Combined from and backtrace field

[astraw]

This implements support for #[from] and #[backtrace] attributes used together on the same field. The purpose is to forward the backtrace() implementation to the source rather than capturing a new backtrace at conversion time.

This was suggested in @dtolnay's comment "I would be prepared to consider a PR that does ..." #93 (review).

[astraw]

Oh, I just saw that the tests are failing. Let me investigate...

[astraw]

I pushed a new commit in which the tests pass on my machine.

(I missed that the relevant tests only ran in nightly and thus they didn't actually run in my initial PR. The implementation remains unchanged.)

[dtolnay]

#[from] is not necessary. It should be sufficient for a field to be #[source] (or just be named source, which is implicitly #[source]).

[dtolnay]

The changes on the next 20 lines here can be replaced with a simpler change that does the same thing and minimizes repetition:

     };
-    let combinator = if type_is_option(backtrace_field.ty) {
+    let combinator = if source == backtrace {
+        source_backtrace
+    } else if type_is_option(backtrace_field.ty) {
         quote! {

[dtolnay]

Thiserror permits the source field to be optional. This code should handle that like in the case above this one.

thiserror/impl/src/expand.rs

Lines 212 to 220 in 031fea6

	let source_backtrace = if type_is_option(source_field.ty) {
	quote_spanned! {source.span()=>
	#varsource.as_ref().and_then(|source| source.as_dyn_error().backtrace())
	}
	} else {
	quote_spanned! {source.span()=>
	#varsource.as_dyn_error().backtrace()
	}
	};

[dtolnay]

I don't think from_field is the right field to check here — it should be source_field. Otherwise this will generate noncompilable code in the case of #[source] #[backtrace] inside an enum, because the generated code would treat the source field as if its type were Backtrace.

Suggested change

	let source = variant.from_field().map(|f| &f.member);
	let source = variant.source_field().map(|f| &f.member);

[dtolnay]

Thanks for the PR! I am merging this with my own fixes from a separate PR.