Merge #8840

8840: fix: false positive "Missing match arm" when a tuple pattern is shorter than scrutinee type. r=Veykril a=iDawer

![Screenshot_20210515_003035](https://user-images.githubusercontent.com/7803845/118320023-2bcb7380-b4eb-11eb-9de6-d8762f981dc2.jpg)
Match checking diagnostic shouldn't fire when there is type mismatches.

rust-analyzer fd109fb58 2021-05-10 dev

(This is part of the preparation for https://github.com/rust-analyzer/rust-analyzer/pull/8717)

Co-authored-by: Dawer <7803845+iDawer@users.noreply.github.com>

crates/hir_ty/src/diagnostics/match_check.rs

@@ -1119,6 +1119,7 @@ fn main() {
         (true, false, true) => (),
         (true) => (),
     }
+    match (true, false) { (true,) => {} }
     match (0) { () => () }
     match Unresolved::Bar { Unresolved::Baz => () }
 }

crates/hir_ty/src/infer/pat.rs

@@ -126,11 +126,12 @@ impl<'a> InferenceContext<'a> {
                     _ => &[],
                 };
 
-                let (pre, post) = match ellipsis {
-                    Some(idx) => args.split_at(idx),
-                    None => (&args[..], &[][..]),
+                let ((pre, post), n_uncovered_patterns) = match ellipsis {
+                    Some(idx) => {
+                        (args.split_at(idx), expectations.len().saturating_sub(args.len()))
+                    }
+                    None => ((&args[..], &[][..]), 0),
                 };
-                let n_uncovered_patterns = expectations.len().saturating_sub(args.len());
                 let err_ty = self.err_ty();
                 let mut expectations_iter =
                     expectations.iter().map(|a| a.assert_ty_ref(&Interner)).chain(repeat(&err_ty));

crates/hir_ty/src/tests/patterns.rs

@@ -732,7 +732,7 @@ fn foo(tuple: (u8, i16, f32)) {
             111..112 'a': u8
             114..115 'b': i16
             124..126 '{}': ()
-            136..142 '(a, b)': (u8, i16, f32)
+            136..142 '(a, b)': (u8, i16)
             137..138 'a': u8
             140..141 'b': i16
             146..161 '{/*too short*/}': ()