What matches

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A dependency is a candidate when it comes from Hex (a git or path dependency never is), is not Nerves tooling (nerves, its own dependencies, nerves_system_*, nerves_toolchain_*, nerves_bootstrap, precompiled itself) and is not in :except. A build of a candidate is installed only when every rule holds:

  1. Target. The build is pure_elixir (it serves every system and plain Elixir), or it was made for this project's Nerves system at the exact system version in mix.lock.
  2. Elixir. Same major and minor as the running Elixir, and the running version is at least the one the build was made with. Elixir code compiled on 1.20.1 runs on 1.20.3, not the other way round; versions compare as versions, so a 1.21.0 build is not served to 1.21.0-rc.0, while a 1.21.0-rc.0 build is served to 1.21.0.
  3. OTP. The same OTP major. What a build inlines at compile time from OTP headers (record layouts) can change between majors.
  4. Dependency env. The build was compiled in the env the dependency compiles in here: prod, unless the project's deps entry overrides it with env:.
  5. Checksum. The build's package checksum equals the dependency's checksum in mix.lock.
  6. Dependencies. Every dependency the build was compiled against is in mix.lock at the same version and checksum, and every dependency the package declares (optional ones included) that the lock has was in the build.
  7. compile_env. Every Application.compile_env value the build read equals the project's config for that key.
  8. Compile-time inputs. A dependency whose own sources read Mix.target() at compile time (module level in lib/, anywhere in mix.exs) gets no pure_elixir build on a device target. It gets a system build only when the build was made under the project's target name: the recorded one, or else the system's conventional one (nerves_system_rpi4 → rpi4). A dependency that reads an environment variable at module level in lib/ never matches, nor does one whose sources cannot be scanned (they do not parse, or are over 2 MB a file or 20 MB a package). Not covered: environment variables read in a dependency's mix.exs, and Application.get_env in a module body (Elixir warns about it). Builds record neither, so such a dependency can be served a build made with other values.

When several builds pass, the one already installed wins (its bytes stay as they are), then the one made on the running Elixir patch, the closest older patch, pure_elixir before a system build, and the newest run. When none passes, the line gives the rule the build that got furthest broke, for example MISS built with Elixir 1.20.3, running 1.20.1.