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Merged
dtolnay merged 2 commits into from
Aug 15, 2025
Merged

Parse const trait bound with bound lifetimes #1902

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1cb3da9
Combine conditionally const type param bound parsing with trait bound…
dtolnay Aug 15, 2025
d7a0db1
Parse const trait bound with bound lifetimes
dtolnay Aug 15, 2025
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74 changes: 42 additions & 32 deletions src/generics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -765,8 +765,6 @@ pub(crate) mod parsing {
return input.parse().map(TypeParamBound::Lifetime);
}

let begin = input.fork();

#[cfg(feature = "full")]
{
if input.peek(Token![use]) {
Expand All @@ -784,38 +782,20 @@ pub(crate) mod parsing {
}
}

let begin = input.fork();

let content;
let (paren_token, content) = if input.peek(token::Paren) {
(Some(parenthesized!(content in input)), &content)
} else {
(None, input)
};

let is_conditionally_const = cfg!(feature = "full") && content.peek(token::Bracket);
let is_unconditionally_const = cfg!(feature = "full") && content.peek(Token![const]);
if is_conditionally_const {
let conditionally_const;
let bracket_token = bracketed!(conditionally_const in content);
conditionally_const.parse::<Token![const]>()?;
if !allow_const {
let msg = "`[const]` is not allowed here";
return Err(Error::new(bracket_token.span.join(), msg));
}
} else if is_unconditionally_const {
let const_token: Token![const] = content.parse()?;
if !allow_const {
let msg = "`const` is not allowed here";
return Err(Error::new(const_token.span, msg));
}
}

let mut bound: TraitBound = content.parse()?;
bound.paren_token = paren_token;

if is_conditionally_const || is_unconditionally_const {
Ok(TypeParamBound::Verbatim(verbatim::between(&begin, input)))
} else {
if let Some(mut bound) = TraitBound::do_parse(content, allow_const)? {
bound.paren_token = paren_token;
Ok(TypeParamBound::Trait(bound))
} else {
Ok(TypeParamBound::Verbatim(verbatim::between(&begin, input)))
}
}

Expand Down Expand Up @@ -850,7 +830,33 @@ pub(crate) mod parsing {
#[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
impl Parse for TraitBound {
fn parse(input: ParseStream) -> Result<Self> {
let allow_const = false;
Self::do_parse(input, allow_const).map(Option::unwrap)
}
}

impl TraitBound {
fn do_parse(input: ParseStream, allow_const: bool) -> Result<Option<Self>> {
let mut lifetimes: Option<BoundLifetimes> = input.parse()?;

let is_conditionally_const = cfg!(feature = "full") && input.peek(token::Bracket);
let is_unconditionally_const = cfg!(feature = "full") && input.peek(Token![const]);
if is_conditionally_const {
let conditionally_const;
let bracket_token = bracketed!(conditionally_const in input);
conditionally_const.parse::<Token![const]>()?;
if !allow_const {
let msg = "`[const]` is not allowed here";
return Err(Error::new(bracket_token.span.join(), msg));
}
} else if is_unconditionally_const {
let const_token: Token![const] = input.parse()?;
if !allow_const {
let msg = "`const` is not allowed here";
return Err(Error::new(const_token.span, msg));
}
}

let modifier: TraitBoundModifier = input.parse()?;
if lifetimes.is_none() && matches!(modifier, TraitBoundModifier::Maybe(_)) {
lifetimes = input.parse()?;
Expand All @@ -876,12 +882,16 @@ pub(crate) mod parsing {
}
}

Ok(TraitBound {
paren_token: None,
modifier,
lifetimes,
path,
})
if is_conditionally_const || is_unconditionally_const {
Ok(None)
} else {
Ok(Some(TraitBound {
paren_token: None,
modifier,
lifetimes,
path,
}))
}
}
}

Expand Down
7 changes: 5 additions & 2 deletions tests/repo/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,9 +27,12 @@ static EXCLUDE_FILES: &[&str] = &[
"src/tools/clippy/tests/ui/trait_duplication_in_bounds.rs",
"src/tools/rust-analyzer/crates/test-utils/src/minicore.rs",

// TODO: conditionally const trait bound with lifetime bindar: `T: for<'a> [const] async Trait<'a>`
// https://github.com/dtolnay/syn/issues/1888
// TODO: async trait bounds: `T: async Trait`
// https://github.com/dtolnay/syn/issues/1901
"src/tools/rust-analyzer/crates/parser/test_data/parser/inline/ok/for_binder_bound.rs",

// TODO: bound lifetime with lifetime bound: `T: for<'a: 'b> Trait<'a>`
// https://github.com/dtolnay/syn/issues/1900
"tests/ui/traits/const-traits/conditionally-const-trait-bound-syntax.rs",

// TODO: non-lifetime binders: `where for<'a, T> &'a Struct<T>: Trait`
Expand Down
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