Custom ctx override for derive macro #106
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| Original file line number | Diff line number | Diff line change |
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| use scroll_derive::{Pread, Pwrite, SizeWith}; | ||
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| /// An example of using a method as the value for a ctx in a derive. | ||
| struct EndianDependent(Endian); | ||
| impl EndianDependent { | ||
| fn len(&self) -> usize { | ||
| match self.0 { | ||
| scroll::Endian::Little => 5, | ||
| scroll::Endian::Big => 6, | ||
| } | ||
| } | ||
| } | ||
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| #[derive(Debug, PartialEq)] | ||
| struct VariableLengthData { | ||
| buf: Vec<u8>, | ||
| } | ||
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| impl<'a> TryFromCtx<'a, usize> for VariableLengthData { | ||
| type Error = scroll::Error; | ||
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| fn try_from_ctx(from: &'a [u8], ctx: usize) -> Result<(Self, usize), Self::Error> { | ||
| let offset = &mut 0; | ||
| let buf = from.gread_with::<&[u8]>(offset, ctx)?.to_owned(); | ||
| Ok((Self { buf }, *offset)) | ||
| } | ||
| } | ||
| impl<'a> TryIntoCtx<usize> for &'a VariableLengthData { | ||
| type Error = scroll::Error; | ||
| fn try_into_ctx(self, dst: &mut [u8], ctx: usize) -> Result<usize, Self::Error> { | ||
| let offset = &mut 0; | ||
| for i in 0..(ctx.min(self.buf.len())) { | ||
| dst.gwrite(self.buf[i], offset)?; | ||
| } | ||
| Ok(*offset) | ||
| } | ||
| } | ||
| impl SizeWith<usize> for VariableLengthData { | ||
| fn size_with(ctx: &usize) -> usize { | ||
| *ctx | ||
| } | ||
| } | ||
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| #[derive(Debug, PartialEq, Pread, Pwrite, SizeWith)] | ||
| #[repr(C)] | ||
| struct Data { | ||
| id: u32, | ||
| timestamp: f64, | ||
| // You can fix the ctx regardless of what is passed in. | ||
| #[scroll(ctx = BE)] | ||
| arr: [u16; 2], | ||
| // You can use arbitrary expressions for the ctx. | ||
| // You have access to the `ctx` parameter of the `{pread/gread}_with` inside the expression. | ||
| // TODO(implement) you have access to previous fields. | ||
| // TODO(check) will this break structs with fields named `ctx`?. | ||
| #[scroll(ctx = EndianDependent(ctx.clone()).len())] | ||
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There was a problem hiding this comment. one last thing I'll say: i think the example is actually better if EndianDependent is the ctx for VariableLength data, and inside of it's impl, it calls That is clearer and more idiomatic scroll, imho, if there is such a thing :D However, i understand that you are showing off arbitrary expressions are callable in the ctx here, which is very cool and probably worth retaining as you've written it. Side note: I'm surprised you have to Side Side note: if the ctx does need to be cloned, and you don't clone, how terrible is the error message presented to the user (at derive time)? I imagine it could be pretty bad? There was a problem hiding this comment. But if you follow the directions: The fn size_with(ctx: &::scroll::Endian)but the try_{from,into}_ctx derives make this code: fn try_into_ctx(
self,
dst: &mut [u8],
ctx: ::scroll::Endian,
) ->and fn try_from_ctx(
src: &'a [u8],
ctx: ::scroll::Endian,
) ->Yeah, its a little hacky :) If |
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| custom_ctx: VariableLengthData, | ||
| } | ||
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| use scroll::{ | ||
| ctx::{SizeWith, TryFromCtx, TryIntoCtx}, | ||
| Endian, Pread, Pwrite, BE, LE, | ||
| }; | ||
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| fn main() { | ||
| let bytes = [ | ||
| 0xefu8, 0xbe, 0xad, 0xde, 0, 0, 0, 0, 0, 0, 224, 63, 0xad, 0xde, 0xef, 0xbe, 0xaa, 0xbb, | ||
| 0xcc, 0xdd, 0xee, | ||
| ]; | ||
| let data: Data = bytes.pread_with(0, LE).unwrap(); | ||
| println!("data: {data:?}"); | ||
| assert_eq!(data.id, 0xdeadbeefu32); | ||
| assert_eq!(data.arr, [0xadde, 0xefbe]); | ||
| let mut bytes2 = vec![0; ::std::mem::size_of::<Data>()]; | ||
| bytes2.pwrite_with(data, 0, LE).unwrap(); | ||
| let data: Data = bytes.pread_with(0, LE).unwrap(); | ||
| let data2: Data = bytes2.pread_with(0, LE).unwrap(); | ||
| assert_eq!(data, data2); | ||
| // Not enough bytes because of ctx dependent length being too long. | ||
| assert!(bytes.pread_with::<Data>(0, BE).is_err()) | ||
| } | ||
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this is so wild hah, i have to think about usecases/what it means, or if there's some kind of hygiene issue that a different design might not have, but it's very interesting! is the fact that
ctxis available within the scope of the pread something you have made available or is it a property of derive with field overrides in macros?There was a problem hiding this comment.
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It's because the user's expression is effectively copy-pasted without hygiene and so the parameters of the trait method are accessible. It would take more work to remove this ability, since we'd have to parse for an instance of a
ctxident and actively block it.There was a problem hiding this comment.
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This also means that if only
Pwritederive is used (for example), they won't get an error usingselfordstidents.