Merge #3387

3387: Type inference for slice patterns r=flodiebold a=JoshMcguigan

Fixes #3043 

Notes to reviewer:

1. This only works if `expected` is `Ty::Apply`. I'm not sure of the implications of this.
1. This only works if the slice pattern only has a prefix. I think this means it doesn't work for subslice patterns, which are currently only available behind a feature flag.

Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>

crates/ra_hir_ty/src/infer/pat.rs

@@ -185,6 +185,23 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.write_pat_ty(pat, bound_ty);
                 return inner_ty;
             }
+            Pat::Slice { prefix, slice: _slice, suffix } => {
+                let (container_ty, elem_ty) = match &expected {
+                    ty_app!(TypeCtor::Array, st) => {
+                        (TypeCtor::Array, st.as_single().clone())
+                    },
+                    ty_app!(TypeCtor::Slice, st) => {
+                        (TypeCtor::Slice, st.as_single().clone())
+                    },
+                    _ => (TypeCtor::Slice, Ty::Unknown),
+                };
+
+                for pat_id in prefix.iter().chain(suffix) {
+                    self.infer_pat(*pat_id, &elem_ty, default_bm);
+                }
+
+                Ty::apply_one(container_ty, elem_ty)
+            }
             _ => Ty::Unknown,
         };
         // use a new type variable if we got Ty::Unknown here

crates/ra_hir_ty/src/tests/patterns.rs

@@ -53,8 +53,9 @@ fn test(x: &i32) {
     [140; 141) 'g': {unknown}
     [144; 145) 'e': {unknown}
     [158; 205) 'if let...     }': ()
-    [165; 170) '[val]': {unknown}
-    [173; 176) 'opt': {unknown}
+    [165; 170) '[val]': [{unknown}]
+    [166; 169) 'val': {unknown}
+    [173; 176) 'opt': [{unknown}]
     [177; 205) '{     ...     }': ()
     [191; 192) 'h': {unknown}
     [195; 198) 'val': {unknown}
@@ -136,6 +137,94 @@ fn test() {
     );
 }
 
+#[test]
+fn infer_pattern_match_slice() {
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let slice: &[f64] = &[0.0];
+    match slice {
+        &[] => {},
+        &[a] => {
+            a;
+        },
+        &[b, c] => {
+            b;
+            c;
+        }
+        _ => {}
+    }
+}
+"#),
+    @r###"
+    [11; 210) '{     ...   } }': ()
+    [21; 26) 'slice': &[f64]
+    [37; 43) '&[0.0]': &[f64; _]
+    [38; 43) '[0.0]': [f64; _]
+    [39; 42) '0.0': f64
+    [49; 208) 'match ...     }': ()
+    [55; 60) 'slice': &[f64]
+    [71; 74) '&[]': &[f64]
+    [72; 74) '[]': [f64]
+    [78; 80) '{}': ()
+    [90; 94) '&[a]': &[f64]
+    [91; 94) '[a]': [f64]
+    [92; 93) 'a': f64
+    [98; 124) '{     ...     }': ()
+    [112; 113) 'a': f64
+    [134; 141) '&[b, c]': &[f64]
+    [135; 141) '[b, c]': [f64]
+    [136; 137) 'b': f64
+    [139; 140) 'c': f64
+    [145; 186) '{     ...     }': ()
+    [159; 160) 'b': f64
+    [174; 175) 'c': f64
+    [195; 196) '_': &[f64]
+    [200; 202) '{}': ()
+    "###
+    );
+}
+
+#[test]
+fn infer_pattern_match_arr() {
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let arr: [f64; 2] = [0.0, 1.0];
+    match arr {
+        [1.0, a] => {
+            a;
+        },
+        [b, c] => {
+            b;
+            c;
+        }
+    }
+}
+"#),
+    @r###"
+    [11; 180) '{     ...   } }': ()
+    [21; 24) 'arr': [f64; _]
+    [37; 47) '[0.0, 1.0]': [f64; _]
+    [38; 41) '0.0': f64
+    [43; 46) '1.0': f64
+    [53; 178) 'match ...     }': ()
+    [59; 62) 'arr': [f64; _]
+    [73; 81) '[1.0, a]': [f64; _]
+    [74; 77) '1.0': f64
+    [79; 80) 'a': f64
+    [85; 111) '{     ...     }': ()
+    [99; 100) 'a': f64
+    [121; 127) '[b, c]': [f64; _]
+    [122; 123) 'b': f64
+    [125; 126) 'c': f64
+    [131; 172) '{     ...     }': ()
+    [145; 146) 'b': f64
+    [160; 161) 'c': f64
+    "###
+    );
+}
+
 #[test]
 fn infer_adt_pattern() {
     assert_snapshot!(