Auto merge of #114494 - est31:extend_useless_ptr_null_checks, r=jackh726

Make useless_ptr_null_checks smarter about some std functions

This teaches the `useless_ptr_null_checks` lint that some std functions can't ever return null pointers, because they need to point to valid data, get references as input, etc.

This is achieved by introducing an `#[rustc_never_returns_null_ptr]` attribute and adding it to these std functions (gated behind bootstrap `cfg_attr`).

Later on, the attribute could maybe be used to tell LLVM that the returned pointer is never null. I don't expect much impact of that though, as the functions are pretty shallow and usually the input data is already never null.

Follow-up of PR #113657

Fixes #114442

compiler/rustc_lint/src/lints.rs

@@ -635,6 +635,8 @@ pub enum PtrNullChecksDiag<'a> {
         #[label]
         label: Span,
     },
+    #[diag(lint_ptr_null_checks_fn_ret)]
+    FnRet { fn_name: Ident },
 }
 
 // for_loops_over_fallibles.rs