[1]: https://staff.fnwi.uva.nl/p.vanemdeboas/knuthnote.pdf

Inconclusive tests to increase automation and coverage accomplish less than nothing. The opportunity cost, the loss of trust, the conflation of the manual testing surface area, are all worse than doing nothing.

The point of testing is to establish a level of trust. That level doesn’t need to be 100%. It can’t be 100% regardless. So you do what you can, and you solve the remaining problem a different way. But you need to know it’s there to address it, and the smoke and mirrors fight that.

All languages are built on top of an ISA, whether directly (assembler), or by mapping it to an abstract machine. The you have the dreaded “eval” function. Then because a lot of software lives on top of an OS, there’s all the virtualization that is going on (timesharing, virtual memory, filesystems,…). Memory safety is nice, but it doesn’t help you when the attacker send valid, but malicious data from a compromised source, or by exploiting a logic error.

And for kernel code, where everything is accessible to everyone, it’s much a matter of discipline, especially with so much hardware interrupts and syscalls from userspace going around. Most kernel code has very short lifecycle, it’s the data that is long lived, and it’s a shapeless blob in memory that you have to trust the other party kept in good shape.

https://en.wikipedia.org/wiki/Zero-one-infinity_rule

The rule is about data structures but I feel it has wider applicability user choices in software. You should either give the user no choices, give them individual binary choices, or let them pick anything that could theoretically be valid and design your system to assume they'll sometimes pick invalid options which will need to be handled appropriately.

I say that as someone who sees Knuth as problematic in other respects starting in the 2010’s. Physics governs so much of computational complexity now that I think the idealized form he presents has transitioned from merely holding us back to being crippling. Only game designers, hyperscalers, and an esoteric few of the rest of us seem to understand the vast gulf between theory and practice that has developed in the five orders of magnitude that compute power has experienced since his first edition.

Many, many things that are considered constant in his world are instead logarithmic to the size of the data being managed, or in the case of very large data sets, order n^1/2 because of speed of light and thermodynamics. That vastly reorders the toolbox, because second shelf algorithms now rival or beat the top shelf ones and some options even farther down now become interesting for novel situations.