Merge #11484

11484: Infer the array size for slice patterns r=flodiebold a=ChayimFriedman2

Fixes #11478 (feat or fix?),

I don't know much about the type-checking in RA, so maybe I did some obvious mistake.

Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>

crates/hir_ty/src/infer/pat.rs

@@ -6,6 +6,7 @@
 use hir_def::{
     expr::{BindingAnnotation, Expr, Literal, Pat, PatId, RecordFieldPat},
     path::Path,
+    type_ref::ConstScalar,
 };
 use hir_expand::name::Name;
 
@@ -14,7 +15,8 @@
         Adjust, Adjustment, AutoBorrow, BindingMode, Expectation, InferenceContext, TypeMismatch,
     },
     lower::lower_to_chalk_mutability,
-    static_lifetime, Interner, Substitution, Ty, TyBuilder, TyExt, TyKind,
+    static_lifetime, ConcreteConst, ConstValue, Interner, Substitution, Ty, TyBuilder, TyExt,
+    TyKind,
 };
 
 impl<'a> InferenceContext<'a> {
@@ -232,16 +234,28 @@ pub(super) fn infer_pat(
                     self.infer_pat(pat_id, &elem_ty, default_bm);
                 }
 
-                let pat_ty = match expected.kind(Interner) {
+                if let &Some(slice_pat_id) = slice {
+                    let rest_pat_ty = match expected.kind(Interner) {
+                        TyKind::Array(_, length) => {
+                            let length = match length.data(Interner).value {
+                                ConstValue::Concrete(ConcreteConst {
+                                    interned: ConstScalar::Usize(length),
+                                }) => length.checked_sub((prefix.len() + suffix.len()) as u64),
+                                _ => None,
+                            };
+                            TyKind::Array(elem_ty.clone(), crate::consteval::usize_const(length))
+                        }
+                        _ => TyKind::Slice(elem_ty.clone()),
+                    }
+                    .intern(Interner);
+                    self.infer_pat(slice_pat_id, &rest_pat_ty, default_bm);
+                }
+
+                match expected.kind(Interner) {
                     TyKind::Array(_, const_) => TyKind::Array(elem_ty, const_.clone()),
                     _ => TyKind::Slice(elem_ty),
                 }
-                .intern(Interner);
-                if let &Some(slice_pat_id) = slice {
-                    self.infer_pat(slice_pat_id, &pat_ty, default_bm);
-                }
-
-                pat_ty
+                .intern(Interner)
             }
             Pat::Wild => expected.clone(),
             Pat::Range { start, end } => {

crates/hir_ty/src/tests/patterns.rs

@@ -890,6 +890,32 @@ fn main() {
     );
 }
 
+#[test]
+fn slice_pattern_correctly_handles_array_length() {
+    check_infer(
+        r#"
+fn main() {
+    let [head, middle @ .., tail, tail2] = [1, 2, 3, 4, 5];
+}
+    "#,
+        expect![[r#"
+            10..73 '{     ... 5]; }': ()
+            20..52 '[head,...tail2]': [i32; 5]
+            21..25 'head': i32
+            27..38 'middle @ ..': [i32; 2]
+            36..38 '..': [i32; 2]
+            40..44 'tail': i32
+            46..51 'tail2': i32
+            55..70 '[1, 2, 3, 4, 5]': [i32; 5]
+            56..57 '1': i32
+            59..60 '2': i32
+            62..63 '3': i32
+            65..66 '4': i32
+            68..69 '5': i32
+        "#]],
+    );
+}
+
 #[test]
 fn pattern_lookup_in_value_ns() {
     check_types(