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Replace ~const syntax with [const] conditionally const syntax #1896

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Merged
dtolnay merged 2 commits into from
Aug 14, 2025
Merged

Replace ~const syntax with [const] conditionally const syntax #1896

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7cfc20e
Rename tilde-const to conditionally-const
dtolnay Aug 14, 2025
f6827c0
Replace `~const` syntax with `[const]` conditionally const syntax
dtolnay Aug 14, 2025
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43 changes: 23 additions & 20 deletions src/generics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -514,7 +514,9 @@ ast_struct! {
#[cfg(feature = "parsing")]
pub(crate) mod parsing {
use crate::attr::Attribute;
use crate::error::{self, Result};
#[cfg(feature = "full")]
use crate::error;
use crate::error::{Error, Result};
use crate::ext::IdentExt as _;
use crate::generics::{
BoundLifetimes, ConstParam, GenericParam, Generics, LifetimeParam, PredicateLifetime,
Expand Down Expand Up @@ -715,11 +717,11 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = true;
let allow_conditionally_const = true;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
if !input.peek(Token![+]) {
Expand Down Expand Up @@ -752,16 +754,16 @@ pub(crate) mod parsing {
impl Parse for TypeParamBound {
fn parse(input: ParseStream) -> Result<Self> {
let allow_precise_capture = true;
let allow_tilde_const = true;
Self::parse_single(input, allow_precise_capture, allow_tilde_const)
let allow_conditionally_const = true;
Self::parse_single(input, allow_precise_capture, allow_conditionally_const)
}
}

impl TypeParamBound {
pub(crate) fn parse_single(
input: ParseStream,
#[cfg_attr(not(feature = "full"), allow(unused_variables))] allow_precise_capture: bool,
allow_tilde_const: bool,
allow_conditionally_const: bool,
) -> Result<Self> {
if input.peek(Lifetime) {
return input.parse().map(TypeParamBound::Lifetime);
Expand Down Expand Up @@ -793,21 +795,21 @@ pub(crate) mod parsing {
(None, input)
};

let is_tilde_const =
cfg!(feature = "full") && content.peek(Token![~]) && content.peek2(Token![const]);
if is_tilde_const {
let tilde_token: Token![~] = content.parse()?;
let const_token: Token![const] = content.parse()?;
if !allow_tilde_const {
let msg = "`~const` is not allowed here";
return Err(error::new2(tilde_token.span, const_token.span, msg));
let is_conditionally_const = cfg!(feature = "full") && content.peek(token::Bracket);
if is_conditionally_const {
let conditionally_const;
let bracket_token = bracketed!(conditionally_const in content);
conditionally_const.parse::<Token![const]>()?;
if !allow_conditionally_const {
let msg = "`[const]` is not allowed here";
return Err(Error::new(bracket_token.span.join(), msg));
}
}

