Add ui test for function_casts_as_integer lint

Author: GuillaumeGomez

tests/ui/lint/function_casts_as_integer.fixed

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+#![deny(function_casts_as_integer)]
+#![allow(unused_variables)] // For the rustfix-ed code.
+
+fn foo() {}
+
+fn main() {
+    let x = foo as fn() as usize; //~ ERROR: function_casts_as_integer
+    let x = String::len as for<'a> fn(&'a String) -> usize as usize; //~ ERROR: function_casts_as_integer
+    // Ok.
+    let x = foo as fn() as usize;
+}

tests/ui/lint/function_casts_as_integer.rs

@@ -0,0 +1,13 @@
+//@ run-rustfix
+
+#![deny(function_casts_as_integer)]
+#![allow(unused_variables)] // For the rustfix-ed code.
+
+fn foo() {}
+
+fn main() {
+    let x = foo as usize; //~ ERROR: function_casts_as_integer
+    let x = String::len as usize; //~ ERROR: function_casts_as_integer
+    // Ok.
+    let x = foo as fn() as usize;
+}

tests/ui/lint/function_casts_as_integer.stderr

@@ -0,0 +1,29 @@
+error: direct cast of function item into an integer
+  --> $DIR/function_casts_as_integer.rs:9:17
+   |
+LL |     let x = foo as usize;
+   |                 ^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/function_casts_as_integer.rs:3:9
+   |
+LL | #![deny(function_casts_as_integer)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+help: first cast to a function pointer `fn()`
+   |
+LL |     let x = foo as fn() as usize;
+   |                 +++++++
+
+error: direct cast of function item into an integer
+  --> $DIR/function_casts_as_integer.rs:10:25
+   |
+LL |     let x = String::len as usize;
+   |                         ^^^^^^^^
+   |
+help: first cast to a function pointer `for<'a> fn(&'a String) -> usize`
+   |
+LL |     let x = String::len as for<'a> fn(&'a String) -> usize as usize;
+   |                         ++++++++++++++++++++++++++++++++++
+
+error: aborting due to 2 previous errors
+