The title is slightly misleading, and so is Linus' response here. This RFC never claimed to be in shape to be immediately mergeable into the mainline kernel as-is. Miguel (the author of the patch) has replied to this mail (and in other places in the thread) that the Rust alloc() is currently called only as a temporary measure to speed up development, and all panic() calls from allocation failures are just as temporary. This is all because the Rust code that hooks into the kernel memory allocation functions is not yet usable.
The main point of this RFC is that "the [in-kernel Rust] support is good enough that prototyping modules can start today." There's no point in making long arguments about alpha-quality design shortcuts on an alpha-quality prototype that are also explicitly mentioned by the patch authors to be of alpha quality.
EDIT: Thanks for the child comments; it seems that Linus is simply not aware of all the specifics and is asking for more information and/or decided to look at the code before reading the full mail thread.
The main point of this RFC is that "the [in-kernel Rust] support is good enough that prototyping modules can start today." There's no point in making long arguments about alpha-quality design shortcuts on an alpha-quality prototype that are also explicitly mentioned by the patch authors to be of alpha quality.
See e.g. https://lkml.org/lkml/2021/4/14/1130 and https://lkml.org/lkml/2021/4/14/1023
EDIT: Thanks for the child comments; it seems that Linus is simply not aware of all the specifics and is asking for more information and/or decided to look at the code before reading the full mail thread.