Allow non-inline literals to be used as printf format

[kerams]

Implements fsharp/fslang-suggestions#638, also extending it to IL constants, because why not.

I wanted to implement this as a type-directed conversion, but I wasn't sure whether I'd have to duplicate the logic from TcFormatStringExpr in MethodCalls.fs, because CheckExpressions is not available there. Since inline literal strings don't use this mechanism either, I chose the straightforward approach.

[T-Gro]

Left minor comments, happy and approving already.

Out of curiosity (and definitely not needed to do if this does not work):
Since we:

• Can derive literals from other literals using basic arithmetical operators (at least for numbers)
• Can have non-inline literals in printfn

Could we:

• Support printfn for a "+"-appended sequence of string literals?
  i.e. having string-literal snippets, and reusing them togeher by the "+" operator.

In order to have better string-reuse of repeatable snippets.

[kerams]

Support printfn for a "+"-appended sequence of string literals?

Yeah, that would be nice. However, string constants aren't folded outside of specific contexts (attributes, literal definitions, ...), so a more general solution would perhaps be better. I think I've seen a conditional compilation directive that disables constant folding of certain kind somewhere?

System.Console.WriteLine ("sadasd"+"Av")

compiles to

Console.WriteLine(string.Concat("sadasd","Av"));

whereas the C# compiler will concatenate the strings at compile time.

i.e. having string-literal snippets, and reusing them togeher by the "+" operator.

Interesting side-effect of these changes! While you can't concatenate the parts inline, you can still achieve reuse with a bit more verbosity

let [<Literal>]coordinatesFormatSnippet="(%f, %f, %f)"let [<Literal>]playerPositionFormat="Player is at "+ coordinatesFormatSnippet
let [<Literal>]objectPositionFormat="Object is at "+ coordinatesFormatSnippet
letprintPlayerPosition p =
printfn playerPositionFormat p.X p.Y p.Z

It would also be cool to support interpolation in this context, but that'd be straying a bit too far from the original suggestion.

let [<Literal>]coordinatesFormatSnippet="(%f, %f, %f)"letprintPlayerPosition p =
printfn $"Player is at {coordinatesFormatSnippet}" p.X p.Y p.Z

[T-Gro]

Support printfn for a "+"-appended sequence of string literals?

Yeah, that would be nice.

The reuse example is already good, I like it.

I think the next level to allow this inline would be a separate issue (not sure if this needs a lang suggestion or not) to "Treat inline + concatenation of any string literals as a string literal as well" , doing the concat already at the compiler.

It does have downsides though - will increase binary codesize for keeping that combined string in, in exchange for 1 less allocation at runtime.
(unless there is codebase which massively "+"s string literals into various combinations, it should not be noticeable of course)

[kerams]

Found a bug here, please don't merge.

[kerams]

Ready now.