Move the HIR CFG to rustc_ast_borrowck

[matthewjasper]

No new code should be using it.

[rust-highfive]

r? @zackmdavis

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[Centril]

This is great! 🎉 -- Cannot wait to ditch ::cfg.

r? @Centril @bors r+

[bors]

📌 Commit 10f46b6 has been approved by Centril

[llogiq]

So...how do we access librustc_ast_borrowck from clippy? Changing the use statement in our cognitive_complexity lint to use librustc_ast_borrowck::cfg::CFG doesn't work because cargo doesn't find librustc_ast_borrowck.

[tesuji]

Probably just add extern crate rustc_ast_borrowck; in
https://github.com/rust-lang/rust-clippy/blob/f30bf69ec7af4366e64916f32566f3a894edd593/clippy_lints/src/lib.rs#L22-L38
I tried this earlier but I cannot sort out this error:

% cargo check --all --all-targets --                                                                                                                                                                                                                                      
   Compiling clippy-mini-macro-test v0.2.0 (/home/lzutao/forked/rust/clippy/mini-macro)                                                                                                                                                                                         
   Compiling clippy_lints v0.0.212 (/home/lzutao/forked/rust/clippy/clippy_lints)                                                                                                                                                                                               
error[E0616]: field `graph` of struct `rustc_ast_borrowck::cfg::CFG` is private                                                                                                                                                                                                 
  --> clippy_lints/src/cognitive_complexity.rs:51:17                                                                                                                                                                                                                            
   |                                                                                                                                                                                                                                                                            
51 |         let n = cfg.graph.len_nodes() as u64;                                                                                                                                                                                                                              
   |                 ^^^^^^^^^                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                                                                
error[E0616]: field `graph` of struct `rustc_ast_borrowck::cfg::CFG` is private                                                                                                                                                                                                 
  --> clippy_lints/src/cognitive_complexity.rs:52:17                                                                                                                                                                                                                            
   |                                                                                                                                                                                                                                                                            
52 |         let e = cfg.graph.len_edges() as u64;                                                                                                                                                                                                                              
   |                 ^^^^^^^^^                                                                                                                                                                                                                                                  

[matthewjasper]

I'll open a PR to remove the use of CFG from clippy, it's not going to be around for that much longer.