builder...build syntax (experimental)

[danlehmann]

This patch-set:

• Breaks up parsing into two phases
• Emits a new builder, which guarantees that all writable fields are set

The new syntax is optional, so it's hidden behind a feature flag

[danlehmann]

To anyone reviewing: The first 2 patches are just a refactoring to prepare the third one. The third one is the actually new thing.

[a6f]

For a register with a mix of read-only and read-write fields, perhaps it would make sense to allow constructing a builder from an existing value, instead of the compile-time default.

[danlehmann]

Good idea! We could allow this as a function while the mask is still 0. Something like this:

let a = A::builder()
  .with_overwritten_raw_value(0x12345678)
  .with_foo(false)
  .with_bar(false)
  .build();

Though I can't quite think of a good name for this function

[johnbwang]

I personally don't see too much value of having something that requires me to fill out the every field. Most registers I work with don't require me to update everything. In fact, often times registers have sensible defaults and I only need to change a few things.

[hecatia-elegua]

Pretty nice idea, iirc from Java, builders were often done like this.
You could remove the prefix with_ here, though.
How does the asm output look?

[danlehmann]

Pretty nice idea, iirc from Java, builders were often done like this.

Java wouldn't give you any of those compile time guarantees though :)

You could remove the prefix with_ here, though.
I tried that, but I didn't like how it looks completely different compared to the other functions. The upside of keeping the with_xx prefixes is that it's easier to upgrade existing DEFAULT.with_foo(false) to builder().with_foo(false).with_bar(false).build().

I think, consistency here is a win.

How does the asm output look?
Well, it's const, so you can always put the result into a constant and use that. In which case the asm output is a constant :) I'll check the assembly for the case where a variable is being passed it.

[danlehmann]

How does the asm output look?

Just to make sure, I look at the generated assembly. It seems optimal: If everything passed in is a constant, the result will simply be there. Otherwise, it performs few branches - I didn't see any function calls