chore(docs): RFC 0015 introduce no_std#1964
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Propose a workspace policy that prefers `no_std + alloc` and uses `std` only where it earns its keep. Motivated by signal safety, smaller artifacts, and tighter dependency hygiene.
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Pull request overview
Adds a new RFC document (0015) proposing an opportunistic workspace policy to prefer no_std + alloc over std where it benefits consumers (signal safety, smaller artifacts, dependency hygiene), with concrete crate conventions, CI enforcement, initial migration candidates, and discussion of drawbacks/alternatives.
Changes:
- Adds
docs/RFCs/0015-no-std.mddescribing the rationale and policy for opportunisticno_stdadoption. - Documents per-crate conventions (feature flags, crate-root attributes, import patterns, dependencies, errors).
- Lists initial migration candidates (e.g.,
libdd-library-config,libdd-crashtrackercollector half) and out-of-scope crates.
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| Concretely, four things make `no_std` attractive for this workspace: | ||
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| 1. **Signal safety by construction.** `core` and `alloc` (with a signal-safe allocator) are made of pure functions, integer math, and stack-allocated data. None of `std`'s mutex, thread-local, environment, file-descriptor, or panic-handler machinery is reachable. Code that runs in async-signal contexts — crashtracker, profiling samplers, anything called from a signal handler — is *much* easier to keep correct when `std::` is simply not in the import graph. The compiler enforces what code review otherwise has to. | ||
| 2. **Smaller artifacts.** Embedders linking libdatadog statically pay for everything `std` pulls in, whether they use it or not. `no_std + alloc` lets us ship the same functionality with substantially less code in the final binary, and noticeably faster compiles in the tree. | ||
| 3. **Frequently, it's a mechanical change.** A surprising amount of "make this `no_std`" work is replacing `std::` with `core::` and adding `extern crate alloc;`. yaml/yaml-serde#8 is a recent example: a near-mechanical patch turned an `std` crate into a `no_std + alloc` crate without changing its API. Many of our internal crates are in the same shape. |
| ## Drawbacks | ||
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| - **Build matrix grows.** Each opted-in crate adds a `--no-default-features` build to CI. Real but bounded. | ||
| - **no_std rust ecosystem is large, but not all crates support it** Code willing to support `no_std` might have to do extra work to align dependencies with `no_std` requirements. |
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| Concretely, four things make `no_std` attractive for this workspace: | ||
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| 1. **Signal safety by construction.** `core` and `alloc` (with a signal-safe allocator) are made of pure functions, integer math, and stack-allocated data. None of `std`'s mutex, thread-local, environment, file-descriptor, or panic-handler machinery is reachable. Code that runs in async-signal contexts — crashtracker, profiling samplers, anything called from a signal handler — is *much* easier to keep correct when `std::` is simply not in the import graph. The compiler enforces what code review otherwise has to. |
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Not really. Just because you don't call signal-unsafe items doesn't make what you do signal safe. Yes, it's certainly easier, but it's not automatic. You still have to worry about reentrancy, atomicity, etc.
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Yeah, I framed it like it solves all problems :D
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| ## The thesis | ||
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| **Prefer `no_std + alloc`. Use `std` only where it is earning its keep.** |
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Why do we want alloc? We also shouldn't panic, and tons of users of alloc panic on allocation failure. What's the motivation for including alloc in the recommendation?
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I just found no_std + alloc is super easy to use and solving all my problems, and you really can have some safe'ish alloc implementations. and
Ideally people could target no_std and add alloc on top if needed.
tl;dr; alloc does make no_std quite bearable
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What does this PR do?
Adds RFC 0015 proposing a workspace policy: prefer
no_std + alloc; usestdonly where it earns its keep.Motivation
no_stdis, for a lot of what libdatadog does, the better Rust.coremakes async-signal-unsafe items unreachable by construction (relevant for crashtracker and profiling samplers), embedders linking statically pay for less code, and once a crate isno_stdevery new dependency comes with a forced "what does this drag in?" review.Many migrations are mechanical (
std::→core::, see yaml/yaml-serde#8). #1770 is the in-flight reference implementation forlibdd-library-config.How to test the change?
Read it. Comment if you disagree.