Deprecate trailing decimal.

DIPs/1NNN-GED.md

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+# Deprecate Trailing Decimal Point
+
+| Field | Value |
+|-----------------|-----------------------------------------------------------------|
+| DIP: | (number/id -- assigned by DIP Manager) |
+| Review Count: | 0 (edited by DIP Manager) |
+| Author: | Garrett D'Amore garrett@damore.org |
+| Implementation: | (links to implementation PR if any) |
+| Status: | Draft |
+
+## Abstract
+
+This proposes to deprecate the floating point syntax
+where a floating number may use a trailing decimal point
+by itself.
+
+
+## Contents
+* [Rationale](#rationale)
+* [Prior Work](#prior-work)
+* [Description](#description)
+* [Breaking Changes and Deprecations](#breaking-changes-and-deprecations)
+* [Reference](#reference)
+* [Copyright & License](#copyright--license)
+* [Reviews](#reviews)
+
+## Rationale
+The syntax for floating points permits a trailing decimal point with no further
+value afterwards, which complicates lexing and parsing D source code involving
+these literals.
+
+It also leads to potential ambiguity, such as in the following syntax:
+
+ `z = x[123..y];`
+
+This could be naively parsed as a range expression (from 123 to the value contained in y),
+or as a reference to property y of the floating point literal 123.0.
+
+The lexical grammar also has a special rule to insist that 1.f is not legal (or
+rather it can only mean the f property of integer 1), but this requires special
+handling in the grammar.
+
+This ambiguity makes it much harder for naive tooling (such as regular expression
+based syntax highlighters) to discriminate, and requires lexers to perform extra
+lookahead and processing when encountering a bare decimal. This adds a very small,
+but non-zero, extra cost to processing D source code.
+
+This is further complicated by the fact that whitespace in a property expression is legal.
+For example, consider the following:
+
+```d
+auto a1 = 1 . sizeof; // legal, evaluates to sizeof int 
+auto a2 = 1. sizeof; // compilation error
+auto a3 = 'c' . sizeof; // legal, evaluates to sizeof char 
+auto a4 = 'c'. sizeof; // also legal, evaluates to sizeof char
+```
+
+This ambiguity makes writing tools harder, and makes it harder for humans to
+understand the resulting code, while oferring no tangible benefit of its own.
+
+As another tidbit of inconsistency, the syntax for hexadecimal floating points
+does not permit a lone trailing decimal point. Consider:
+
+```d
+ auto a1 = 1.; // legal same as 1.0f
+ auto a2 = 0x1.0; // legal, also 1.0f
+ auto a3 = 0x1.; // compilation error
+```
+
+The author of this DIP encountered these problems and had to write special
+code to workaround them while creating a Tree-Sitter grammar for D. 
+(That grammar can be found at https://github.com/gdamore/tree-sitter-d/ and
+the workarounds for this problem are to be found in the `src/scanner.c` file
+located therein.)
+
+## Prior Work
+
+N/A.
+
+## Description
+
+The current grammar for floating point decimal literals (2.13) is:
+
+```
+DecimalFloat:
+ LeadingDecimal .
+ LeadingDecimal . DecimalDigitsNoStartingUS
+ LeadingDecimal . DecimalDigitsNoStartingUS DecimalExponent
+ . DecimalDigitsNoStartingUS
+ . DecimalDigitsNoStartingUS DecimalExponent
+ LeadingDecimal DecimalExponent
+```
+
+This should be broken into two separate rules:
+
+```
+DecimalFloat:
+ LeadingDecimal . DecimalDigitsNoStartingUS
+ LeadingDecimal . DecimalDigitsNoStartingUS DecimalExponent
+ . DecimalDigitsNoStartingUS
+ . DecimalDigitsNoStartingUS DecimalExponent
+ LeadingDecimal DecimalExponent
+``` 
+
+and
+
+```
+DecimalFloatDeprecated:
+ LeadingDecimal .
+```
+
+The `DecimalFloatDeprecated` rule should be linked to a deprecation warning
+when it is encountered, and may at some point in the future ultimately be
+removed. 
+
+## Breaking Changes and Deprecations
+
+As proposed above, we would mark the use of a trailing decimal for floating
+points deprecated, and issue a warning.
+
+The resolution for most users would simply be to add a zero after the
+decimal point. The deprecation warning could (should) suggest that.
+
+As with all deprecations, making this enforced would be opt-in for the user,
+at least until such time as it is actually removed.
+
+## Reference
+
+The author's Tree-Sitter Grammar for D is located at
+https://github.com/gdamore/tree-sitter-d. 
+
+## Copyright & License
+Copyright (c) 2022 by the D Language Foundation
+
+Licensed under [Creative Commons Zero 1.0](https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt)
+
+## Reviews
+The DIP Manager will supplement this section with a summary of each review stage
+of the DIP process beyond the Draft Review.