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

if is_tilde_const {
if is_conditionally_const {
Ok(TypeParamBound::Verbatim(verbatim::between(&begin, input)))
} else {
Ok(TypeParamBound::Trait(bound))
Expand All @@ -818,11 +820,12 @@ pub(crate) mod parsing {
input: ParseStream,
allow_plus: bool,
allow_precise_capture: bool,
allow_tilde_const: bool,
allow_conditionally_const: bool,
) -> Result<Punctuated<Self, Token![+]>> {
let mut bounds = Punctuated::new();
loop {
let bound = Self::parse_single(input, allow_precise_capture, allow_tilde_const)?;
let bound =
Self::parse_single(input, allow_precise_capture, allow_conditionally_const)?;
bounds.push_value(bound);
if !(allow_plus && input.peek(Token![+])) {
break;
Expand All @@ -833,7 +836,7 @@ pub(crate) mod parsing {
|| input.peek(Token![?])
|| input.peek(Lifetime)
|| input.peek(token::Paren)
|| input.peek(Token![~]))
|| (allow_conditionally_const && input.peek(token::Bracket)))
{
break;
}
Expand Down Expand Up @@ -993,11 +996,11 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = true;
let allow_conditionally_const = true;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
if !input.peek(Token![+]) {
Expand Down
20 changes: 14 additions & 6 deletions src/item.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1208,11 +1208,11 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = true;
let allow_conditionally_const = true;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
if input.peek(Token![where]) || input.peek(Token![=]) || input.peek(Token![;]) {
Expand Down Expand Up @@ -2233,8 +2233,12 @@ pub(crate) mod parsing {
}
supertraits.push_value({
let allow_precise_capture = false;
let allow_tilde_const = true;
TypeParamBound::parse_single(input, allow_precise_capture, allow_tilde_const)?
let allow_conditionally_const = true;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_conditionally_const,
)?
});
if input.peek(Token![where]) || input.peek(token::Brace) {
break;
Expand Down Expand Up @@ -2305,8 +2309,12 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = false;
TypeParamBound::parse_single(input, allow_precise_capture, allow_tilde_const)?
let allow_conditionally_const = false;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_conditionally_const,
)?
});
if input.peek(Token![where]) || input.peek(Token![;]) {
break;
Expand Down
4 changes: 2 additions & 2 deletions src/path.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -379,11 +379,11 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = true;
let allow_conditionally_const = true;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
if !input.peek(Token![+]) {
Expand Down
22 changes: 11 additions & 11 deletions src/ty.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -404,11 +404,11 @@ pub(crate) mod parsing {
bounds.push_punct(plus);
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = false;
let allow_conditionally_const = false;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
}
Expand Down Expand Up @@ -480,11 +480,11 @@ pub(crate) mod parsing {
bounds.push_punct(plus);
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = false;
let allow_conditionally_const = false;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
}
Expand Down Expand Up @@ -551,11 +551,11 @@ pub(crate) mod parsing {
}
bounds.push_value({
let allow_precise_capture = false;
let allow_tilde_const = false;
let allow_conditionally_const = false;
TypeParamBound::parse_single(
input,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?
});
}
Expand Down Expand Up @@ -849,12 +849,12 @@ pub(crate) mod parsing {
allow_plus: bool,
) -> Result<Punctuated<TypeParamBound, Token![+]>> {
let allow_precise_capture = false;
let allow_tilde_const = false;
let allow_conditionally_const = false;
let bounds = TypeParamBound::parse_multiple(
input,
allow_plus,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?;
let mut last_lifetime_span = None;
let mut at_least_one_trait = false;
Expand Down Expand Up @@ -899,12 +899,12 @@ pub(crate) mod parsing {
pub(crate) fn parse(input: ParseStream, allow_plus: bool) -> Result<Self> {
let impl_token: Token![impl] = input.parse()?;
let allow_precise_capture = true;
let allow_tilde_const = false;
let allow_conditionally_const = false;
let bounds = TypeParamBound::parse_multiple(
input,
allow_plus,
allow_precise_capture,
allow_tilde_const,
allow_conditionally_const,
)?;
let mut last_nontrait_span = None;
let mut at_least_one_trait = false;
Expand All @@ -929,7 +929,7 @@ pub(crate) mod parsing {
}
}
TypeParamBound::Verbatim(_) => {
// ~const Trait
// `[const] Trait`
at_least_one_trait = true;
break;
}
Expand Down
62 changes: 3 additions & 59 deletions tests/repo/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -32,75 +32,18 @@ static EXCLUDE_FILES: &[&str] = &[
"tests/ui/traits/const-traits/auxiliary/minicore.rs",
"tests/ui/traits/const-traits/const-via-item-bound.rs",

