refactor: forbid clippy::indexing_slicing

It is impossible to allow this in the new code now.

deltachat-contact-tools/src/lib.rs

@@ -15,7 +15,7 @@
     clippy::explicit_into_iter_loop,
     clippy::cloned_instead_of_copied
 )]
-#![cfg_attr(not(test), warn(clippy::indexing_slicing))]
+#![cfg_attr(not(test), forbid(clippy::indexing_slicing))]
 #![allow(
     clippy::match_bool,
     clippy::mixed_read_write_in_expression,

deltachat-jsonrpc/src/lib.rs

@@ -1,4 +1,5 @@
 #![recursion_limit = "256"]
+#![cfg_attr(not(test), forbid(clippy::indexing_slicing))]
 pub mod api;
 pub use yerpc;
 

format-flowed/src/lib.rs

@@ -8,6 +8,7 @@
 //! is assumed to be set to "no".
 //!
 //! For received messages, DelSp parameter is honoured.
+#![cfg_attr(not(test), forbid(clippy::indexing_slicing))]
 
 /// Wraps line to 72 characters using format=flowed soft breaks.
 ///

src/lib.rs

@@ -16,7 +16,7 @@
     clippy::explicit_into_iter_loop,
     clippy::cloned_instead_of_copied
 )]
-#![cfg_attr(not(test), warn(clippy::indexing_slicing))]
+#![cfg_attr(not(test), forbid(clippy::indexing_slicing))]
 #![allow(
     clippy::match_bool,
     clippy::mixed_read_write_in_expression,