csharplang/proposals/format.md

# Efficent Formatting

## Summary
This combination of features will allow for effecient and customizable formatting of `string` values.

## Motivation
The allocation of `string` and `params` values dominates the performance of many text based applications like MSBuild. 
In order to maintain efficiency such applications often need to abandon popular features like `params`, `string` 
interpolation, etc ... This set of language features will enable applications to avoid the extra allocation overhead
while continuing to use these features. 

## Detailed Design 
There are a set of features that will be used here to achieve these results:

- Expanding `params` to support more effecient types than array. 
- Allowing for developers to customize how `string` interpolation is achieved. 

### params Span
The language will allow for `params` in a method signature to have the types `Span<T>` and `ReadOnlySpan<T>`. The same
rules will apply to `params Span<T>` that apply to `params T[]`:

- Can't overload where the only difference is a `params` keyword.
- Can invoke by passing a series of `T` arguments or a single `Span<T>` argument.
- Must be the last parameter in a method signature.
- Etc ... 

The advantage this variant of `params` provides is it gives the compiler great flexbility in how it allocations the
backing storage for the `Span<T>` value. With a `params T[]` the compiler must allocate a new `T[]` for every 
invocation of a `params` method because it must assume the callee stored and reused the parameter. Given the 
`Span<T>` and `ReadOnlySpan<T>` types are `ref struct` the callee cannot store the argument. Hence the compiler can
safely re-use the value. 

One such potential implementation is the following. Consider all `params` invocation of a given type in a method 
body. The compiler could allocate an array which has a size equal to the largest `params` invocation and use that for
all of the invocations by creating appropriately sized `Span<T>` instances over the array. For example:

``` csharp
static class OneAllocation {
    static void Use(params Span<string> spans) {
        ...
    }

    static void Go() {
        Use("jaredpar");
        Use("hello", "world");
        Use("a", "longer", "set");
    }
}
```

The compiler could choose to emit the body of `Go` as follows:

``` csharp
    static void Go() {
        var args = new string[3];
        args[0] = "jaredpar";
        Use(new Span<int>(args, start: 0, length: 1));

        args[0] = "hello";
        args[1] = "world";
        Use(new Span<int>(args, start: 0, length: 2));

        args[0] = "a";
        args[1] = "longer";
        args[2] = "set";
        Use(new Span<int>(args, start: 0, length: 3));
   }
```

This can siginficantly reduce the number of arrays allocated in an application. Allocations can be even further 
reduced if the runtime provides utilities for smarter stack allocation of arrays.

This optimization cannot always be applied though. Even though the callee cannot capture the `params` argument it can 
still be captured in the caller when there is a `ref` or a `out / ref` parameter that is itself a `ref struct`
type. 

``` csharp
static class SneakyCapture {
    static ref int M(params Span<T> span) => ref span[0];

    static void Oops() {
        // This now holds onto the memory backing the Span<T> 
        ref int r = ref M(42);
    }
}
```

These cases are statically dectable though. It potentially occurs whenever there is a `ref` return or a `ref struct`
parameter passed by `out` or `ref`. In such a case the compiler must allocate a fresh `T[]` for every invocation. 

### params IEnumerable
The language will allow for `params` in a method signature to have the type `IEnumerable<T>`. The same rules will apply 
to `params IEnumerable<T>` that apply to `params T[]` (see above for listing of rules).

The compiler will invoke a `params IEnumerable<T>` method exactly as it invokes a `params T[]` method. A new array will
be allocated for every call site and passed to the callee.

### params VariantCollection
The language will allow for `params` in a method signature to have the type `VariantCollection`. This elements of a 
`VariantCollection` are considered to be of type `Variant`. The same rules will apply to `params VariantCollection` 
that apply to `params Variant[]` (see above for listing of rules).

The `Variant` value type is a [suggested addition](https://github.com/dotnet/corefxlab/pull/2595) to CoreFX that has a
conversion from any type. For the majority of common framework types (`int`, `double`, `TimeSpan`, etc ..) the 
conversion is allocation free.

Methods which take a `params` of hetrogeneous data today must declare as `params object[]`. This means that every 
value type passed to the method incurs a boxing allocation (in addition to the array the compiler must allocate). The
`Variant` and `VariantCollection` type allow such methods to avoid allocations in the most common paths. Invocations
like `Console.WriteLine("hello {0] {1}", someInt, DateTime.Now)` can be invoked without any boxing or array allocations
here.

