match single prefix slice

Author: JoshMcguigan

crates/ra_hir_ty/src/infer/pat.rs

@@ -12,7 +12,7 @@ use hir_expand::name::Name;
 use test_utils::tested_by;
 
 use super::{BindingMode, InferenceContext};
-use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor};
+use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor, ApplicationTy};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     fn infer_tuple_struct_pat(
@@ -185,6 +185,20 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.write_pat_ty(pat, bound_ty);
                 return inner_ty;
             }
+            Pat::Slice { prefix, slice, suffix } => {
+                if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Slice, parameters }) = expected {
+                    match (prefix.as_slice(), slice, suffix.as_slice()) {
+                        ([prefix_pat_id], None, []) => {
+                            let ty = self.infer_pat(*prefix_pat_id, &parameters.0[0], default_bm);
+
+                            Ty::apply_one(TypeCtor::Slice, ty)
+                        },
+                        _ => Ty::Unknown,
+                    }
+                } else {
+                    Ty::Unknown
+                }
+            }
             _ => Ty::Unknown,
         };
         // use a new type variable if we got Ty::Unknown here

crates/ra_hir_ty/src/tests/patterns.rs

@@ -136,6 +136,39 @@ fn test() {
     );
 }
 
+#[test]
+fn infer_pattern_match_slice() {
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let slice: &[f64] = &[0.0];
+    match slice {
+        &[a] => {
+            a;
+        }
+        _ => {}
+    }
+}
+"#),
+    @r###"
+    [11; 129) '{     ...   } }': ()
+    [21; 26) 'slice': &[f64]
+    [37; 43) '&[0.0]': &[f64; _]
+    [38; 43) '[0.0]': [f64; _]
+    [39; 42) '0.0': f64
+    [49; 127) 'match ...     }': ()
+    [55; 60) 'slice': &[f64]
+    [71; 75) '&[a]': &[f64]
+    [72; 75) '[a]': [f64]
+    [73; 74) 'a': f64
+    [79; 105) '{     ...     }': ()
+    [93; 94) 'a': f64
+    [114; 115) '_': &[f64]
+    [119; 121) '{}': ()
+    "###
+    );
+}
+
 #[test]
 fn infer_adt_pattern() {
     assert_snapshot!(