Add new lint: strlen_on_c_strings

tests/ui/strlen_on_c_strings.rs

@@ -0,0 +1,16 @@
+#![warn(clippy::strlen_on_c_strings)]
+#![allow(dead_code)]
+#![feature(rustc_private)]
+extern crate libc;
+
+use std::ffi::{CStr, CString};
+
+fn main() {
+    // CString
+    let cstring = CString::new("foo").expect("CString::new failed");
+    let len = unsafe { libc::strlen(cstring.as_ptr()) };
+
+    // CStr
+    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    let len = unsafe { libc::strlen(cstr.as_ptr()) };
+}

tests/ui/strlen_on_c_strings.stderr

@@ -0,0 +1,25 @@
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:11:24
+   |
+LL |     let len = unsafe { libc::strlen(cstring.as_ptr()) };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::strlen-on-c-strings` implied by `-D warnings`
+help: try this (you might also need to get rid of `unsafe` block in some cases):
+   |
+LL |     let len = unsafe { cstring.as_bytes().len() };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:15:24
+   |
+LL |     let len = unsafe { libc::strlen(cstr.as_ptr()) };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try this (you might also need to get rid of `unsafe` block in some cases):
+   |
+LL |     let len = unsafe { cstr.to_bytes().len() };
+   |                        ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+