Don't allow subdiagnostic to use variables from their parent Tangentially related to https://github.com/rust-lang/rust/issues/151366 This is PR 1/2 for structured diagnostics, will do the unstructured ones next. After the second PR I will be able to remove some code that should compensate for this PR being positive. Regardless of this PR having a positive diff, I feel that subdiagnostics being able to use variables from their parent is very confusing, so this is for the better,. r? @jdonszelmann