TryFormat with format strings as ReadOnlySpan<char> or string

[stephentoub]

We've approved and added a bunch of TryFormat methods for doing the equivalent of ToString, but rendering into a supplied Span<char> instead of allocating a string. Many of these TryFormat methods accept a string format argument, just as does ToString. @justinvp raised the question of whether these should instead be ReadOnlySpan<char> format.

Example scenario:
His scenario was in looking at changing the implementation of string.Format, StringBuilder.AppendFormat, etc.: today they allocate substrings when parsing out formats from holes, e.g. "blah {0:D4} blah {1:[##-##-##]}", in order to supply that format string to the formatting API. We can achieve this internally for these methods; the question is whether we should expose that publicly so that other scenarios can benefit from it as well, if there are other scenarios.

Another scenario:
https://github.com/dotnet/corefx/blob/master/src/System.Runtime.Numerics/src/System/Numerics/BigNumber.cs#L578
BigInteger creates a format string to use when formatting its _sign value, in order to specify the number of digits. If format supported ReadOnlySpan<char>, we could use stack space instead of allocating a string.

The main downside to this change is it makes the methods a bit harder to use, in particular because there's no implicit cast from string to ReadOnlySpan<char>, so instead of passing in a format like "D4", you need to do "D4".AsReadOnlySpan(). TryFormat methods on the following types would be affected:

BigInteger // TryFormat not yet implemented
Byte
DateTime
DateTimeOffset
Decimal
Double // TryFormat not yet implemented
Guid
Int16
Int32
Int64
SByte
Single // TryFormat not yet implemented
TimeSpan
UInt16
UInt32
UInt64

Options

1. Leave the design as-is. We could using string format, we can address string.Format/StringBuilder.AppendFormat internally, and rare public uses can allocate strings when needed.
2. Change the methods to use ReadOnlySpan<char> format instead of string format. Callers use AsReadOnlySpan() when they have a string.
3. Change the methods to use ReadOnlySpan<char> format instead of string format, and add an implicit cast on string from string to ReadOnlySpan<char>.
4. Keep the existing string format methods, and add overloads that take ReadOnlySpan<char>.

[terrajobst]

Video

We seem to gravitate to (2) because it doesn't require an implicit conversion, but also doesn't prevent us from adding it in the future.

@stephentoub has filed a separate issue to discuss (3) dotnet/corefx#25413.

[karelz]

FYI: The API review discussion was recorded - see https://youtu.be/o5JFZtgaJLs?t=1593 (18 min duration)