matrix literals or const constructors about nalgebra

[tpdickso]

Can you elaborate on what you mean by "types like all matrixes"? Owned matrices of constant size appear to expose const constructors for sizes up to 6x6. Not every matrix can be initialized in a const context because their data may not be const; for example, you can't have a dynamic matrix in a const, because its storage is a Vec, which would require a dynamic allocation.

I suspect the issue with making identity const is that it's a generic function over the scalar field; it invokes Zero::zero and One::one to populate the relevant entries of the matrix. Without a stable way to parameterize over specifically-const implementations of these trait functions I don't think it's possible to produce the relevant const? annotation on identity. Something like from_diagonal but that allows specifying both the on-diagonal and off-diagonal entries would be possible, but currently you can explicitly write new(1.0, 0.0, 0.0, 1.0) to get a const identity matrix.

I don't believe Rust has custom literals at all, but you can get a similar effect using the matrix! macro. It invokes new and so should be usable in a const context when invoking a const new. So you should also be able to write matrix![1.0, 0.0; 0.0, 1.0] to produce a 2x2 identity matrix.