VRL coalesce assignment panics

[fuchsnj]

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Problem

The following VRL code causes a panic

.(a|b) = 5

thread 'main' panicked at 'attempt to subtract with overflow', lib/vrl/compiler/src/expression/assignment.rs:250:37

I suspect this was caused by #13317

It seems to work fine with 0.23

Version

master / 2d62397

[fuchsnj]

The following also panics

.@foo = 5

I believe the root cause is that the diagnostic code is assuming that length of string generated from the Display impl of FieldBuf is identical to the raw source code, which isn't necessarily true.

In the above case, the Display implementation is incorrect and unnecessarily quotes the field name, which makes the string longer than the source code, causing an underflow / panic.

I would also consider it valid for the Display implementation to remove quotes if the user unnecessarily add quotes. The diagnostics probably shouldn't rely on this for calculating spans.