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Fix grammar rules containing or pertaining to bounds #2257

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Fix grammar rules containing or pertaining to bounds #2257

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feba243
Fix grammar rules containing bounds
fmease May 5, 2026
e8c2309
Revise section about `dyn` in Rust 2015
fmease May 5, 2026
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2 changes: 1 addition & 1 deletion src/items/type-aliases.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@ r[items.type]
r[items.type.syntax]
```grammar,items
TypeAlias ->
`type` IDENTIFIER GenericParams? ( `:` Bounds )?
`type` IDENTIFIER GenericParams? ( `:` Bounds? )?
WhereClause?
( `=` Type WhereClause?)? `;`
```
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2 changes: 1 addition & 1 deletion src/paths.md
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Expand Up @@ -68,7 +68,7 @@ GenericArgsBinding ->
IDENTIFIER GenericArgs? `=` Type

GenericArgsBounds ->
IDENTIFIER GenericArgs? `:` Bounds
IDENTIFIER GenericArgs? `:` Bounds?
```

r[paths.expr.intro]
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4 changes: 2 additions & 2 deletions src/types/impl-trait.md
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Expand Up @@ -3,9 +3,9 @@ r[type.impl-trait]

r[type.impl-trait.syntax]
```grammar,types
ImplTraitType -> `impl` Bounds
ImplTraitType -> `impl` Bounds?

ImplTraitTypeOneBound -> `impl` TraitBound
ImplTraitTypeOneBound -> `impl` TraitBound?
```

r[type.impl-trait.intro]
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8 changes: 5 additions & 3 deletions src/types/trait-object.md
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Expand Up @@ -3,9 +3,9 @@ r[type.trait-object]

r[type.trait-object.syntax]
```grammar,types
TraitObjectType -> `dyn`? Bounds
TraitObjectType -> Bounds | `dyn` Bounds?

TraitObjectTypeOneBound -> `dyn`? TraitBound
TraitObjectTypeOneBound -> TraitBound | `dyn` TraitBound?
```

r[type.trait-object.intro]
Expand Down Expand Up @@ -37,7 +37,9 @@ r[type.trait-object.syntax-edition2021]

r[type.trait-object.syntax-edition2018]
> [!EDITION-2018]
> In the 2015 edition, if the first bound of the trait object is a path that starts with `::`, then the `dyn` will be treated as a part of the path. The first path can be put in parenthesis to get around this. As such, if you want a trait object with the trait `::your_module::Trait`, you should write it as `dyn (::your_module::Trait)`.
> In the 2015 edition, `dyn` must be followed by [PathIdentSegment][grammar-PathIdentSegment], [LIFETIME_OR_LABEL][grammar-LIFETIME_OR_LABEL], `for`, `(` or `?` to be interpreted as the start of a trait object type. Otherwise, it will be interpreted as a regular identifier.
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@fmease fmease May 5, 2026 •
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Originally I wrote Otherwise, it will be interpreted as a regular identifier and thus as the start of a type path. However the final clause wasn't correct strictly speaking: In Rust 2015 in a type context, dyn that's not followed by one of those tokens could start a TypePath (e.g., dyn, dyn::X), a TraitObjectType (e.g., dyn+, dyn + X) or a MacroInvocation (e.g., dyn!(), dyn::m!()), so I've retracted that statement again.

I don't feel like mentioning TypePath, TraitObjectType & MacroInvocation in this sentence since it can easily become outdated. The consequences should be self-evident anyway.

View changes since the review

>
> Most notably, `dyn`, `dyn::T` and `dyn<T>` will all be treated as type paths. The first path can be put in parenthesis to get around this. As such, if you want a trait object with the trait `::module::Trait`, you should write it as `dyn (::module::Trait)`.
>
> Beginning in the 2018 edition, `dyn` is a true keyword and is not allowed in paths, so the parentheses are not necessary.

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