Compile interpolated strings properly for better performance

[charlesroddie]

The code

let world = "world"
let helloWorld = $"hello {world}" compiles to

Currently compiles to (see SharpLab)

object[] array = new object[1];
array[0] = "world";
PrintfModule.PrintFormatToStringThen(new PrintfFormat<string, Unit, string, string, string>("Hello %P()", array, null));

Instead, it should compile to one of these, and testing suggests that Impl2 is better

let helloWorldImpl1() =
    String.Format("Hello {0}", "world")
let helloWorldImpl2() =
    let sb = Text.StringBuilder()
    sb.Append("hello") |> ignore
    sb.Append(world) |> ignore
    sb.ToString()

Pros and Cons

The advantages of making this adjustment to F# are

1. Performance.

https://github.com/charlesroddie/BenchmarksFsharp/tree/InterpolatedStrings

Method	Mean	Error	StdDev	Gen0	Allocated
InterpolatedStringCurrent	88.20 ns	1.764 ns	1.650 ns	0.0219	184 B
Impl1	36.03 ns	0.757 ns	0.632 ns	0.0057	48 B
Impl2	18.30 ns	0.416 ns	0.841 ns	0.0181	152 B

2. Complex generic instantiations are not surfaced, helping the size of apps when AOT compiled. We previously found that this was several MB! But would need to retest with current NativeAOT.

3. The current code requires reflection, making it harder for compilers to reason about, contributing to poor performance, contributing to 2 by bringing in more reflection data, and being incompatible with reflection-free modes.

The disadvantages of making this adjustment to F# are ...

None

Extra information

Estimated cost (XS, S, M, L, XL, XXL):

M

Related suggestions: (put links to related suggestions here)

#919
dotnet/fsharp#13398

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• [ x ] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• [ x ] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
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Please tick all that apply:

• [ x ] This is not a breaking change to the F# language design
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[kerams]

#1108 should be preferred. Implementing string builders for simple cases, while valuable, could result in redundant work otherwise.

[charlesroddie]

Ah you are right. Closing as a duplicate and I'll past the info there.