Design updates for #2015 numeric type literal syntax (#2410)

Edit numeric literal design for the literal type proposal and add reference where #2015 was mentioned.

Closes #2159 

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: josh11b <josh11b@users.noreply.github.com>

docs/design/README.md

@@ -9,8 +9,6 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 > **STATUS:** Up-to-date on 09-Aug-2022, including proposals up through
 > [#1327](https://github.com/carbon-language/carbon-lang/pull/1327).
 
-> FIXME: add #2015
-
 <!-- toc -->
 
 ## Table of contents
@@ -469,6 +467,7 @@ may be limited to integers of at most 128 bits due to LLVM limitations.
 
 > References:
 >
+> -   [Numeric type literal expressions](expressions/literals.md#numeric-type-literals)
 > -   Question-for-leads issue
 >     [#543: pick names for fixed-size integer types](https://github.com/carbon-language/carbon-lang/issues/543)
 > -   Question-for-leads issue
@@ -534,6 +533,7 @@ number.
 
 > References:
 >
+> -   [Numeric type literal expressions](expressions/literals.md#numeric-type-literals)
 > -   Question-for-leads issue
 >     [#543: pick names for fixed-size integer types](https://github.com/carbon-language/carbon-lang/issues/543)
 > -   Question-for-leads issue

docs/design/expressions/README.md

@@ -18,6 +18,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   [Operators](#operators)
 -   [Conversions and casts](#conversions-and-casts)
 -   [`if` expressions](#if-expressions)
+-   [Numeric type literal expressions](#numeric-type-literal-expressions)
 -   [Alternatives considered](#alternatives-considered)
 -   [References](#references)
 
@@ -298,6 +299,16 @@ fn Run(args: Span(StringView)) {
 
 `if` expressions are analogous to `?:` ternary expressions in C and C++.
 
+## Numeric type literal expressions
+
+Carbon's syntax provides a simple way to represent different types of integers
+and floating-point numbers. Each type is identified with a keyword-like syntax,
+prefixed with either `i`, `u`, or `f` followed by a multiple of 8, representing
+the size in bits of the data type.
+
+These are referred to as
+[numeric type literals](literals.md#numeric-type-literals).
+
 ## Alternatives considered
 
 Other expression documents will list more alternatives; this lists alternatives

docs/design/expressions/literals.md

@@ -0,0 +1,72 @@
+# Literal expressions
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Numeric type literals](#numeric-type-literals)
+    -   [Usage](#usage)
+    -   [Alternatives considered](#alternatives-considered)
+-   [References](#references)
+
+<!-- tocstop -->
+
+## Overview
+
+This document is intended to cover all literal expressions, excluding numeric,
+floats and strings, which are covered in the
+[Lexical Conventions](../lexical_conventions/README.md) section. For now, the
+document explains the numeric type literals.
+
+## Numeric type literals
+
+Carbon has a simple keyword-like syntax of `iN`, `uN`, and `fN` for two's
+complement integers, unsigned integers, and
+[IEEE-754](https://en.wikipedia.org/wiki/IEEE_754) floating-point numbers,
+respectively. Here, `N` can be a positive multiple of 8, including the common
+power-of-two sizes (for example, `N = 8, 16, 32`).
+
+Examples of this syntax include:
+
+-   `i16` - A 16-bit two's complement signed integer type
+-   `u32` - A 32-bit unsigned integer type
+-   `f64` - A 64-bit IEEE-754 binary floating-point number type
+
+### Usage
+
+```carbon
+package sample api;
+
+fn Sum(x: i32, y: i32) -> i32 {
+  return x + y;
+}
+
+fn Main() -> i32 {
+  return Sum(4, 2);
+}
+```
+
+In the above example, `Sum` has parameters `x` and `y`, each of which is typed
+as a 32-bit two's complement signed integer. `Main` then returns the output of
+`Sum` as a 32-bit two's complement signed integer.
+
+### Alternatives considered
+
+-   [C++ LP64 convention](/proposals/p2015.md#c-lp64-convention)
+-   [Type name with length suffix](/proposals/p2015.md#type-name-with-length-suffix)
+-   [Uppercase suffixes](/proposals/p2015.md#uppercase-suffixes)
+-   [Additional bit sizes](/proposals/p2015.md#additional-bit-sizes)
+
+## References
+
+-   Issue
+    [#543: pick names for fixed-size integer types](https://github.com/carbon-language/carbon-lang/issues/543)
+-   Proposal
+    [#2015: Numeric type literal syntax](https://github.com/carbon-language/carbon-lang/pull/2015)