Improve error messages for missing JsonSchema trait in ABI generation

[Copilot]

When custom types used in contract function signatures lack JsonSchema implementation, ABI generation fails with an unhelpful compiler error that doesn't explain the fix.

Changes

Enhanced error detection (cargo-near-build/src/cargo_native/compile.rs)

• Detect JsonSchema trait bound errors during compilation
• Display actionable hint box with two fix options:
  • #[near(serializers = [json])] for near-sdk 5.0+
  • Manual JsonSchema derive from schemars

Added test (integration-tests/tests/abi/json_schema.rs)

• test_schema_vec_of_custom_types() demonstrates correct usage of Vec<CustomType>

Updated CHANGELOG

Example

Before:

error[E0277]: the trait bound `Attestation: JsonSchema` is not satisfied
   --> src/lib.rs:224:10

After:

error[E0277]: the trait bound `Attestation: JsonSchema` is not satisfied
   --> src/lib.rs:224:10
 │
 │ ╭─────────────────────────────────────────────────────────────────╮
 │ │ HINT: The error above indicates that a type used in your       │
 │ │ contract's public interface does not implement the JsonSchema  │
 │ │ trait, which is required for ABI generation.                   │
 │ │                                                                 │
 │ │ To fix this, add the JsonSchema derive to your struct/enum:    │
 │ │                                                                 │
 │ │ Option 1 (Recommended - works with near-sdk 5.0+):             │
 │ │   #[near(serializers = [json])]                                │
 │ │   pub struct YourType { ... }                                  │
 │ ╰─────────────────────────────────────────────────────────────────╯

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>ABI not generated when functions return a vector of objects or regular JSON</issue_title>
<issue_description>When running cargo near build --embed-abi as well as cargo near abi on the following contract sources:
https://github.com/archetype-org/attestation-registry/blob/main/src/lib.rs

I get the following error:

error[E0277]: the trait bound `Attestation: JsonSchema` is not satisfied
│    --> src/lib.rs:224:10
│     |
│ 224 |     ) -> Attestations {
│     |          ^^^^^^^^^^^^ the trait `JsonSchema` is not implemented for `Attestation`
│     |
│     = help: the following other types implement trait `JsonSchema`:
│               bool
│               char
│               isize
│               i8
│               i16
│               i32
│               i64
│               i128
│             and 165 others
│     = note: required for `std::vec::Vec<Attestation>` to implement `JsonSchema`
│ note: required by a bound in `SchemaGenerator::subschema_for`
│    --> /Users/c8r7/.cargo/registry/src/index.crates.io-6f17d22bba15001f/schemars-0.8.16/src/gen.rs:221:38
│     |
│ 221 |     pub fn subschema_for<T: ?Sized + JsonSchema>(&mut self) -> Schema {
│     |                                      ^^^^^^^^^^ required by this bound in `SchemaGenerator::subschema_for`
│ 
│ error[E0277]: the trait bound `Attestation: JsonSchema` is not satisfied
│    --> src/lib.rs:241:10
│     |
│ 241 |     ) -> Attestation {
│     |          ^^^^^^^^^^^ the trait `JsonSchema` is not implemented for `Attestation`
│     |
│     = help: the following other types implement trait `JsonSchema`:
│               bool
│               char
│               isize
│               i8
│               i16
│               i32
│               i64
│               i128
│             and 165 others
│ note: required by a bound in `SchemaGenerator::subschema_for`
│    --> /Users/c8r7/.cargo/registry/src/index.crates.io-6f17d22bba15001f/schemars-0.8.16/src/gen.rs:221:38
│     |
│ 221 |     pub fn subschema_for<T: ?Sized + JsonSchema>(&mut self) -> Schema {
│     |                                      ^^^^^^^^^^ required by this bound in `SchemaGenerator::subschema_for`
│ 
│ For more information about this error, try `rustc --explain E0277`.
│ error: could not compile `attestation-registry` (lib) due to 2 previous errors

Similar code also fails due to the same reason:
https://github.com/NEAR-DevHub/neardevhub-contract

error[E0277]: the trait bound `access_control::AccessControl: JsonSchema` is not satisfied
 │    --> src/access_control/mod.rs:22:46
 │     |
 │ 22  |     pub fn get_access_control_info(&self) -> &AccessControl {
 │     |                                              ^^^^^^^^^^^^^^ the trait `JsonSchema` is not implemented for `access_control::AccessControl`
 │     |
 │     = help: the following other types implement trait `JsonSchema`:
 │               &'a T
 │               &'a mut T
 │               ()
 │               (T0, T1)
 │               (T0, T1, T2)
 │               (T0, T1, T2, T3)
 │               (T0, T1, T2, T3, T4)
 │               (T0, T1, T2, T3, T4, T5)
 │             and 163 others
 │     = note: required for `&access_control::AccessControl` to implement `JsonSchema`
 │ note: required by a bound in `SchemaGenerator::subschema_for`
 │    --> /Users/ebraem/.cargo/registry/src/index.crates.io-6f17d22bba15001f/schemars-0.8.15/src/gen.rs:221:38
 │     |
 │ 221 |     pub fn subschema_for<T: ?Sized + JsonSchema>(&mut self) -> Schema {
 │     |                                      ^^^^^^^^^^ required by this bound in `SchemaGenerator::subschema_for`
 │
 │ error[E0277]: the trait bound `RulesList: JsonSchema` is not satisfied
 │    --> src/access_control/mod.rs:34:51
 │     |
 │ 34  |     pub fn set_restricted_rules(&mut self, rules: RulesList) {
 │     |                                                   ^^^^^^^^^ the trait `JsonSchema` is not implemented for `RulesList`
 │     |
 │     = help: the following other types implement trait `JsonSchema`:
 │               &'a T
 │               &'a mut T
 │               ()
 │               (T0, T1)
 │               (T0, T1, T2)
 │               (T0, T1, T2, T3)
 │               (T0, T1, T2, T3, T4)
 │               (T0, T1, T2, T3, T4, T5)
 │             and 163 others

I believe this is because the abigen does not support the same types of schema traits as borsh. Is this a limitation with the standard or will support be added in the future.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ABI not generated when functions return a vector of objects or regular JSON #136

---

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[r-near]

@copilot the error message is nice, but couldn't we also be smart about this and support it in the same way borsh does?

[Copilot]

@copilot the error message is nice, but couldn't we also be smart about this and support it in the same way borsh does?

The automatic schema derivation like borsh does happens in the near-sdk macros during compilation, not in cargo-near. When users apply #[near] to a struct, near-sdk auto-derives BorshSchema. Similarly, #[near(serializers = [json])] auto-derives JsonSchema.

The issue here is that the user is using manual derives (#[derive(Serialize, Deserialize)]) instead of the #[near] macro, which bypasses the automatic JsonSchema derivation. Since cargo-near runs after macro expansion, we can't retroactively add the trait implementation.

The enhanced error message in this PR is the most helpful approach cargo-near can provide - it clearly guides users to add #[near(serializers = [json])] or manually derive JsonSchema.

For fully automatic support like borsh, that would need to be implemented in near-sdk itself to handle cases where users use manual derives. Would you like me to create an issue in the near-sdk-rs repo to discuss this enhancement?