postgres-shared: phf =0.7.21 causes severe version conflicts

[fschutt]

I happen to depend on the git version of svgdom, which has phf = "0.7.22" listed:

https://github.com/RazrFalcon/svgtypes/blob/master/Cargo.toml#L20

However, the currently released version of postgres 0.6.0 has phf = "=0.7.21" listed. The = leads to a version conflict between the two:

rust-postgres/postgres-shared/Cargo.toml

Line 20 in 4eecb59

phf = "=0.7.22"

results in:

    Updating git repository `https://github.com/maps4print/azul`
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Updating git repository `https://github.com/fschutt/glium`
    Updating git repository `https://github.com/RazrFalcon/usvg`
    Updating git repository `https://github.com/servo/webrender`
    Updating git repository `https://github.com/fschutt/glutin`
    Updating git repository `https://github.com/fschutt/winit`
    Updating git repository `https://github.com/RazrFalcon/svgdom`
    Updating git repository `https://github.com/RazrFalcon/svgtypes`
    Updating git repository `https://github.com/RazrFalcon/xmlparser`
error: failed to select a version for `phf`.
    ... required by package `svgtypes v0.1.1 (https://github.com/RazrFalcon/svgtypes?rev=a794d41#a794d416)`
    ... which is depended on by `svgdom v0.13.0 (https://github.com/RazrFalcon/svgdom?rev=968db94#968db94b)`
    ... which is depended on by `usvg v0.2.0 (https://github.com/RazrFalcon/usvg?rev=803c4f7bfa1775e7f3824173ac05793a8a4adc5c#803c4f7b)`
    ... which is depended on by `azul v0.1.0 (https://github.com/maps4print/azul?branch=mapedit_menus#870c588c)`
    ... which is depended on by `mapedit2 v0.1.0 (file:///C:/Users/fschutt/Development/mapedit2)`
versions that meet the requirements `^0.7.22` are: 0.7.22

all possible versions conflict with previously selected packages.

  previously selected package `phf v0.7.21`
    ... which is depended on by `postgres-shared v0.3.0`
    ... which is depended on by `postgres v0.14.2`
    ... which is depended on by `mapedit2 v0.1.0 (file:///C:/Users/fschutt/Development/mapedit2)`

failed to select a version for `phf` which could resolve this conflict

So I can't use the postgres crate at all if I happen to also use svgdom. What I don't get: phf is just a hashing crate, why is the = required? Why can't it just use semver? Without the = I can build it without any problems, because it'll select 0.7.22 as the version for phf.

Why is the "=0.7.21" required, is it possible to change it to just be "0.7.21"?

[sfackler]

This is due to the somewhat weird way phf is structured - we run phf_codegen to build the phf data structure and check the generated file in. In theory, there could be changes to the internal representation of a PhfMap within a semver-compatible release which would break things if the runtime version differed from the codegen version.

This level of paranoia probably isn't warranted though. I'll bump the version and switch to a normal semver dependency.