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, 4 insertions, 0 deletions

diff --git a/docs/markdown/snippets/unstable-rust-module.md b/docs/markdown/snippets/unstable-rust-module.md
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+## Untable Rust module
+
+A new unstable module has been added to make using rust with meson easier.
+Currently it adds a single function to ease defining rust tests.