Optimization for string interpolation - simple scenarios: unfolding constants and lowering to concatenation

[abonie]

Seems like there is a lot we could do to improve perf of string interpolation. One good example is fsharp/fslang-suggestions#1108, but also some low hanging fruits to start with could be unfolding compile time constants, e.g.:

[<Literal>]
let x = "hello"

[<Literal>]
let y = "world"

let z = $"{x} {y}!"

in the above, the last line could be lowered to let z = "hello world!"

Another one could be lowering to System.String.Concat when it applies, e.g. let f (x: string) (y: string) = $"{x},{y}." could be rewritten as let f (x: string) (y: string) = System.String.Concat(x, ",", y, "."), because we know that all expression holes are filled with objects of type string and there are at most 4 pieces to concatenate.

[abonie]

For the second one, here is difference in allocations from calling the first version of the function vs. the second one 1k times in a loop. Using concat:

vs. string interpolation:

[charlesroddie]

I would simplify this to: implement string interpolation as String.Concat in all scenarios. With no helper method around {x} unless one is known to be required. Dealing with constants and other special cases can be treated as a subsequent optimization step.