Error types should not implement source _and_ print inner error in Display

[jonhoo]

SdkError, and possibly other error types within the SDK, currently expose inner errors both through Error::source and fmt::Display:

https://github.com/awslabs/smithy-rs/blob/692bf94ba30f570365f559c67d6a9981c037c371/rust-runtime/aws-smithy-http/src/result.rs#L176-L206

The recommendation in the ecosystem is to either return inner errors through Error::source or print the inner error in Display, but never both (see rust-lang/project-error-handling#27 (comment), rust-lang/project-error-handling#23, rust-lang/api-guidelines#210), as otherwise errors end up being duplicated whenever error reporters print error backtraces. For example, I recently got an error trace that ended in:

    6: failed to construct request: Failed to load credentials from the credentials provider: An error occurred while loading credentials: io error: error trying to connect: invalid certificate: UnknownIssuer
    7: Failed to load credentials from the credentials provider: An error occurred while loading credentials: io error: error trying to connect: invalid certificate: UnknownIssuer

[jonhoo]

The same is the case for CredentialsError:

https://github.com/awslabs/smithy-rs/blob/a46e4899cc854e02e2c25d20797a497c67ad28f4/aws/rust-runtime/aws-types/src/credentials/provider.rs#L106-L144

[rcoh]

yeah this is definitely the case currently—is there a plan to add backtrace reporting by default? I worry about customers not using a "fancy" printer missing error context

[rcoh]

I guess this may guide us towards "use display but not source"?

[jdisanti]

Implemented as part of #1926