Add unsafe checks

[CohenArthur]

• Dereferencing raw pointers unsafe: Add UnsafeCheck visitor #1410
• Calls to unsafe functions unsafe: Check for unsafe function/method calls #1427
• Forbid calls to asm!() in safe contexts #1569
• Use of mutable static unsafe: check uses of static variables #1417
• Use of extern static unsafe: check uses of static variables #1417
• Access to a union's field unsafe: Add checks for union field accesses #1416
• Call to functions with #[target(feature)] attribute
• Initializing type with rustc_layout_scalar_valid_range attribute
• Mutation of layout constrained field
• Borrow of layout constrained field

I believe the last two are related to the Freeze lang item which we do not support yet.

From @bjorn3:

• Borrow of a packed struct's field (currently back compat lint in rustc, will be a hard error in the future)

[bjorn3]

Mutation of layout constrained field

What do you mean with a layout constrained field? A field of a packed struct? Or a field of a type with the rustc_layout_scalar_valid_range_start or rustc_layout_scalar_valid_range_end attributes? Neither is related to Freeze.

[CohenArthur]

Mutation of layout constrained field

What do you mean with a layout constrained field? A field of a packed struct? Or a field of a type with the rustc_layout_scalar_valid_range_start or rustc_layout_scalar_valid_range_end attributes? Neither is related to Freeze.

I'm referring to these two unsafe operations which seems to be triggered only in some cases like this one

I didn't look much into it, I had just read your comment on #182 and assumed this had to do with Freeze. I might be completely off the mark, sorry about that.

[bjorn3]

I'm referring to these two unsafe operations which seems to be triggered only in some cases like this one

I see. In that case I have another item to add to the list:

• Borrow of a packed struct's field (currently back compat lint in rustc, will be a hard error in the future)

[CohenArthur]

Thank you! Adding it to the list to make sure I don't miss it