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Allow new-lines in all interpolations

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This article is a feature specification. The specification serves as the design document for the feature. It includes proposed specification changes, along with information needed during the design and development of the feature. These articles are published until the proposed spec changes are finalized and incorporated in the current ECMA specification.

There may be some discrepancies between the feature specification and the completed implementation. Those differences are captured in the pertinent language design meeting (LDM) notes.

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Summary

The language today non-verbatim and verbatim interpolated strings ($"" and $@"" respectively). The primary sensible difference for these is that a non-verbatim interpolated string works like a normal string and cannot contain newlines in its text segments, and must instead use escapes (like \r\n). Conversely, a verbatim interpolated string can contain newlines in its text segments (like a verbatim string), and doesn't escape newlines or other character (except for "" to escape a quote itself).

This is all reasonable and will not change with this proposal.

What is unreasonable today is that we extend the restriction on 'no newlines' in a non-verbatim interpolated string beyond its text segments into the interpolations themselves. This means, for example, that you cannot write the following:

var v = $"Count is\t: { this.Is.A.Really(long(expr))
                            .That.I.Should(
                                be + able)[
                                    to.Wrap()] }.";

Ultimately, the 'interpolation must be on a single line itself' rule is just a restriction of the current implementation. That restriction really isn't necessary, and can be annoying, and would be fairly trivial to remove (see work https://github.com/dotnet/roslyn/pull/54875 to show how). In the end, all it does is force the dev to place things on a single line, or force them into a verbatim interpolated string (both of which may be unpalatable).

The interpolation expressions themselves are not text, and shouldn't be beholden to any escaping/newline rules therin.

Specification change

single_regular_balanced_text_character
-    : '<Any character except / (U+002F), @ (U+0040), \" (U+0022), $ (U+0024), ( (U+0028), ) (U+0029), [ (U+005B), ] (U+005D), { (U+007B), } (U+007D) and new_line_character>'
-    | '</ (U+002F), if not directly followed by / (U+002F) or * (U+002A)>'
+    : <Any character except @ (U+0040), \" (U+0022), $ (U+0024), ( (U+0028), ) (U+0029), [ (U+005B), ] (U+005D), { (U+007B), } (U+007D)>
+    | comment
    ;

LDM Discussions

https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-09-20.md