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Non-derive proc macros are invoked without cfg being resolved. This adds quite a bit complexity to the macro because all of the macro needs to be careful to attach necessary cfgs. This becomes especially tricky for tuple structs. Thus, it is convenient if cfgs are all resolved before expansion. The most optimal way to handle this is via `TokenStream::expand_expr`, but that is still unstable. Implement an approach where we generate two cfg-gated macro invocations with cfg resolved within the invocation. This is the same approach as commit 3445a65 ("internal: rework how `#[pin_data]` handles cfg"). Signed-off-by: Gary Guo <gary@garyguo.net>
Add a test with 26 cfg options to test linear time behaviour of cfg resolution. If the cfg-expansion approach is exponential, this test will cause a timeout. Signed-off-by: Gary Guo <gary@garyguo.net>
BennoLossin
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Aug 5, 2026
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I'm very happy with this strategy to handle cfg's :) I left some small suggestions, with those fixed, you can add my RB.
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| for attr in &self.attrs { | ||
| attr.to_tokens(tokens); | ||
| } |
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Doesn't Vec<T>: ToTokens hold when T: ToTokens?
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| if let Some(this) = &self.this { | ||
| this.to_tokens(tokens); | ||
| } |
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| fn to_tokens(&self, tokens: &mut TokenStream) { | ||
| for attr in &self.attrs { | ||
| attr.to_tokens(tokens); | ||
| } | ||
| if let Some(this) = &self.this { | ||
| this.to_tokens(tokens); | ||
| } | ||
| self.path.to_tokens(tokens); | ||
| self.brace_token.surround(tokens, |tokens| { | ||
| self.fields.to_tokens(tokens); | ||
| if let Some((dotdot, expr)) = &self.rest { | ||
| dotdot.to_tokens(tokens); | ||
| expr.to_tokens(tokens); | ||
| } | ||
| }); | ||
| if let Some((question, ty)) = &self.error { | ||
| question.to_tokens(tokens); | ||
| ty.to_tokens(tokens); | ||
| } |
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I usually prefer to exhaustively match in functions like these. With the other two suggestions:
Suggested change
| fn to_tokens(&self, tokens: &mut TokenStream) { | |
| for attr in &self.attrs { | |
| attr.to_tokens(tokens); | |
| } | |
| if let Some(this) = &self.this { | |
| this.to_tokens(tokens); | |
| } | |
| self.path.to_tokens(tokens); | |
| self.brace_token.surround(tokens, |tokens| { | |
| self.fields.to_tokens(tokens); | |
| if let Some((dotdot, expr)) = &self.rest { | |
| dotdot.to_tokens(tokens); | |
| expr.to_tokens(tokens); | |
| } | |
| }); | |
| if let Some((question, ty)) = &self.error { | |
| question.to_tokens(tokens); | |
| ty.to_tokens(tokens); | |
| } | |
| fn to_tokens(&self, tokens: &mut TokenStream) { | |
| let Self { attrs, this, path, brace_token, fields, rest, error } = self; | |
| attrs.to_tokens(tokens); | |
| this.to_tokens(tokens); | |
| path.to_tokens(tokens); | |
| brace_token.surround(tokens, |tokens| { | |
| fields.to_tokens(tokens); | |
| if let Some((dotdot, expr)) = rest { | |
| dotdot.to_tokens(tokens); | |
| expr.to_tokens(tokens); | |
| } | |
| }); | |
| if let Some((question, ty)) = error { | |
| question.to_tokens(tokens); | |
| ty.to_tokens(tokens); | |
| } | |
| } |
The tuples might also support ToTokens, I haven't checked, in that case, this could be even shorter.
| expand(initializer, default_error, pinned, dcx) | ||
| } | ||
|
|
||
| pub(crate) fn expand( |
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should we make this private now?
mqqz
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to mqqz/pin-init
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this pull request
Aug 28, 2026
Extend the initializer syntax so that a field can be named by an index,
addressing tuple struct fields the same way a struct expression does:
pin_init!(Foo { 0: value, 1 <- initializer })
Tuple fields are not exposed by a `let` binding to the fields after them,
since they have no name to bind; `_0` would shadow a user variable.
`cfg` needs different treatment from named fields. Non-derive proc macros
are invoked before cfg is resolved, so the macro cannot know whether a
field survives, and dropping a tuple field renumbers every field after it.
That cannot be expressed by attaching a `cfg` attribute to the initializer
of a single field.
Resolve tuple field cfgs up front instead, by generating two cfg-gated
invocations of the macro with one field resolved in each. This is the
approach of commit 3445a65 ("internal: rework how `#[pin_data]`
handles cfg"), and it is linear because only one of the two branches is
ever expanded. Named fields do not renumber, so they keep using the
existing attribute-based handling.
Link: Rust-for-Linux#165
Suggested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>
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Non-derive proc macros are invoked without cfg being resolved. This adds quite a bit complexity to the macro because all of the macro needs to be careful to attach necessary cfgs. This becomes especially tricky for tuple structs. Thus, it is convenient if cfgs are all resolved before expansion.
The most optimal way to handle this is via
TokenStream::expand_expr, but that is still unstable. Implement an approach where we generate two cfg-gated macro invocations with cfg resolved within the invocation.This is the same approach as #161 but for init macros. This should hopefully also help #155 by removing the cfg limitation.