Show unused code from multi-crate Rust projects

The released version 0.1.0 does not compile now:

error[E0432]: unresolved imports `syn::visit`, `syn::visit`
  --> /home/efanzh/.cargo/registry/src/github.com-1ecc6299db9ec823/warnalyzer-0.1.0/src/mute.rs:88:11
   |
88 |     use syn::visit::{visit_item, self};
   |              ^^^^^               ^^^^ no `visit` in the root
   |              |
   |              could not find `visit` in `syn`

error[E0433]: failed to resolve: could not find `visit` in `syn`
   --> /home/efanzh/.cargo/registry/src/github.com-1ecc6299db9ec823/warnalyzer-0.1.0/src/mute.rs:105:22
    |
105 |     impl<'ast, 'a> syn::visit::Visit<'ast> for Visitor<'a> {
    |                         ^^^^^ could not find `visit` in `syn`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: failed to compile `warnalyzer v0.1.0`, intermediate artifacts can be found at `/tmp/cargo-installCYhgjx`

Caused by:
  could not compile `warnalyzer`

To learn more, run the command again with --verbose.

trait Abc {
    fn a();
    fn b();
}

struct Bcd;

impl Abc for Bcd {
    fn a() {}
    fn b() {} // considered unused even though it is required
}

fn main() {
    Bcd::a();
}

save-analysis is going away. For the short to mid term future, we can pin nightlies but with increased MSRV requirements, this will cause issues.

Some more info on it in this reddit thread.

Private items are already warned about by rustc's dead_code lint, and because of the bugs around macros, almost all warnings about private code are false positives.

Right now, warnalyzer regards the main function of a program as unused. It would be great if it could somehow recognize that something is an entry point and record it as used.

Probably most of these are because -Z save-analysis is buggy, but I don't know warnalyzer well enough to tell myself. Also, I want you to know this tool has been super useful :heart: and I'm going to make a PR removing dead code shortly; it just happens to also be very buggy at the same time :laughing: I'm going to put them all in one issue to avoid spamming your bug tracker, but happy to separate them out if you think it's useful.

You should be able to reproduce with RUSTFLAGS_BOOTSTRAP="-Z save-analysis" x.py check on https://github.com/rust-lang/rust/commit/2b5c78488e1a9bbc27ecf96d351e880a808077a0.

• compiler/rustc/src/main.rs:1:4: unused Function 'main' - clearly wrong, this is the entrypoint of the compiler
• compiler/rustc/src/main.rs:1:1: unused Mod '' - I think this is trying to warn about the crate root? there's at least one bug here
• compiler/rustc_ast/src/ast_like.rs:114:8: unused Method 'visit' - this is a trait function and can't be used. There are some implementations of the trait which are used.
• compiler/rustc_apfloat/src/ieee.rs:166:5: unused Type 'Single' - this is defined in a macro, which you mentioned in the readme is buggy.
• compiler/rustc_apfloat/src/lib.rs:64:10: unused Type 'Copy' - this is a derived trait, not a type.
• compiler/rustc_apfloat/src/lib.rs:576:14: unused Type '$t' - this should show what $t is, since the macro could be called multiple times.
• compiler/rustc_ast/src/ast.rs:163:20: unused Field '0' - this is for GenericArgs::AngleBracketed(AngleBracketedArgs). Not sure what's going on here, these are used quite a lot.
• compiler/rustc_codegen_ssa/src/common.rs:109:5: unused Mod 'temp_stable_hash_impls' - the impl in this mod is used, even though it has no other items.
• compiler/rustc_hir/src/hir.rs:3040:12: unused Method 'hir_id' - this is used outside of a macro, in compiler/rustc_middle/src/hir/map/mod.rs:304:28.
• compiler/rustc_hir/src/definitions.rs:94:12: unused Method 'enumerated_keys_and_path_hashes' - this is used in compiler/rustc_metadata/src/rmeta/encoder.rs:462:62
• compiler/rustc_middle/src/mir/mod.rs:2472:12: unused Method 'projections_and_spans' - this is used in compiler/rustc_middle/src/mir/mod.rs:2472:12: unused Method 'projections_and_spans':403

There are a bunch more macro issues, but I didn't report them all since you say in the readme they're imperfectly supported.

https://github.com/rust-analyzer/rust-analyzer

API docs: https://rust-analyzer.github.io/rust-analyzer/ra_ide_api/index.html

Not sure whether it's up to the task yet.