Fix match ergonomics in closure parameters

Fixes #4476.

crates/ra_hir_ty/src/tests/patterns.rs

@@ -520,3 +520,53 @@ fn main() {
         105..107 '()': ()
     ")
 }
+
+#[test]
+fn match_ergonomics_in_closure_params() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "fn_once"]
+trait FnOnce<Args> {
+    type Output;
+}
+
+fn foo<T, U, F: FnOnce(T) -> U>(t: T, f: F) -> U { loop {} }
+
+fn test() {
+    foo(&(1, "a"), |&(x, y)| x); // normal, no match ergonomics
+    foo(&(1, "a"), |(x, y)| x);
+}
+"#),
+        @r###"
+    94..95 't': T
+    100..101 'f': F
+    111..122 '{ loop {} }': U
+    113..120 'loop {}': !
+    118..120 '{}': ()
+    134..233 '{     ... x); }': ()
+    140..143 'foo': fn foo<&(i32, &str), i32, |&(i32, &str)| -> i32>(&(i32, &str), |&(i32, &str)| -> i32) -> i32
+    140..167 'foo(&(...y)| x)': i32
+    144..153 '&(1, "a")': &(i32, &str)
+    145..153 '(1, "a")': (i32, &str)
+    146..147 '1': i32
+    149..152 '"a"': &str
+    155..166 '|&(x, y)| x': |&(i32, &str)| -> i32
+    156..163 '&(x, y)': &(i32, &str)
+    157..163 '(x, y)': (i32, &str)
+    158..159 'x': i32
+    161..162 'y': &str
+    165..166 'x': i32
+    204..207 'foo': fn foo<&(i32, &str), &i32, |&(i32, &str)| -> &i32>(&(i32, &str), |&(i32, &str)| -> &i32) -> &i32
+    204..230 'foo(&(...y)| x)': &i32
+    208..217 '&(1, "a")': &(i32, &str)
+    209..217 '(1, "a")': (i32, &str)
+    210..211 '1': i32
+    213..216 '"a"': &str
+    219..229 '|(x, y)| x': |&(i32, &str)| -> &i32
+    220..226 '(x, y)': (i32, &str)
+    221..222 'x': &i32
+    224..225 'y': &&str
+    228..229 'x': &i32
+    "###
+    );
+}