Add a no_prefix feature

[sfackler]

There are some targets that we still force a prefix since they have
issues when their libc malloc is replaced in this way.

Closes #39

[SimonSapin]

I think it can be unexpected to silently use a prefix when the no_prefix feature is enabled. Shouldn’t the build script fail with an appropriate message in that case, to force users who would rely on this to do something platform-specific?

[gnzlbg]

I'm going to hold on a new release until @SimonSapin issues in the last two PRs are resolved. @SimonSapin mind you opening an issue?

[sfackler]

@SimonSapin my concern about failing the build is that there's no way to enable a feature on a target-specific basis IIRC, so if you want to have jemalloc replace libc malloc on Linux, you're not going to be able to run on OSX or whatever.

[SimonSapin]

I was thinking of enabling the feature in a [target.'cfg(…)'.dependencies] section, but I wonder if that actually works in Cargo.

[gnzlbg]

my concern about failing the build is that there's no way to enable a feature on a target-specific basis IIRC

I think this is possible: https://github.com/gnzlbg/slice_deque/blob/master/Cargo.toml#L24

[SimonSapin]

Depending on a crate or not at all based on cfg is possible.

I don’t know if conditionally enabling a feature based on cfg actually works or if the feature ends up being always enabled. I vaguely remember a Cargo bug like this but I couldn’t easily find it in the tracker and might be confusing it with something else.

[gnzlbg]

I vaguely remember a Cargo bug like this

Maybe rust-lang/cargo#4866 ?

There are issues with feature/dependency resolution when doing target specific things, but enabling/disabling the feature works IIRC.

[SimonSapin]

I thought rust-lang/cargo#4866 wasn’t it based on the title and first message, but it does discuss [target.'cfg(…)'.dependencies] in later comments. So yeah, the scenario I had mind is indeed buggy in Cargo. The test case below enables both features.

Since conditionally-enabled feature doesn’t work, I suppose the best we can do is rename the feature to no_prefix_on_supported_platforms, or override_malloc_on_supported_platforms? https://github.com/alexcrichton/jemallocator/pull/56

Test case

Cargo.toml

[package]
name = "foo"
version = "0.1.0"

[dependencies]
a = {path = "./a"}

[target.'cfg(windows)'.dependencies]
a = {path = "./a", features = ["b"]}

[target.'cfg(unix)'.dependencies]
a = {path = "./a", features = ["c"]}

src/main.rs

extern crate a;

fn main() {
    a::b();
    a::c();
}

a/src/lib.rs

#[cfg(feature = "b")]
pub fn b() {}

#[cfg(feature = "c")]
pub fn c() {}

a/Cargo.toml

[package]
name = "a"
version = "0.1.0"

[features]
b = []
c = []

[sfackler]

Those feature names seem reasonable to me.