Compatibility with ESP-IDF V5

[ivmarkov]

The new major release of the ESP-IDF (V5) has extended the time_t type from 32 to 64 bits. Unfortunately, this brings the challenge how to simultaneously support older ESP-IDF releases (current stable is V4.4.2) and the V5 one with the libc (and Rust STD) crates.

After dismissing the option to introduce 5 (or more) additional ESP-IDF targets, we settled on the introduction of a rustc cfg flag, as the most lightweight option: espidf_time64:

• This flag needs to be enabled by the user for ESP-IDF V5 (for example, by setting it in the [build] section of .config/cargo.toml or using one of the other methods to pass flags to the Rust compiler);
• This flag should not be set by the user for earlier ESP-IDF versions (we might reverse the logic once ESP-IDF V5 becomes more main-stream in future).

The good news in all that is that it is impossible for the user to set the flag to the wrong value as his/her build will simply fail. This is because compiling for the ESP-IDF framework does require a hard dependency on the esp-idf-sys crate, which - besides exposing autogenerated (with bindgen) APIs which are a super-set of the libc APIs - also does the heavy-lifting of compiling the ESP-IDF framework itself and emitting the relevant Rust linker flags and a lot of other things. So when compiling esp-idf-sys, we are checking the compatibility of the libc's type_t with the bindgen-ed one inside esp-idf-sys where the latter is the source of truth, so to say.

[rust-highfive]

r? @Amanieu

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[JohnTitor]

Makes sense, thanks for the detailed explanation!
@bors r+

[bors]

📌 Commit ba7ff45 has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit ba7ff45 with merge 8531d01...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing 8531d01 to master...