modules: Add an unstable-rust module

Like other language specific modules this module is module for holding
rust specific helpers. This commit adds a test() function, which
simplifies using rust's internal unittest mechanism.

Rust tests are generally placed in the same code files as they are
testing, in contrast to languages like C/C++ and python which generally
place the tests in separate translation units. For meson this is
somewhat problematic from a repetition point of view, as the only
changes are generally adding --test, and possibly some dependencies.

The rustmod.test() method provides a mechanism to remove the repatition:
it takes a rust target, copies it, and then addes the `--test` option,
then creates a Test() target with the `rust` protocol. You can pass
additional dependencies via the `dependencies` keyword. This all makes
for a nice, DRY, test definition.

1 files changed, 35 insertions, 0 deletions

diff --git a/docs/markdown/Rust-module.md b/docs/markdown/Rust-module.md
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+---
+short-description: Rust language integration module
+authors:
+    - name: Dylan Baker
+      email: dylan@pnwbakers.com
+      years: [2020]
+...
+
+# Unstable Rust module
+
+*(new in 0.57.0)*
+
+**Note** Unstable modules make no backwards compatible API guarantees.
+
+The rust module provides helper to integrate rust code into meson. The goal
+is to make using rust in meson more pleasant, while still remaining mesonic,
+this means that it attempts to make rust work more like meson, rather than
+meson work more like rust.
+
+## Functions
+
+### test(name: string, target: library | executable, dependencies: []Dependency)
+
+This function creates a new rust unittest target from an existing rust based
+target, which may be a library or executable. It does this by copying the
+sources and arguments passed to the original target and adding the `--test`
+argument to the compilation, then creates a new test target which calls that
+executable, using the rust test protocol.
+
+This accepts all of the keyword arguments as the
+[`test`](Reference-manual.md#test) function except `protocol`, it will set
+that automatically.
+
+Additional, test only dependencies may be passed via the dependencies
+argument.