feat: Improved error message for unexpected return type

[ghost]

Description

better return type error message

Problem

Resolves #2105

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kevaundray]

For totality, can you provide images of the before and after?

[ghost]

@kevaundray

before:

error: Expected type Field, found type ()
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:1:4
  │
1 │ fn main() -> Field {
  │    ----

Error: Aborting due to 2 previous errors

after (empty function):

error: Expected return type Field, but a value of type () was actually returned
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:1:4
  │
1 │ fn main() -> Field {}
  │    ----
  │
  = implicitly returns `()` as its body has no tail or `return` expression

after (non-empty function):

error: Expected return type Field, but a value of type () was actually returned
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:1:4
  │
1 │ fn main() -> Field {
  │    ----

Perhaps it's necessary to store a span for the return type and point it specifically to this location, rather than just the function name.

[ghost]

for comparison with Rust:

noir:

error: Expected return type (), but a value of type Field was actually returned
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:1:4
  │
1 │ fn main() -> () {
  │    ----

rust:

 --> src/lib.rs:2:5
  |
1 | fn lotsofRead() -> () {
  |                    -- expected `()` because of return type
2 |     42
  |     ^^ expected `()`, found integer

probably need something like this

[jfecher]

Perhaps it's necessary to store a span for the return type and point it specifically to this location, rather than just the function name.

Can you include that as part of this PR? Where this error message is generated we should have access to the function body which you can match on and get the location of the last expression if it is a block expression, or just get the location of the overall body if not.

[kevaundray]

@f01dab1e can you give an updated before and after with the return type span?

[ghost]

@kevaundray

error: expected type (), found type Field
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:3:11
  │
3 │ fn main() {
  │           - expected () because of return type
4 │     2
  │     - expected type (), found type Field
  │
  = help: try adding a return type: `-> Field`

Error: Aborting due to 1 previous error

error: expected type (), found type Field
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:3:14
  │
3 │ fn main() -> () {
  │              -- expected () because of return type
4 │     2
  │     - expected type (), found type Field

Error: Aborting due to 1 previous error

error: expected type Field, found type ()
  ┌─ /home/work/subject/oliver/noir/h/src/main.nr:3:14
  │
3 │ fn main() -> Field {
  │              ----- expected Field because of return type
  │
  = implicitly returns `()` as its body has no tail or `return` expression

[jfecher]

Some very good improvements to the error messages here