Spanned values don't play well with dotted notation: getting "invalid type: string [..], expected a borrowed string"

[yannham]

I'm using Spanned to deserialize TOML to Nickel (a configuration language) while preserving spans as much as possible, as Nickel adds validation capabilities and we'd like to link back validation errors to the precise piece of TOML data that failed.

To do so, we define a bespoke datastructure that is more or less like a TOML value but with Spanned appropriately sprinkled, and write a custom deserializer using serde_untagged. You can find the type definition and the deserializer here: https://github.com/tweag/nickel/blob/927ee23993747b7851e51bcfe3eb3e685ba4ebb1/core/src/serialize.rs#L491-L582

However, when deserializing the following file:

[foo.bar]
baz = "qux"

This gives the following surprising error:

error: toml parse error: TOML parse error at line 1, column 1
  |
1 | [foo.bar]
  | ^
invalid type: string "bar", expected a borrowed string



in `foo`

It's surprising because we never try to deserialize a borrowed string: all strings, both as terminal values and keys, are owned in SpannedValue (NickelString is a simple wrapper around String). Also, any TOML file without dotted notation is parsed fine. After some experimentation, it seems that this happens when trying to deserialize the value (and not the key) of the outer map, that is the value associated to foo.

I suspect that there are some shenanigans around getting the location of the nested map {bar = {baz = "qux"}}. It seems that the spanned deserializer of toml-rs tries to deserialize markers as borrowed string (https://github.com/toml-rs/toml/blob/b05e8c489be8ebfc0acacc1ec3556d95cd8d2198/crates/serde_spanned/src/spanned.rs#L161) but it also expects a very precise structure, so I'm not entirely sure what's going on here.

The issue is that I don't see any easy work-around: once we've tried to deserialize the content of a map as spanned (which is entirely legit for files that don't have the dotted notation), there doesn't seem to be anyway to retry the same deserialization at a different type.