// TODO: conditionally const trait bounds: `T: [const] Trait`
// https://github.com/dtolnay/syn/issues/1883
"library/core/src/array/mod.rs",
"library/core/src/cell.rs",
"library/core/src/clone.rs",
"library/core/src/cmp.rs",
"library/core/src/cmp/bytewise.rs",
"library/core/src/convert/mod.rs",
"library/core/src/net/socket_addr.rs",
"library/core/src/num/nonzero.rs",
"library/core/src/ops/deref.rs",
"library/core/src/ops/function.rs",
"library/core/src/ops/index.rs",
"library/core/src/ops/try_trait.rs",
"library/core/src/ptr/const_ptr.rs",
"library/core/src/ptr/mut_ptr.rs",
"library/core/src/ptr/non_null.rs",
"library/core/src/result.rs",
"library/core/src/slice/cmp.rs",
"library/core/src/slice/index.rs",
"library/core/src/slice/mod.rs",
"library/core/src/str/mod.rs",
"library/core/src/str/traits.rs",
"src/tools/rust-analyzer/crates/parser/test_data/parser/inline/ok/type_param_bounds.rs",
"tests/run-make/const-trait-stable-toolchain/const-super-trait.rs",
"tests/rustdoc/constant/const-effect-param.rs",
"tests/rustdoc/inline_cross/auxiliary/const-effect-param.rs",
"tests/ui/traits/const-traits/assoc-type-const-bound-usage-0.rs",
"tests/ui/traits/const-traits/call-generic-in-impl.rs",
"tests/ui/traits/const-traits/call-generic-method-chain.rs",
"tests/ui/traits/const-traits/call-generic-method-dup-bound.rs",
"tests/ui/traits/const-traits/call-generic-method-pass.rs",
"tests/ui/traits/const-traits/conditionally-const-assoc-fn-in-trait-impl.rs",
"tests/ui/traits/const-traits/conditionally-const-inherent-assoc-const-fn.rs",
"tests/ui/traits/const-traits/conditionally-const-trait-bound-assoc-tys.rs",
"tests/ui/traits/const-traits/const-assoc-bound-in-trait-wc.rs",
"tests/ui/traits/const-traits/const-closure-trait-method.rs",
"tests/ui/traits/const-traits/const-closures.rs",
"tests/ui/traits/const-traits/const-drop-bound.rs",
"tests/ui/traits/const-traits/const-drop.rs",
"tests/ui/traits/const-traits/const-fn-trait-bound-issue-104314.rs",
"tests/ui/traits/const-traits/dont-prefer-param-env-for-infer-self-ty.rs",
"tests/ui/traits/const-traits/function-pointer-does-not-require-const.rs",
"tests/ui/traits/const-traits/inherent-impl-const-bounds.rs",
"tests/ui/traits/const-traits/issue-92111.rs",
"tests/ui/traits/const-traits/issue-92230-wf-super-trait-env.rs",
"tests/ui/traits/const-traits/item-bound-entailment.rs",
"tests/ui/traits/const-traits/minicore-works.rs",
"tests/ui/traits/const-traits/non-const-op-in-closure-in-const.rs",
"tests/ui/traits/const-traits/predicate-entailment-passes.rs",
"tests/ui/traits/const-traits/specialization/const-default-const-specialized.rs",
"tests/ui/traits/const-traits/specialization/issue-95187-same-trait-bound-different-constness.rs",
"tests/ui/traits/const-traits/specialization/non-const-default-const-specialized.rs",
"tests/ui/traits/const-traits/specialization/specialize-on-conditionally-const.rs",
"tests/ui/traits/const-traits/super-traits.rs",
"tests/ui/traits/const-traits/trait-where-clause-run.rs",
"tests/ui/traits/const-traits/trait-where-clause-self-referential.rs",
"tests/ui/unpretty/ast-const-trait-bound.rs",

// TODO: unconditionally const trait bound syntax: `T: const FnOnce<Arg, Output = Ret>`
// https://github.com/dtolnay/syn/issues/1885
"library/core/src/intrinsics/mod.rs",
"tests/ui/generic-const-items/const-trait-impl.rs",
"tests/ui/traits/const-traits/assoc-type-const-bound-usage-1.rs",
"tests/ui/traits/const-traits/const-bound-in-host.rs",
"tests/ui/traits/const-traits/const-drop.rs",
"tests/ui/traits/const-traits/const-in-closure.rs",
"tests/ui/traits/const-traits/dont-ice-on-const-pred-for-bounds.rs",
"tests/ui/traits/const-traits/imply-always-const.rs",
"tests/ui/traits/const-traits/minicore-const-fn-early-bound.rs",
"tests/ui/traits/const-traits/predicate-entailment-passes.rs",
"tests/ui/traits/const-traits/unconstrained-var-specialization.rs",

// TODO: conditionally const impl Trait: `impl [const] FnOnce()`
Expand All @@ -111,6 +54,7 @@ static EXCLUDE_FILES: &[&str] = &[
"tests/ui/traits/const-traits/const-closure-issue-125866-pass.rs",
"tests/ui/traits/const-traits/const-cond-for-rpitit.rs",
"tests/ui/traits/const-traits/const-impl-trait.rs",
"tests/ui/traits/const-traits/dont-prefer-param-env-for-infer-self-ty.rs",

// TODO: conditionally const trait bound with lifetime bindar: `T: for<'a> [const] async Trait<'a>`
// https://github.com/dtolnay/syn/issues/1888
Expand Down
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