Enhance CA1305 analyzer to cover interpolated strings

[Alex-Sob]

Describe the problem you are trying to solve

The existing CA1305: Specify IFormatProvider rule does no seem to cover interpolated strings. Currently compiler emits no warning if a value that can be formatted is used inside an interpolation expression:

var text = $"Date: {DateTime.Now}";

Describe suggestions on how to achieve the rule

The existing analyzer could be enhanced, so that it would report diagnostic and recommend using new string.Create(IFormatProvider, ref DefaultInterpolatedStringHandler) overloads if type of the value used in interpolation expression implements ISpanFormattable (I'm aware that it's actually used instead of calling ToString). Does that make sense? If so, I could try contributing myself as I have experience writing analyzers for Microsoft projects.

Examples

If type of the value implements IFormattable/ISpanFormattable which are types that default interpolated string handler special-cases when formatting a value, then compiler could recommend a call to string.Create with format provider supplied explicitly:

// Violation
var text = $"Date: {DateTime.Now}";

// Fix
text = string.Create(CultureInfo.CurrentCulture, $"Date: {DateTime.Now}"); // Or whatever culture is appropriate

If an interpolated string is implicitly cast to FormattableString, then string.Create would also be a more efficient alternative:

// Violation
FormattableString formattableString = $"Date: {DateTime.Now}";
var text = formattableString.ToString(CultureInfo.CurrentCulture);

// Fix
text = string.Create(CultureInfo.CurrentCulture, $"Date: {DateTime.Now}");

Or

// Violation
FormattableString formattableString = $"Date: {DateTime.Now}";
var text = FormattableString.Invariant(formattableString);

// Fix
text = string.Create(CultureInfo.InvariantCulture, $"Date: {DateTime.Now}");

In case of IFormattable, it allows passing null as a format provider. Maybe this could be addressed as well, but it's not related specifically to interpolated strings:

IFormattable formattable = $"Date: {DateTime.Now}";
var text = formattable.ToString("format", null);

[mavasani]

Moving to dotnet/runtime for triage. I guess the primary question would be whether we want to enhance the existing rule or use a different rule ID?

[ghost]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Describe the problem you are trying to solve

The existing CA1305: Specify IFormatProvider rule does no seem to cover interpolated strings. Currently compiler emits no warnings if a value that can be formatted is used inside an interpolation expression:

var formatted = $"Date: {DateTime.Now}";

Describe suggestions on how to achieve the rule

The existing analyzer could be enhanced, so that it would report diagnostic and recommend using new string.Create(IFormatProvider, ref DefaultInterpolatedStringHandler) overloads if an object of a type containing ToString(IFormatProvider) or an object implementing ISpanFormattable (I'm aware that it's actually used instead of calling ToString on values) is injected into an interpolated string. Does that make sense? If so, I could try contributing myself as I have experience writing analyzers for Microsoft projects.

Author:	Alex-Sob
Assignees:	-
Labels:	area-System.Runtime, untriaged, needs-area-label
Milestone:	-

[tannergooding]

This needs more thought/consideration, namely updating the top post to more clearly provide some example code showing what's happening today vs what's expected to happen and be suggested by the analyzer.

We should take into account https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated#implicit-conversions-and-how-to-specify-iformatprovider-implementation and the different ways format providers can be provided alongside string interpolation

[Alex-Sob]

@tannergooding I added some examples to the description.