Remove azure_identity::TokenCredential trait in favor of azure_core::TokenCredential

[thovoll]

azure_core::TokenCredential
azure_identity::TokenCredential

We should probably get rid of the TokenCredential in azure_identity. This seems to match the .Net SDK as well.

This implies that the token credential implementations need to switch to the TokenCredential trait from azure_core.

Further, if we decide to implement #426, it would be nice to avoid having to downcast errors produced by a TokenCredential when creating a client, in order to enable exhaustive matching.

[cataggar]

They are not quite the same. See the comments:

azure-sdk-for-rust/sdk/identity/src/token_credentials/mod.rs

Lines 20 to 27 in 7bd9193

	/// Represents a credential capable of providing an OAuth token.
	/// Same as [azure_core::TokenCredential](azure_core::TokenCredential), except a more specific error is returned.
	#[async_trait::async_trait]
	pub trait TokenCredential: Send + Sync {
	type Error;
	/// Gets a `TokenResponse` for the specified resource
	async fn get_token(&self, resource: &str) -> Result<azure_core::TokenResponse, Self::Error>;
	}

[cataggar]

it would be nice to avoid having to downcast errors produced by a TokenCredential when creating a client, in order to enable exhaustive matching

Can you give an example, so I can better understand what you are trying to do?

[thovoll]

Thanks @cataggar, I did see the comment in the code. @rylev and I looked at this code together this morning in the context of trying to determine feasibility of implementing #426 (not needed for 0.1.0 I think but I was in the area anyway).

Taking the azure_identity::TokenCredential into clients is causing some problems because all the TokenCredential implementations have different Error types. I'm not good enough yet at Rust errors to be able to fully articulate @rylev's arguments so I'll ask him to also chime in here.

[rylev]

The reason that having a single unified error type and not allowing individual implementations to specfify different errors is that we want to be able to use this trait as a trait object (e.g., Box<dyn TokenCredential>) which we cannot do unless the error types are unified. The flexibility of having individual error types per implementation is not needed and makes things complicated and/or impossible.

It's my belief we should unify on one error type (something like TokenCredentialError) that is the union of all possible error cases from getting a token.

[thovoll]

In #426, we want to be able to pass any TokenCredential to a client. This is easy in C# because it has only unchecked exceptions, but requires more careful design in Rust.

[cataggar]

Fixed by #781.