Conditionally compile contracts instead of deciding at run-time

[dawidl022]

The initial implementation of contracts, despite requiring a compiler flag to enable runtime checking of contracts, still compiled contracts into function definitions, even when the compiler flag was disabled. This meant that contracts could not be safely added to functions without breaking optimisations, or even without potentially changing the behaviour of the function. This was blocking the addition of contracts to standard library functions in #136578.

This change guards macro expansion of the built-in contract macros with the contract-checks compiler flag. Additionally, it removes the contract_checks compiler intrinsic that was used to determine whether contract checks should be executed at runtime. Now, when contracts checks are compiled into the body of a function, they will always be executed.

The change is motivated by the following discussion: #144438 (comment)

Contracts tracking issue: #128044

Known limitations:

• When contract-checks are disabled, contracts will not be parsed or type checked, meaning that they are susceptible to becoming out of sync with the rest of the codebase.

[rustbot]

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[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

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[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
fmt: checked 6297 files
tidy check
tidy error: found output file `/checkout/tests/ui/contracts/internal_machinery/contract-lang-items.unchk_fail_post.stderr` for unexpected revision `unchk_fail_post` of test `/checkout/tests/ui/contracts/internal_machinery/contract-lang-items.rs`
tidy error: found output file `/checkout/tests/ui/contracts/internal_machinery/contract-lang-items.unchk_pass.stderr` for unexpected revision `unchk_pass` of test `/checkout/tests/ui/contracts/internal_machinery/contract-lang-items.rs`
Checking tidy rustdoc_json...
No error code explanation was removed!
tidy: Skipping binary file check, read-only filesystem
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
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added 291 packages in 35s
Running eslint on rustdoc JS files
info: ES-Check: there were no ES version matching errors!  🎉
some tidy checks failed
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1634:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1239:29

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[saethlin]

This was blocking the addition of contracts to standard library functions in #136578.

Hunh? All I can see in that PR is contract checks changing the optimized MIR of functions which is expected and should be fine. Disabled contract check MIR should be cleaned out as part of lowering to LLVM, or it should be immediately cleaned up by LLVM because it is under the equivalent of an if false.