It doesn't link two versions of `rand-core`. That's not even possible with rust (you can only link two semver-incompatible versions of the same crate). And dependency specifications in Rust don't work like that - unless you explicitly override it, all dependencies are semver constraints, so "0.9.0" will happily match "0.9.3".
They are different:
"0": ">=0.0.0, <1.0.0"
"0.9": ">=0.9.0, <1.0.0"
"0.9.3": ">=0.9.3, <1.0.0"
The reason for this is that unless otherwise specified, the ^ operator is used, so "0.9" is actually "^0.9", which then gets translated into the kind of range specifier I showed above.
There are other operators you can use, these are the common ones:
(default) ^ Semver compatible, as described above
>= Inclusive lower bound only
< Exclusive upper bound only
= Exact bound
For this reason, if you are writing a library, you should in almost all cases stick to regular semver-compatible dependency specifications.
For binaries, it is more common to want exact control over versions and you don't have downstream consumers for whom your exact constraints would be a nightmare.
Note that in the output, there is rand 0.9.0, and two instances of rand_core 0.9.3. You may have thought it selected two versions because you missed the _core there.
> So there's no difference at all between "0", "0.9" and "0.9.3" in cargo.toml
No, there is a difference, in particular, they all specify different minimum bounds.
The trick is that these are using the ^ operator to match, which means that the version "0.9.3" will satisfy all of those constraints, and so Cargo will select 0.9.3 (the latest version at the time I write this comment) as the one version to satisfy all of them.
Cargo will only select multiple versions when it's not compatible, that is, if you had something like "1.0.0" and "0.9.0".
> Surely there's some way to actually require an exact version?
Yes, you'd have to use `=`, like `=0.9.3`. This is heavily discouraged because it would lead to a proliferation of duplication in dependency versions, which aren't necessarily unless you are trying to avoid some sort of specific bugfix. This is sometimes done in applications, but basically should never be done in libraries.
Sorry, I don't understand the "^ operator" in this context. Do I understand correctly that cargo will basically select the latest release that matches within a major version, so if I have two crates that specify "0.8" and "0.7.1" as dependencies then the compiler will use "0.8.n" for both? And then if I add a new dependency that specifies "0.9.5", all three crates would use "0.9.5"? Assuming I have that right, I'm quite surprised that it works in practice.
It’s all good. Let me break it down.
Semver specifies versions. These are the x.y.z (plus other optional stuff) triples you see. Nothing should be complicated there.
Tools that use semver to select versions also define syntax for defining which versions are acceptable. npm calls these “ranges”, cargo calls them “version requirements”, I forget what other tools call them. These are what you actually write in your Cargo.toml or equivalent. These are not defined by the semver specification, but instead, by the tools. They are mostly identical across tools, but not always. Anyway, they often use operators to define the ranges (that’s the name I’m going to use in this post because I think it makes the most sense.) So for example, ‘>3.0.0’ means “any version where x >= 3.” “=3.0.0” means “any version where x is 3, y is 0, and z is 0” which 99% of the time means only one version.
When you write “0.9.3” in a Cargo.toml, you’re writing a range, not a version. When you do not specify an operator, Cargo treats that as if you use the ^ operator. So “0.9.3” is equivalent to “^0.9.3” what does ^ do? It means two things, one if x is 0 and one if x is nonzero. Since “^0.9.3” has x of zero, this range means “any version where x is 0, y is 9, and z is >= 3.” Likewise, “0.9” is equivalent to “^0.9.0” which is “any version where x is 0, y is 9, and z is >=0.”
Putting these two together:
0.9.0 satisfies the latter, but not the former
0.9.1 satisfies the latter, but not the former
0.9.2 satisfies the latter, but not the former
0.9.3 satisfies both
If we had “0.8” and “0.7.1”, no version could satisfy both simultaneously, as “y must be 8” and “y must be 7” would conflict. Cargo would give you both versions in this case, whichever y=8 and y=7 versions have the highest z at the time.
Within a crate graph, for any given major version of a crate (eg. D v1) only a single minor version can exist. So if B depends on D v1.x, and C depends on D v2.x, then two versions of D will exist. If B depends on Dv1.2 and C depends on Dv1.3, then only Dv1.3 will exist.
I'm over-simplifying a few things here:
1. Semver has special treatment of 0.x versions. For these crates the minor version depends like the major version and the patch version behaves like the minor version. So technically you could have v0.1 and v0.2 of a crate in the same crate graph.
2. I'm assuming all dependencies are specified "the default way", ie. as just a number. When a dependency looks like "1.3", cargo actually treats this as "^1.3", ie. the version must be at least 1.3, but can be any semver compatible version (eg. 1.4). When you specify an exact dependency like "=1.3" instead, the rules above still apply (you still can't have 1.3 and 1.4 in the same crate graph) but cargo will error if no version can be found that satisfies all constraints, instead of just picking a version that's compatible with all dependents.
can does not mean must. Cargo attempts to unify (aka deduplicate) dependencies where possible, and in this case, it can find a singular version that satisfies the entire thing.
So there's no difference at all between "0", "0.9" and "0.9.3" in cargo.toml (Since semver says only major version numbers are breaking)? As a decently experienced Rust developer, that's deeply surprising to me.
What if devs don't do a good job of versioning and there is a real incompatibility between 0.9.3 and 0.9.4? Surely there's some way to actually require an exact version?