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Confusing diagnostic when trying to define a field in a trait #161815

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#162103
Closed
Confusing diagnostic when trying to define a field in a trait#161815
#162103
Assignees
raushan728
Labels
A-diagnosticsArea: Messages for errors, warnings, and lintsA-parserArea: The lexing & parsing of Rust source code to an ASTT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

Description

@wischi-chr
wischi-chr
opened on Aug 26, 2026

Code

trait MyTrait {
  field_name: String,
}

Current output

error: expected one of `!` or `::`, found `:`
  --> src/main.rs:25:15
   |
24 | trait MyTrait {
   |               - while parsing this item list starting here
25 |     field_name: String,
   |               ^ expected one of `!` or `::`
26 | }
   | - the item list ends here

Desired output

Maybe something like "fields not supported/allowed in traits, use getter/setter function instead"

Rust Version

rustc 1.97.0 (2d8144b78 2026-07-07)
binary: rustc
commit-hash: 2d8144b7880597b6e6d3dfd63a9a9efae3f533d3
commit-date: 2026-07-07
host: x86_64-unknown-linux-gnu
release: 1.97.0
LLVM version: 22.1.6

Activity

  1. wischi-chr
    added
    A-diagnosticsArea: Messages for errors, warnings, and lints
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    on Aug 26, 2026
  2. fmease
    added
    A-parserArea: The lexing & parsing of Rust source code to an AST
    on Aug 26, 2026

  3. raushan728 commented on Aug 27, 2026

    @raushan728
    raushan728
    on Aug 27, 2026
    Contributor

    @rustbot claim

  4. rustbot
    assigned
    raushan728
    on Aug 27, 2026

  5. raushan728 commented on Aug 27, 2026

    @raushan728
    raushan728
    on Aug 27, 2026
    Contributor

    Hi! I'm new to Rust and was investigating this issue. I created a UI test to see how rustc handles invalid items inside traits. Just let me know if I missed anything or wrong

    Full code and output 
    fn main() {}

trait Trait1 {
    field_name: String,
    //~^ ERROR expected one of `!` or `::`, found `:`
}

trait Trait2 {
    pub pub_field_name: String,
    //~^ ERROR visibility `pub` is not followed by an item
    //~| ERROR non-item in item list
}

trait Trait3 {
    mut value: i32,
    //~^ ERROR non-item in item list
}

trait Trait4 {
    static VALUE: i32 = 5;
    //~^ ERROR associated `static` items are not allowed
}

trait Trait5 {
    impl SomeType {
    //~^ ERROR implementation is not supported in `trait`s or `impl`s
        fn method(&self) {}
    }
}

trait Trait6 {
    mod my_module;
    //~^ ERROR module is not supported in `trait`s or `impl`s
}

trait Trait7 {
    const CONST_VALUE;
    //~^ ERROR missing type for `const` item
}

trait Trait8 {
    pub type Item;
    //~^ ERROR visibility qualifiers are not permitted here
}

trait Trait9 {
    self: String,
    //~^ ERROR expected one of `!` or `::`, found `:`
}

trait Trait10 {
    // This is actually completely valid syntax in Rust!
    fn foo<T>(&self);
}

trait Trait11 {
    pub(self) fn private_foo(&self);
    //~^ ERROR visibility qualifiers are not permitted here
}
    error: expected one of `!` or `::`, found `:`
  --> $DIR/trait-invalid-items.rs:4:15
   |
LL | trait Trait1 {
   |              - while parsing this item list starting here
LL |     field_name: String,
   |               ^ expected one of `!` or `::`
LL | }
   | - the item list ends here

error: visibility `pub` is not followed by an item
  --> $DIR/trait-invalid-items.rs:8:5
   |
LL |     pub pub_field_name: String,
   |     ^^^ the visibility
   |
   = help: you likely meant to define an item, e.g., `pub fn foo() {}`

error: non-item in item list
  --> $DIR/trait-invalid-items.rs:8:9
   |
LL | trait Trait2 {
   |              - item list starts here
LL |     pub pub_field_name: String,
   |         ^^^^^^^^^^^^^^ non-item starts here
LL | }
   | - item list ends here

error: non-item in item list
  --> $DIR/trait-invalid-items.rs:16:5
   |
LL | trait Trait3 {
   |              - item list starts here
LL |     mut value: i32,
   |     ^^^ non-item starts here
LL | }
   | - item list ends here

error: associated `static` items are not allowed
  --> $DIR/trait-invalid-items.rs:20:5
   |
LL |     static VALUE: i32 = 5;
   |     ^^^^^^^^^^^^^^^^^^^^^^

error: implementation is not supported in `trait`s or `impl`s
  --> $DIR/trait-invalid-items.rs:24:5
   |
LL |     impl SomeType {
   |     ^^^^^^^^^^^^^
   |
   = help: consider moving the implementation out to a nearby module scope

error: module is not supported in `trait`s or `impl`s
  --> $DIR/trait-invalid-items.rs:31:5
   |
LL |     mod my_module;
   |     ^^^^^^^^^^^^^^
   |
   = help: consider moving the module out to a nearby module scope

