Version trouble at handlebar-rust

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1

This has me puzzled. I'm building something that uses "handlebars" from within a dependency that's not mine. It's pulling in handlebars 5.1.2, which is current. Fails to compile with error:

error[E0599]: no method named `message` found for enum `ErrorVariant` in the current scope
   --> /home/john/.cargo/registry/src/index.crates.io-6f17d22bba15001f/handlebars-5.1.2/src/template.rs:644:27
    |
644 |                 e.variant.message().to_string(),
    |                           ^^^^^^^ method not found in `ErrorVariant<Rule>`

This is a version problem. ErrorVariant is from crate "pest", and only newer versions of ErrorVariant have message() as a method. In earlier versions, it was a field.

So, time to look at cargo.lock and cargo.toml files.
Version of "handlebars" pulled in is 5.1.2. Current.
Version of "pest" pulled in is "2.1.3". Not current; current in cargo.io is 2.7.9.

Checking "handlebars" Cargo.toml: pest = "2.1.3" That old version of "pest" doesn't have .message(), so handlebars won't compile.

So far, it just looks like a botched dependency.

So, to reproduce the problem for a bug report, I build "handlebars" from

git clone https://github.com/sunng87/handlebars-rust.git

That builds fine. When I look at its Cargo.lock file, I find

[[package]]
name = "pest"
version = "2.7.9"

Why did Cargo pick that later version, when the Cargo.toml file for "handlebar-rust" has

pest = "2.1.0"

Not seeing any workspace-level files or overrides.

2

Cargo considers that versions are equivalent if their left-most non-zero number is the same - see here:
https://doc.rust-lang.org/cargo/reference/resolver.html#semver-compatibility

So if Cargo.toml says "2.1.0", but "2.7.9" is available, Cargo would go for "2.7.9".

What happens if you run cargo update on your project

3

Oh, right. Default is not "="

It works. I forgot that I'd started from a branch of an existing checkout, not a new checkout.

4

Does that mean pest's 2.7.9 (or something between that and 2.1.3) is a breaking change?

Or if it's strictly additional and doesn't break existing code, did handlebars fail to notice that they relied on a new feature and bump their dependency version of pest accordingly?

What can we do to avoid (as authors of our own crates) that kind of problem in the future?

5

This. It it a kind of change for which Semver uses minor version bump, and here minor version have indeed changed.

Possible way is to check regularly with something like cargo-minimal-versions.

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6

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