When emitting the code for a `params VariantCollection` invocation the compiler will look for an overload of 
`VariantCollection.Create(in Variant v1, in Variant v2, ..., in Variant vn)` where `n` matches the number of arguments 
being passed to the parameter. Each argument will be passed exactly as if the developer wrote the
`VariantCollection.Create` call with the arguments. For example:

``` csharp
class VariantCall { 
    static void Write(params VariantCollection collection) { ... } 

    static void Use() {
        Write(1, DateTime.UtcNow)

        // Compiler will evaluate as 
        Write(VariantCollection.Create(1, DateTime.UtcNow))

        // Compiler will eventually emit as 
        Write(VariantCollection.Create((Variant)1, (Variant)DateTime.UtcNow))
    }
}
```

In the case a `VariantCollection.Create` method with the correct number of parameters does not exist the compiler will
attempt to invoke `Create(Variant[] array)` instead. A fresh `Variant[]` will be allocated for every invocation in this
case. In the case `Create(Variant[] array)` does not exist the compiler will issue an error.

### params overload resolution changes
This proposal means the language now has four variants of `params` where before it had one. It is also sensible for 
methods to define overloads of methods that differ only on `params` declarations. 

Consider that `StringBuilder.AppendFormat` would certainly add a `params VariantCollection` overload in addition to the
`params object[]`. This would allow it to substantially improve performance by reducing boxing and collection 
allocations without requiring any changes to the calling code. 

To facilitate this the language will introduce the following overload resolution tie breaking rule. When the candidate
methods differ only by the `params` parameter then the canditates will be preferred in the following order:

1. `VariantCollection`
1. `ReadOnlySpan<T>`
1. `Span<T>`
1. `T[]`
1. `IEnumerable<T>`

This order is the most to the least effecient for the general case.

### Efficient interpolated strings
Interpolated strings are a popular yet innefecient feature in C#. The most common syntax, using an interpolated string
as a `string`, translates into a `string.Format` call. That will inccur boxing allocations for all value types, 
intermediate `string` allocations as the implementation largely uses `object.ToString` for formatting as well as
array allocations once the number of arguments exceeds the amount of parameters on the "fast" overloads of 
`string.Format`. 

Developers are able to customize the behavior of interpolated strings with `FormattableString`. This contains the data
which goes into an interpolated string: the format strings and the arguments as an array. This though still has the 
boxing and argument array allocation as well as the allocation for `FormattableString` (it's an `abstract class`). Hence
it's of little use to applications which are allocation heavy in `string` formatting.

To make interopolated string formatting efficient the language will recognize a new type: 
`System.ValueFormattableString`. All interpolated strings will have a target type conversion to this type. This will 
be implemented by translating the interpolated string into the call `ValueFormattableString.Create` exactly as is done
for `FormattableString.Create` today. The language will support all `params` options described in this document when
looking for the most suitable `ValueFormattableString.Create` method. 

``` csharp
readonly struct ValueFormattableString {
    public static ValueFormattableString Create(Variant v) { ... } 
    public static ValueFormattableString Create(string s) { ... } 
    public static ValueFormattableString Create(string s, params VariantCollection collection) { ... } 
    public static ValueFormattableString Create(string s, Variant v) { ... }
}

class ConsoleEx { 
    static void Write(ValueFormattableString f) { ... }
}

class Program { 
    static void Main() { 
        ConsoleEx.Write(42);
        ConsoleEx.Write("hello {DateTime.UtcNow}");

        // Translates into 
        ConsoleEx.Write(ValueFormattableString.Create((Variant)42));
        ConsoleEx.Write(ValueFormattableString.Create(
            "hello {0}", 
            VariantCollection.Create((Variant)DateTime.UtcNow));
    }
}
```

Overload resolution rules will be changed to prefer `ValueFormattableString` over `string` when the argument is an 
interpolated string. This means it will be valuable to have overloads which differ only on `string` and 
`ValueFormattableString`. Such an overload today with `FormattableString` is not valauble as the compiler will always
prefer the `string` version (unless the developer uses an explicit cast). 

## Open Issuess

### ValuableFormattableString breaking change
The change to prefer `ValueFormattableString` during overload resolution over `string` is a breaking change. It is
possible for a developer to have defined a type called `ValueFormattableString` today and use it in method overloads
with `string`. This proposed change would cause the compiler to pick a different overload once this set of features
was implemented. 

The possibility of this seems reasonably low. The type would need the full name `System.ValueFormattableString` and it 
would need to have `static` methods named `Create`. Given that developers are strongly discouraged from defining any
type in the `System` namespace this break seems like a reasonable compromise.

### open issue1

## Considerations

### consideration 1
## Future Considerations

CLR helper for stack allocating arrays 

## Misc
Related issues
- https://github.com/dotnet/csharplang/issues/1757
- https://github.com/dotnet/csharplang/issues/179
- https://github.com/dotnet/corefxlab/pull/2595