A reproducible build means the following (as quoted from the reproducible builds project site):
Reproducible builds are a set of software development practices that create a verifiable path from human readable source code to the binary code used by computers.
Roughly what this means is that if two different people compile the project from source, their outputs are bitwise identical to each other. This allows people to verify that binaries downloadable from the net actually come from the corresponding sources and have not, for example, had malware added to them.
Meson aims to support reproducible builds out of the box with zero additional work (assuming the rest of the build environment is set up for reproducibility). If you ever find a case where this is not happening, it is a bug. Please file an issue with as much information as possible and we'll get it fixed.
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