error: missing type for `const` item
  --> $DIR/trait-invalid-items.rs:36:22
   |
LL |     const CONST_VALUE;
   |                      ^
   |
help: provide a type for the item
   |
LL |     const CONST_VALUE: <type>;
   |                      ++++++++

error[E0449]: visibility qualifiers are not permitted here
  --> $DIR/trait-invalid-items.rs:41:5
   |
LL |     pub type Item;
   |     ^^^ help: remove the qualifier
   |
   = note: trait items always share the visibility of their trait

error: expected one of `!` or `::`, found `:`
  --> $DIR/trait-invalid-items.rs:47:9
   |
LL | trait Trait9 {
   |              - while parsing this item list starting here
LL |     self: String,
   |         ^ expected one of `!` or `::`
LL | }
   | - the item list ends here

error[E0449]: visibility qualifiers are not permitted here
  --> $DIR/trait-invalid-items.rs:56:5
   |
LL |     pub(self) fn private_foo(&self);
   |     ^^^^^^^^^ help: remove the qualifier
   |
   = note: trait items always share the visibility of their trait

error: aborting due to 11 previous errors

For more information about this error, try `rustc --explain E0449`.

    Mostly is ok here are finding

    fn main() {}

trait Trait1 {
    field_name: String,
    //~^ ERROR expected one of `!` or `::`, found `:`
}

trait Trait2 {
    pub pub_field_name: String,
    //~^ ERROR visibility `pub` is not followed by an item
    //~| ERROR non-item in item list
}

trait Trait3 {
    mut value: i32,
    //~^ ERROR non-item in item list
}

trait Trait4 {
    self: String,
    //~^ ERROR expected one of `!` or `::`, found `:`
}
    error: expected one of `!` or `::`, found `:`
  --> $DIR/trait-invalid-items.rs:4:15
   |
LL | trait Trait1 {
   |              - while parsing this item list starting here
LL |     field_name: String,
   |               ^ expected one of `!` or `::`
LL |
LL | }
   | - the item list ends here

error: visibility `pub` is not followed by an item
  --> $DIR/trait-invalid-items.rs:9:5
   |
LL |     pub pub_field_name: String,
   |     ^^^ the visibility
   |
   = help: you likely meant to define an item, e.g., `pub fn foo() {}`

error: non-item in item list
  --> $DIR/trait-invalid-items.rs:9:9
   |
LL | trait Trait2 {
   |              - item list starts here
LL |     pub pub_field_name: String,
   |         ^^^^^^^^^^^^^^ non-item starts here
...
LL | }
   | - item list ends here

error: non-item in item list
  --> $DIR/trait-invalid-items.rs:15:5
   |
LL | trait Trait3 {
   |              - item list starts here
LL |     mut value: i32,
   |     ^^^ non-item starts here
LL |
LL | }
   | - item list ends here

error: expected one of `!` or `::`, found `:`
  --> $DIR/trait-invalid-items.rs:20:9
   |
LL | trait Trait4 {
   |              - while parsing this item list starting here
LL |     self: String,
   |         ^ expected one of `!` or `::`
LL |
LL | }
   | - the item list ends here

error: aborting due to 5 previous errors

    trait Trait1 {
    field_name: String,
    //~^ ERROR expected one of ! or ::, found :
    }

    needs improvement like

    error: fields are not allowed in traits
help: use a method instead, e.g., `fn field_name(&self) -> &String;`

    trait Trait2 {
    pub pub_field_name: String,
    //^ ERROR visibility pub is not followed by an item
    //    | ERROR non-item in item list
    }

    The first error is OK but the second one "non-item in item list" is confusing. What does "non-item" mean exactly?
    It doesn't tell the user what they did wrong. They don't know what a "non-item" is or why it's wrong.

    trait Trait3 {
    mut value: i32,
    //~^ ERROR non-item in item list
    }

    Same as above. "non-item" is vague.

    trait Trait4 {
    self: String,
    //~^ ERROR expected one of ! or ::, found :
    }

    Need improvement

  6. wischi-chr commented on Aug 27, 2026

    @wischi-chr
    wischi-chr
    on Aug 27, 2026
    Author

    I guess the "non-item" error is the same situation as with the "expected one of ! or ::, found :" that the parser/lexer wasn't built to to handle those items in a trait (that actually shouldn't be there in correct Rust code) and thus can't give good diagnostics.

    I'm not sure how the Rust compiler typically handles that but I guess (if we want to fix that) it would have to parse/allow more items that actually don't belong in a trait (for example a field) to tell the developer that this is not allowed.

  7. jhpratt
    added a commit that references this issue on Sep 23, 2026

    Rollup merge of rust-lang#162103 - raushan728:issues/161815, r=davidtwco

    11682a4
  8. rust-bors
    closed this as completedin 11682a4on Sep 23, 2026
  9. rust-bors
    added a commit that references this issue on Sep 23, 2026

    Unrolled build for #162103 in rollup 163227

    8824bb3
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