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Reserve more numeric types. #907

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Reserve more numeric types. #907

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reserve more primitive types
mahkoh Feb 25, 2015
8120ceb
fixup
mahkoh Feb 25, 2015
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33 changes: 33 additions & 0 deletions text/0000-reserve-types.md
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- Start Date: 2014-06-25
- RFC PR #: (leave this empty)
- Rust Issue #: (leave this empty)

# Summary

Reserve more numeric types.

# Motivation

It is conceivable that Rust will gain support for types such as `f128`, `f16`,
or `u128`. In the interest of backwards compatability, extend the grammar to
reserve these.

Adding them is backwards incompatible because `type PRIMITIVE = T;` is an error.

# Detailed design

Reserve the following type names: `fN`, `uN`, `iN` for `N` a multiple of 8.
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What limit? Or just to, say, 232?

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Whatever the parser can handle.

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The parser can handle arbitrarily large numbers: congruence mod 8 can be determined using only the last 3 digits of a number represented in base 10, meaning string handling works fine, and hence i1234567891234567891234567891234567800 could be checked to be "valid" under that rule.

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Is there any reason to limit ourselves to numbers that are a multiple of 8? Why not just reserve them for all numbers?


Reserve additionally `dN` for decimal floating point numbers.

Reserve additionally `mN` for SSE.

# Drawbacks

Makes the grammar larger for types which we may never use.

# Alternatives

New types could require a flag to be enabled.

C99 uses `_Bool` instead of `bool` because `_T` is reserved.