add a test for tuple coercions

Author: WaffleLapkin

tests/ui/tuple/coercion.rs

@@ -0,0 +1,17 @@
+// This test checks if tuple elements are a coercion site or not.
+// Note that the code here is a degenerate case, but you can get similar effects in real code, when
+// unifying match arms, for example.
+
+fn main() {
+    let _: ((),) = (loop {},);
+
+    ((),) = (loop {},); //~ error: mismatched types
+
+    let x = (loop {},);
+    let _: ((),) = x; //~ error: mismatched types
+
+    let _: (&[u8],) = (&[],);
+
+    let y = (&[],);
+    let _: (&[u8],) = y; //~ error: mismatched types
+}

tests/ui/tuple/coercion.stderr

@@ -0,0 +1,36 @@
+error[E0308]: mismatched types
+  --> $DIR/coercion.rs:8:6
+   |
+LL |     ((),) = (loop {},);
+   |      ^^     ---------- this expression has type `(!,)`
+   |      |
+   |      expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/coercion.rs:11:20
+   |
+LL |     let _: ((),) = x;
+   |            -----   ^ expected `((),)`, found `(!,)`
+   |            |
+   |            expected due to this
+   |
+   = note: expected tuple `((),)`
+              found tuple `(!,)`
+
+error[E0308]: mismatched types
+  --> $DIR/coercion.rs:16:23
+   |
+LL |     let _: (&[u8],) = y;
+   |            --------   ^ expected `(&[u8],)`, found `(&[_; 0],)`
+   |            |
+   |            expected due to this
+   |
+   = note: expected tuple `(&[u8],)`
+              found tuple `(&[_; 0],)`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.