Fix match ergonomics in closure parameters

Fixes #4476.

Author: flodiebold

crates/ra_hir_ty/src/infer/expr.rs

@@ -140,13 +140,13 @@ impl<'a> InferenceContext<'a> {
 
                 let mut sig_tys = Vec::new();
 
-                for (arg_pat, arg_type) in args.iter().zip(arg_types.iter()) {
-                    let expected = if let Some(type_ref) = arg_type {
+                // collect explicitly written argument types
+                for arg_type in arg_types.iter() {
+                    let arg_ty = if let Some(type_ref) = arg_type {
                         self.make_ty(type_ref)
                     } else {
-                        Ty::Unknown
+                        self.table.new_type_var()
                     };
-                    let arg_ty = self.infer_pat(*arg_pat, &expected, BindingMode::default());
                     sig_tys.push(arg_ty);
                 }
 
@@ -158,7 +158,7 @@ impl<'a> InferenceContext<'a> {
                 sig_tys.push(ret_ty.clone());
                 let sig_ty = Ty::apply(
                     TypeCtor::FnPtr { num_args: sig_tys.len() as u16 - 1 },
-                    Substs(sig_tys.into()),
+                    Substs(sig_tys.clone().into()),
                 );
                 let closure_ty =
                     Ty::apply_one(TypeCtor::Closure { def: self.owner, expr: tgt_expr }, sig_ty);
@@ -168,6 +168,12 @@ impl<'a> InferenceContext<'a> {
                 // infer the body.
                 self.coerce(&closure_ty, &expected.ty);
 
+                // Now go through the argument patterns
+                for (arg_pat, arg_ty) in args.iter().zip(sig_tys) {
+                    let resolved = self.resolve_ty_as_possible(arg_ty);
+                    self.infer_pat(*arg_pat, &resolved, BindingMode::default());
+                }
+
                 let prev_diverges = mem::replace(&mut self.diverges, Diverges::Maybe);
                 let prev_ret_ty = mem::replace(&mut self.return_ty, ret_ty.clone());
 

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
+    "###
+    );
+}