, tcx.mk_array(inner_ty, rest))
+                            } else {
+                                struct_span_err!(tcx.sess, pat.span, E0528,
+                                        "pattern requires at least {} elements but array has {}",
+                                        min_len, size)
+                                    .span_label(pat.span,
+                                        format!("pattern cannot match array of {} elements", size))
                                     .emit();
+                                (inner_ty, tcx.types.err)
                             }
-                            (inner_ty, tcx.types.err)
-                        } else if let Some(rest) = size.checked_sub(min_len) {
-                            (inner_ty, tcx.mk_array(inner_ty, rest))
                         } else {
-                            struct_span_err!(tcx.sess, pat.span, E0528,
-                                    "pattern requires at least {} elements but array has {}",
-                                    min_len, size)
-                                .span_label(pat.span,
-                                    format!("pattern cannot match array of {} elements", size))
-                                .emit();
+                            struct_span_err!(
+                                tcx.sess,
+                                pat.span,
+                                E0730,
+                                "cannot pattern-match on an array without a fixed length",
+                            ).emit();
                             (inner_ty, tcx.types.err)
                         }
                     }

src/librustc_typeck/error_codes.rs

@@ -4648,6 +4648,38 @@ fn make_recursive_type() -> impl Sized {
 ```
 "##,
 
+E0730: r##"
+An array without a fixed length was pattern-matched.
+
+Example of erroneous code:
+
+```compile_fail,E0730
+#![feature(const_generics)]
+
+fn is_123<const N: usize>(x: [u32; N]) -> bool {
+    match x {
+        [1, 2, 3] => true, // error: cannot pattern-match on an
+                           //        array without a fixed length
+        _ => false
+    }
+}
+```
+
+Ensure that the pattern is consistent with the size of the matched
+array. Additional elements can be matched with `..`:
+
+```
+#![feature(slice_patterns)]
+
+let r = &[1, 2, 3, 4];
+match r {
+    &[a, b, ..] => { // ok!
+        println!("a={}, b={}", a, b);
+    }
+}
+```
+"##,
+
 }
 
 register_diagnostics! {

src/test/ui/error-codes/E0730.rs

@@ -0,0 +1,11 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn is_123<const N: usize>(x: [u32; N]) -> bool {
+    match x {
+        [1, 2, 3] => true, //~ ERROR cannot pattern-match on an array without a fixed length
+        _ => false
+    }
+}
+
+fn main() {}

src/test/ui/error-codes/E0730.stderr

@@ -0,0 +1,15 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/E0730.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0730]: cannot pattern-match on an array without a fixed length
+  --> $DIR/E0730.rs:6:9
+   |
+LL |         [1, 2, 3] => true,
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0730`.