Add test for unnecessary_refs lint

Author: obeis

tests/ui/consts/offset_from.rs

@@ -1,5 +1,7 @@
 //@ run-pass
 
+#![allow(unnecessary_refs)]
+
 struct Struct {
     field: (),
 }

tests/ui/lint/lint-unnecessary-refs.rs

@@ -0,0 +1,18 @@
+#![deny(unnecessary_refs)]
+
+struct A {
+    a: i32,
+    b: u8,
+}
+
+fn via_ref(x: *const (i32, i32)) -> *const i32 {
+    unsafe { &(*x).0 as *const i32 }
+    //~^ ERROR creating unecessary reference is discouraged [unnecessary_refs]
+}
+
+fn via_ref_struct(x: *const A) -> *const u8 {
+    unsafe { &(*x).b as *const u8 }
+    //~^ ERROR creating unecessary reference is discouraged [unnecessary_refs]
+}
+
+fn main() {}

tests/ui/lint/lint-unnecessary-refs.stderr

@@ -0,0 +1,31 @@
+error: creating unecessary reference is discouraged
+  --> $DIR/lint-unnecessary-refs.rs:9:14
+   |
+LL |     unsafe { &(*x).0 as *const i32 }
+   |              ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/lint-unnecessary-refs.rs:1:9
+   |
+LL | #![deny(unnecessary_refs)]
+   |         ^^^^^^^^^^^^^^^^
+help: consider using `&raw const` for a safer and more explicit raw pointer
+   |
+LL -     unsafe { &(*x).0 as *const i32 }
+LL +     unsafe { &raw const (*x).0 }
+   |
+
+error: creating unecessary reference is discouraged
+  --> $DIR/lint-unnecessary-refs.rs:14:14
+   |
+LL |     unsafe { &(*x).b as *const u8 }
+   |              ^^^^^^^^^^^^^^^^^^^^
+   |
+help: consider using `&raw const` for a safer and more explicit raw pointer
+   |
+LL -     unsafe { &(*x).b as *const u8 }
+LL +     unsafe { &raw const (*x).b }
+   |
+
+error: aborting due to 2 previous errors
+