Make message for &T -> &mut T transmute more accurate

compiler/rustc_lint/src/builtin.rs

@@ -1248,7 +1248,7 @@ fn check_item(&mut self, cx: &LateContext<'_>, it: &hir::Item<'_>) {
     /// [`UnsafeCell`]: https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html
     MUTABLE_TRANSMUTES,
     Deny,
-    "mutating transmuted &mut T from &T may cause undefined behavior"
+    "transmuting &T to &mut T is undefined behavior, even if the reference is unused"
 }
 
 declare_lint_pass!(MutableTransmutes => [MUTABLE_TRANSMUTES]);
@@ -1260,8 +1260,8 @@ fn check_expr(&mut self, cx: &LateContext<'_>, expr: &hir::Expr<'_>) {
             get_transmute_from_to(cx, expr).map(|(ty1, ty2)| (ty1.kind(), ty2.kind()))
         {
             if to_mt == hir::Mutability::Mut && from_mt == hir::Mutability::Not {
-                let msg = "mutating transmuted &mut T from &T may cause undefined behavior, \
-                               consider instead using an UnsafeCell";
+                let msg = "transmuting &T to &mut T is undefined behavior, \
+                    even if the reference is unused, consider instead using an UnsafeCell";
                 cx.struct_span_lint(MUTABLE_TRANSMUTES, expr.span, |lint| lint.build(msg).emit());
             }
         }

src/test/ui/transmute/transmute-imut-to-mut.rs

@@ -4,5 +4,5 @@
 
 fn main() {
     let _a: &mut u8 = unsafe { transmute(&1u8) };
-    //~^ ERROR mutating transmuted &mut T from &T may cause undefined behavior
+    //~^ ERROR transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
 }

src/test/ui/transmute/transmute-imut-to-mut.stderr

@@ -1,4 +1,4 @@
-error: mutating transmuted &mut T from &T may cause undefined behavior, consider instead using an UnsafeCell
+error: transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
   --> $DIR/transmute-imut-to-mut.rs:6:32
    |
 LL |     let _a: &mut u8 = unsafe { transmute(&1u8) };