Read-only pointers, and what C's `const T*` becomes

Decided

D-0037, settled by the language’s author: part of spec §18. Older text cites it as 18.37.

^T is read-only and ^mut T writable, standing to each other as &T/&mut T and []T/[]mut T do: read-only unless mut is written. const T* maps to ^T and T* to ^mut T (§15.2). &T → ^T and &mut T → ^mut T are implicit, anything writable serves where the read-only form is wanted, and nothing gains writability; or gives a &T from a ^T and a &mut T from a ^mut T; .ptr is as writable as its view; the owning pointer is own ^mut T. Behind a ^mut the pointee must match exactly, for the reason C forbids T** → const T**. A ^const T was rejected as the Rust shape, which reads against the rest of the language. Forced by M7: with one writable pointer, every read-only C parameter needed an &mut at the call (§5.2, §5.3)