let chains testing

[est31]

The appearance of #104843 has shown that bugs might still be in hiding for let chains. In order to make the implementation of let chains in rustc more robust, it would be useful to increase the number of places let chains are tested in.

There is an automatted tool to convert uses of the if_chain crate to let chains: https://github.com/Alexendoo/if-to-let-chain

One ways to increase testing would be to run the tool on some of the users of if_chain, e.g. clippy. Maybe this brings up new bugs like #104843.

@rustbot label F-let_chains

[est31]

I ran if-to-let-chain on clippy. The diff was huge: 244 files changed, 8078 insertions(+), 8994 deletions(-). Then I built it with RUSTFLAGs="-Z validate=mir" cargo build --release. There were zero ICEs.

There are more big projects that use if_chain however. Next step is to make a list I guess?

[est31]

deno_lint commit 15729fb04d9fee7c0f68f85f2477f6a2e3feac85 diff 13 files changed, 115 insertions(+), 140 deletions(-). There was one error I had to fix which was because let chains don't have : Type notation. Outside of that, RUSTFLAGs="-Z validate=mir" cargo build --release gave no errors or ICEs.

[est31]

There is rust-lang/rust-clippy#11750 which moved clippy to use let chains now on the master branch.