Add a style guide

Change-Id: I13bf82440cdf5c9268e8cc0d35a1119ebf9a9ed0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/42200
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: David Neto <dneto@google.com>

CONTRIBUTING.md

@@ -20,10 +20,7 @@ again.
 All submissions, including submissions by project members, require review. We
 use [Dawn's Gerrit](https://dawn-review.googlesource.com/) for this purpose.
 
-Tint generally follows the Google C++ style guide. There is a clang-format file
-provided and a `tools/format` script which will run the formatter. There is also
-a `tools/lint` script to run a linter. Overall try to use the same style and
-convention as code around your change.
+Submissions should follow the [Tint style guide](docs/style_guide.md).
 
 ## Community Guidelines
 

docs/style_guide.md

@@ -0,0 +1,50 @@
+# Tint style guide
+
+* Generally, follow the [Chromium style guide for C++](https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/c++/c++.md)
+  which itself is built on the [Google C++ style guide](https://google.github.io/styleguide/cppguide.html).
+
+* Overall try to use the same style and convention as code around your change.
+
+* Code must be formatted. Use `clang-format` with the provided [.clang-format](../.clang-format)
+  file.  The `tools/format` script runs the formatter.
+
+* Code should not have linting errors.
+    The `tools/lint` script runs the linter. So does `git cl upload`.
+
+* Do not use C++ exceptions
+
+* Do not use C++ RTTI.
+   Instead, use `tint::Castable::As<T>()` from
+   [src/castable.h](../src/castable.h)
+
+* Generally, avoid `assert`.  Instead, issue a [diagnostic](../src/diagnostic.h)
+  and fail gracefully, possibly by returning an error sentinel value.
+  Code that should not be reachable should call `TINT_UNREACHABLE` macro
+  and other internal error conditions should call the `TINT_ICE` macro.
+  See [src/debug.h](../src/debug.h)
+
+* Use `type` as part of a name only when the name refers to a type
+  in WGSL or another shader language processed by Tint.  If the concept you are
+  trying to name is about distinguishing between alternatives, use `kind` instead.
+
+## Compiler support
+
+Tint requires C++14.
+
+Tint uses the Chromium build system and will stay synchronized with that system.
+Compiler configurations beyond that baseline is on a best-effort basis.
+We strive to support recent GCC and MSVC compilers.
+
+## Test code
+
+We might relax the above rules rules for test code, since test code
+shouldn't ship to users.
+
+However, test code should still be readable and maintainable.
+
+For test code, the tradeoff between readability and maintainability
+and other factors is weighted even more strongly toward readability
+and maintainability.
+
+
+