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document guidelines for which shims have a place in Miri #4767

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14 changes: 14 additions & 0 deletions CONTRIBUTING.md
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Expand Up @@ -66,6 +66,20 @@ process for such contributions:
This process is largely informal, and its primary goal is to more clearly communicate expectations.
Please get in touch with us if you have any questions!

## Scope of Miri shims

Miri has "shims" to implement functionality that is usually implemented in C libraries which are
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@saethlin saethlin Jan 13, 2026

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Miri also has shims for various SIMD intrinsics, or would you not call those shims?

invoked from Rust code, such as opening files or spawning threads. However, the set of C functions
that Rust code invokes this way is enormous, and for obvious reasons we have no intention of
implementing every C API ever in Miri.
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@saethlin saethlin Jan 13, 2026

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implementing every C API ever in Miri.
implementing every C API ever written in Miri.

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@saethlin saethlin Jan 13, 2026

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"every C API ever" is fine in informal spoken English, but I think it seems weirdly incomplete in such documentation.


At the moment, the general guideline for "could this function have a shim in Miri" is: if the
function can be implemented in a portable way using just what is provided by the Rust standard
library, then yes. Otherwise, we might make exceptions for certain cases if (a) there is a good case
for why Miri should support those APIs, and (b) robust and widely-used portable libraries exist in
the Rust ecosystem. We will generally not add shims to Miri that would require Miri to directly
interact with platform-specific APIs (such as `libc` or `windows-sys`).

## Preparing the build environment

Miri heavily relies on internal and unstable rustc interfaces to execute MIR,
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