Add docs.rs post about fewer targets

posts/inside-rust/2020-05-27-docs-rs-building-fewer-targets.md

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+---
+layout: post
+title: "docs.rs will start building fewer targets"
+author: Joshua Nelson
+team: the docs.rs team <https://www.rust-lang.org/governance/teams/dev-tools#docs-rs>
+---
+
+Starting soon, docs.rs will by default only build one target for crates published to crates.io.
+This is a breaking change, and if you own a crate that is different on different platforms,
+you should consider specifying your targets explicitly.
+
+## What does this change?
+
+Currently, docs.rs builds documentation for all [tier 1 platforms][platform support] by default.
+After this change, docs.rs will only build one platform by default.
+Usually the platform is `x86_64-unknown-linux-gnu`, but this can be configured.
+This only changes the default, you can still opt-in to more targets if you choose.
+
+## Why is this change being made?
+
+Building all 5 tier-one targets means that builds take 5x as long to finish.
+Additionally, it means that docs.rs stores 5x as much documentation, increasing our fixed costs.
+For most crates, the documentation is the same on every platform, so there is no need to build it many times.
+
+## Who will this affect?
+
+This will affect Rust developers who maintain or use a crate which is different on different platforms
+(i.e. uses [conditional compilation]) and also does not explicitly specify the targets it should be built on.
+For example, [winapi](https://docs.rs/winapi/0.3.8/) only has documentation on windows platforms,
+so it needs to specify all windows targets in order for them to show up.
+
+As a quick rule of thumb, if you go to a docs.rs page that looks like `docs.rs/:crate/:version/:target`,
+where `:target` is a tier 1 platform, that page may be affected.
+
+## Will old documentation be deleted?
+
+No, any existing documentation for different targets will remain up.
+There is no plan to delete docs.rs documentation that has already been built.
+
+## How do I make sure my crate doesn't break?
+
+If you are a maintainer of a crate that uses conditional compilation,
+you can explicitly specify your targets in `Cargo.toml` like so:
+
+```toml
+[package.metadata.docs.rs]
+# replace these with the targets you actually want to build
+targets = ["x86_64-unknown-linux-gnu", "x86_64-pc-windows-msvc"]
+```
+
+Any target supported by `rustup add target` is also supported by docs.rs.
+See [the metadata documentation](https://docs.rs/about#metadata) for more details.
+
+If you develop but do not maintain a crate that uses conditional compilation,
+you can open an issue or PR to add these targets.
+
+## When is this change being made?
+
+There is no set time frame yet.
+We can guarantee at least a month (July 1st) before we will implement the change.
+
+## How can I learn more?
+
+More information on targets is available in the [platform support] page.
+You can learn more about the change in [the issue proposing it][docs.rs#343] and [the PR with the implementation][docs.rs#532].
+If you want to discuss the change with the docs.rs team,
+feel free to join the [docs.rs discord channel] or [open an issue][issue link].
+
+
+[platform support]: https://forge.rust-lang.org/release/platform-support.html
+[docs.rs#343]: https://github.com/rust-lang/docs.rs/issues/343
+[docs.rs#532]: https://github.com/rust-lang/docs.rs/issues/532
+[docs.rs discord channel]: https://discord.gg/f7mTXPW
+[conditional compilation]: https://doc.rust-lang.org/reference/conditional-compilation.html#forms-of-conditional-compilation
+[issue link]: (https://github.com/rust-lang/docs.rs/issues/new?title=Changing%20default%20targets)