derive PartialEq on Expression

[robtfm]

in updating naga_oil i found i needed to have the const_expressions arena contain only unique expressions. this is needed because the equality test for Consts and GlobalVariables does not inspect the init field, it just compares handles, so otherwise equal items with equivalent inits that are distinct handles do not test as equal. I rely on the Arena::fetch_or_append to deduplicate consts and globals which uses the derived equality.

adds the expression-eq feature to enable derive of PartialEq for Expression.

i note there are quite a lot of other PartialEq impls that are cfg(test). I don't need any of them but perhaps the feature sohuld be more general and include them as well:

front::glsl::error::ErrorKind
front::glsl::error::Error

front::glsl::lex::LexerResultKind
front::glsl::lex::LexerResult

front::glsl::token::DirectiveKind
front::glsl::token::Directive
front::glsl::token::Token

valid::UniformityDisruptor
valid::compose::ComposeError
valid::expression::ExpressionError

valid::function::CallError
valid::function::AtomicError
valid::function::LocalVariableError
valid::function::FunctionError

[teoxoy]

Thanks for the PR!

I don't think we need to derive PartialEq for the other types in the glsl frontend and validation module.

Are the compilation times so drastic that we should gate this derive behind a feature flag?

[robtfm]

the generated code is ~260 lines, a single run showed negligible impact on compile time. happy to remove the flag and always derive it if you prefer.

[teoxoy]

Let's always derive it.