Improve error messages on failure to derive

[JayKickliter]

This PR uses spans to add source-of-error messages when a rustler derive macro fails.

Example invalid input

use rustler::{NifMap, NifRecord, NifStruct, NifTuple, NifUnitEnum, NifUntaggedEnum};

/// This struct does not implement Encoder/Decoder
pub struct C(i32);

#[derive(NifStruct)]
#[module = "AddStruct"]
struct AddStruct {
    lhs: i32,
    rhs: C,
}

#[derive(NifMap)]
struct AddMap {
    lhs: i32,
    rhs: C,
}

#[derive(NifRecord)]
#[tag = "record"]
struct AddRecord {
    lhs: i32,
    rhs: C,
}

#[derive(NifTuple)]
struct AddTuple {
    lhs: i32,
    rhs: C,
}

#[derive(NifUnitEnum)]
enum UnitEnum {
    FooBar,
    Baz(u8),
}

#[derive(NifUntaggedEnum)]
enum UntaggedEnum {
    Foo(u32),
    Bar(String),
    Baz(i32,i32),
}

Improved messages

error: NifUnitEnum can only be used with enums containing unit variants.
  --> src/nifmap.rs:34:5
   |
34 |     Baz(u8),
   |     ^^^

error: NifUntaggedEnum can only be used with enums that contain all NewType variants.
  --> src/nifmap.rs:41:5
   |
41 |     Baz(i32,i32),
   |     ^^^

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
 --> src/nifmap.rs:9:5
  |
5 | #[derive(NifStruct)]
  |          --------- required by this bound in `<nifmap::AddStruct as rustler::Decoder<'a>>::decode::try_decode_field`
...
9 |     rhs: C,
  |     ^^^ the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
 --> src/nifmap.rs:9:5
  |
3 | pub struct C(i32);
  | ------------------ method `encode` not found for this
...
9 |     rhs: C,
  |     ^^^ method not found in `nifmap::C`
  |
  = help: items from traits can only be used if the trait is implemented and in scope
  = note: the following trait defines an item `encode`, perhaps you need to implement it:
          candidate #1: `rustler::Encoder`

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:15:5
   |
12 | #[derive(NifMap)]
   |          ------ required by this bound in `<nifmap::AddMap as rustler::Decoder<'a>>::decode::try_decode_field`
...
15 |     rhs: C,
   |     ^^^ the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:15:5
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
15 |     rhs: C,
   |     ^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:22:5
   |
18 | #[derive(NifRecord)]
   |          --------- required by this bound in `<nifmap::AddRecord as rustler::Decoder<'a>>::decode::try_decode_index`
...
22 |     rhs: C,
   |     ^^^ the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:22:5
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
22 |     rhs: C,
   |     ^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:28:5
   |
25 | #[derive(NifTuple)]
   |          -------- required by this bound in `<nifmap::AddTuple as rustler::Decoder<'a>>::decode::try_decode_index`
...
28 |     rhs: C,
   |     ^^^ the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:28:5
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
28 |     rhs: C,
   |     ^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`

error: aborting due to 10 previous errors

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `nifmap`.

Current error messages

error: proc-macro derive panicked
  --> src/nifmap.rs:31:10
   |
31 | #[derive(NifUnitEnum)]
   |          ^^^^^^^^^^^
   |
   = help: message: NifUnitEnum can only be used with enums that contain all unit variants.

error: proc-macro derive panicked
  --> src/nifmap.rs:37:10
   |
37 | #[derive(NifUntaggedEnum)]
   |          ^^^^^^^^^^^^^^^
   |
   = help: message: NifUntaggedEnum can only be used with enums that contain all NewType variants.

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
 --> src/nifmap.rs:5:10
  |
5 | #[derive(NifStruct)]
  |          ^^^^^^^^^
  |          |
  |          the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`
  |          required by this bound in `<nifmap::AddStruct as rustler::Decoder<'a>>::decode::try_decode_field`
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
 --> src/nifmap.rs:5:10
  |
3 | pub struct C(i32);
  | ------------------ method `encode` not found for this
4 |
5 | #[derive(NifStruct)]
  |          ^^^^^^^^^ method not found in `nifmap::C`
  |
  = help: items from traits can only be used if the trait is implemented and in scope
  = note: the following trait defines an item `encode`, perhaps you need to implement it:
          candidate #1: `rustler::Encoder`
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:12:10
   |
12 | #[derive(NifMap)]
   |          ^^^^^^
   |          |
   |          the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`
   |          required by this bound in `<nifmap::AddMap as rustler::Decoder<'a>>::decode::try_decode_field`
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:12:10
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
12 | #[derive(NifMap)]
   |          ^^^^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:18:10
   |
18 | #[derive(NifRecord)]
   |          ^^^^^^^^^
   |          |
   |          the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`
   |          required by this bound in `<nifmap::AddRecord as rustler::Decoder<'a>>::decode::try_decode_index`
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:18:10
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
18 | #[derive(NifRecord)]
   |          ^^^^^^^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `nifmap::C: rustler::Decoder<'_>` is not satisfied
  --> src/nifmap.rs:25:10
   |
25 | #[derive(NifTuple)]
   |          ^^^^^^^^
   |          |
   |          the trait `rustler::Decoder<'_>` is not implemented for `nifmap::C`
   |          required by this bound in `<nifmap::AddTuple as rustler::Decoder<'a>>::decode::try_decode_index`
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `encode` found for struct `nifmap::C` in the current scope
  --> src/nifmap.rs:25:10
   |
3  | pub struct C(i32);
   | ------------------ method `encode` not found for this
...
25 | #[derive(NifTuple)]
   |          ^^^^^^^^ method not found in `nifmap::C`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `encode`, perhaps you need to implement it:
           candidate #1: `rustler::Encoder`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 10 previous errors

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `nifmap`.

To learn more, run the command again with --verbose

[evnu]

@JayKickliter thank you!

[JayKickliter]

You're welcome! I've been looking for an excuse to learn a little bit about proc macros.