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Auto merge of #11604 - pkgw:resolver-unification-docs, r=ehuss

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book: describe how the current resolver sometimes duplicates deps

### What does this PR try to resolve?

This updates the book to document the behavior described in #9029, where sometimes Cargo will duplicate a dep when it doesn't have to.

### How should we test and review this PR?

Doc-only change; someone with knowledge of the resolver should read and assess.
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bors 2023-01-25 19:31:56 +00:00
commit 4a2ebfbbf3
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src/doc/src/reference/resolver.md
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@ -7,8 +7,10 @@ result of the resolution is stored in the `Cargo.lock` file which "locks" the
dependencies to specific versions, and keeps them fixed over time.
The resolver attempts to unify common dependencies while considering possibly
conflicting requirements. The sections below provide some details on how these
constraints are handled, and how to work with the resolver.
conflicting requirements. It turns out, however, that in many cases there is no
single "best" dependency resolution, and so the resolver must use heuristics to
choose a preferred solution. The sections below provide some details on how
requirements are handled, and how to work with the resolver.
See the chapter [Specifying Dependencies] for more details about how
dependency requirements are specified.
@ -490,6 +492,35 @@ break the build.
The following illustrates some problems you may experience, and some possible
solutions.
### Unexpected dependency duplication
The resolver algorithm may converge on a solution that includes two copies of a
dependency when one would suffice. For example:
```toml
# Package A
[dependencies]
rand = "0.7"
# Package B
[dependencies]
rand = ">=0.6" # note: open requirements such as this are discouraged
```
In this example, Cargo may build two copies of the `rand` crate, even though a
single copy at version `0.7.3` would meet all requirements. This is because the
resolver's algorithm favors building the latest available version of `rand` for
Package B, which is `0.8.5` at the time of this writing, and that is
incompatible with Package A's specification. The resolver's algorithm does not
currently attempt to "deduplicate" in this situation.
The use of open-ended version requirements like `>=0.6` is discouraged in Cargo.
But, if you run into this situation, the [`cargo update`] command with the
`--precise` flag can be used to manually remove such duplications.
[`cargo update`]: ../commands/cargo-update.md
### SemVer-breaking patch release breaks the build
Sometimes a project may inadvertently publish a point release with a