Add a custom error message for invalid use of default(value = ...) in define_tedge_config!

[jarhodes314]

Proposed changes

https://github.com/thin-edge/thin-edge.io/pull/2398/files/313cc0efaac938fe47b1267d63a3d89cbb31c312..954da3d0246b6e6db4b5cd3d41421f5864ad9088#r1380338718 highlighted a usability issue with define_tedge_config and default values, which can either be provided using default(value = true) or default(variable = "Ipv4Addr::LOCALHOST"). The distinction between these two formats is important, but subtle.

This PR adds a custom error message for the case which caused confusion, pointing the user towards a suggested solution. See the screenshot below (or the added test) for the error that appears:

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Merging #2401 (9e659c7) into main (4bdae95) will decrease coverage by 0.1%.
The diff coverage is 96.1%.

Additional details and impacted files

Files	Coverage Δ
crates/common/tedge_config_macros/impl/src/lib.rs	95.5% <100.0%> (+0.3%)	⬆️
...common/tedge_config_macros/impl/src/input/parse.rs	51.8% <92.3%> (+12.1%)	⬆️

... and 7 files with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
361	0	3	361	100	53m9.085999999s

[didier-wenzek]

Approved. This adds nice support for using the tedge config macro.

[didier-wenzek]

Nitpicking.

To me an expression such as Ipv4Addr::LOCALHOST is a literal, i.e. an expression which evaluates to itself.
Maybe a lack of knowledge on Rust terminology.

That said the hint Perhaps you want to use ... variable = \"Ipv4Addr::LOCALHOST\" is wonderful!

[jarhodes314]

The reference explains quite nicely what a literal is: https://doc.rust-lang.org/beta/reference/expressions/literal-expr.html#:~:text=A%20literal%20expression%20is%20an,or%20some%20other%20evaluation%20rule.

A literal expression is an expression consisting of a single token, rather than a sequence of tokens, that immediately and directly denotes the value it evaluates to, rather than referring to it by name or some other evaluation rule.

Ipv4Addr::LOCALHOST is made of three tokens, and refers to the value by name rather than directly. Worth noting here that LOCALHOST is an associated constant, not an enum variant, which is potentially also confusing.

[didier-wenzek]

Indeed, I missed this single token constraint.