Tracking issue for RFC 2342, "Allow `if` and `match` in constants"

[Centril]

This is a tracking issue for the RFC "Allow if and match in constants" (rust-lang/rfcs#2342).

Please redirect constification of specific functions or issues you want to report to fresh issues and label them appropriately with F-const_if_match so that this issues doesn't get flooded with ephemeral comments obscuring important developments.

Steps:

• Implement the RFC
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge): Stabilize #![feature(const_if_match)] and #![feature(const_loop)] #72437
• let bindings in constants that use && and || short circuiting operations. These are treated as & and | inside const and static items right now.

Unresolved questions:

None

[oli-obk]

1. add a feature gate for it
2. switch and switchInt terminators in https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs#L347 need to have custom code in case the feature gate is active
3. instead of having a single current basic block (https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs#L328) this needs to be some container that has a list of basic blocks it still has to process.

[eddyb]

@oli-obk It's a bit trickier because the complex control-flow means dataflow analysis needs to be employed. I need to get back to @alexreg and figure out how to integrate their changes.

[alexreg]

@eddyb A good starting point would probably be to take my const-qualif branch (minus the top commit), rebase it over master (not going to be fun), and then add data annotation stuff, right?

[mark-i-m]

Any news on this?

[alexreg]

@mark-i-m Alas no. I think @eddyb has been very busy indeed, because I've not even been able to ping him on IRC for the last few weeks hah. Sadly my const-qualif branch doesn't even compile since I last rebased it over master. (I don't believe I've pushed yet though.)

thread 'main' panicked at 'assertion failed: position <= slice.len()', libserialize/leb128.rs:97:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Could not compile `rustc_llvm`.

Caused by:
  process didn't exit successfully: `/Users/alex/Software/rust/build/bootstrap/debug/rustc --crate-name build_script_build librustc_llvm/build.rs --error-format json --crate-type bin --emit=dep-info,link -C opt-level=2 -C metadata=74f2a810ad96be1d -C extra-filename=-74f2a810ad96be1d --out-dir /Users/alex/Software/rust/build/x86_64-apple-darwin/stage1-rustc/release/build/rustc_llvm-74f2a810ad96be1d -L dependency=/Users/alex/Software/rust/build/x86_64-apple-darwin/stage1-rustc/release/deps --extern build_helper=/Users/alex/Software/rust/build/x86_64-apple-darwin/stage1-rustc/release/deps/libbuild_helper-89aaac40d3077cd7.rlib --extern cc=/Users/alex/Software/rust/build/x86_64-apple-darwin/stage1-rustc/release/deps/libcc-ead7d4af4a69e776.rlib` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: build failed
command did not execute successfully: "/Users/alex/Software/rust/build/x86_64-apple-darwin/stage0/bin/cargo" "build" "--target" "x86_64-apple-darwin" "-j" "8" "--release" "--manifest-path" "/Users/alex/Software/rust/src/librustc_trans/Cargo.toml" "--features" " jemalloc" "--message-format" "json"
expected success, got: exit code: 101
thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1085:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failed to run: /Users/alex/Software/rust/build/bootstrap/debug/bootstrap -i build

[alexreg]

Okay, funnily enough, I rebased again just today and it seems to be building all fine now! Looks like there was a regression, and it just got fixed. All over to @eddyb now.

[eddyb]

@alexreg Sorry, I've switched to a local sleep schedule and I see you've pinged me when I wake up but then you're offline all day when I'm awake (ugh timezones).
Should I just make a PR out of your branch? I forgot what we were supposed to do with it?

[alexreg]

@eddyb That's alright heh. You must be off to bed early, since I'm usually on from 8:00PM GMT, but it's all good! :-)