Add a dead code elimination pass #1749
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as per IRC conversation w/ nmatsakis: The only trick is that many functions are "accidentally" exported due to the "export all by default" rule. One solution is to warn anyway if a function in a module with no export decls is unused (within the crate). Adding explicit export decls suppresses the warning. |
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See also #1702 |
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We've discussed in the past inverting the "everything exported by default" rule at crate top-level. That is: only export explicit stuff for linking. The intra-crate is intended to make code less chatty and bureaucratic, but linking is often the appropriate time for bureaucracy. Also note: if you have an explicit export list from a crate, you have a lot fewer roots to run your dead-code analysis from. |
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Somewhat related is #2953 |
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Visiting for triage. There have been lots of changes to privacy and similar, but I think this is still true. |
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I'm thinking of working on this for a class project but I'm unfamiliar with the internal of rustc. tjc has told me to focus on adding warning for now, and then work on eliminating dead code if I still have time, so I have been looking inside librustc/middle to figure out what to do. Since this issue is about adding a new pass and (I presume) the logic is similar to the liveness analysis in librustc/middle/liveness.rs, does that mean I'm supposed to create a new file in librustc/middle/ (say foo.rs) with a top-facing pub fn check_crate that does something similar to liveness.rs, and then call middle::foo::check_crate inside librustc/driver/driver.rs? Also, is there difficulty in checking for unused non-exported functions inside the existing liveness.rs rather than doing a separate pass? (sorry if the above are stupid questions) |
PR for issue #1749 mainly to get some feedback and suggestion. This adds a pass that warns if a function, struct, enum, or static item is never used. For the following code, ```rust pub static pub_static: int = 0; static priv_static: int = 0; static used_static: int = 0; pub fn pub_fn() { used_fn(); } fn priv_fn() { let unused_struct = PrivStruct; } fn used_fn() {} pub struct PubStruct(); struct PrivStruct(); struct UsedStruct1 { x: int } struct UsedStruct2(int); struct UsedStruct3(); pub enum pub_enum { foo1, bar1 } enum priv_enum { foo2, bar2 } enum used_enum { foo3, bar3 } fn foo() { bar(); let unused_enum = foo2; } fn bar() { foo(); } fn main() { let used_struct1 = UsedStruct1 { x: 1 }; let used_struct2 = UsedStruct2(1); let used_struct3 = UsedStruct3; let t = used_static; let e = foo3; } ``` it would add the following warnings: ```rust /home/ktt3ja/test.rs:2:0: 2:28 warning: code is never used: `priv_static`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:2 static priv_static: int = 0; ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ /home/ktt3ja/test.rs:6:0: 6:48 warning: code is never used: `priv_fn`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:6 fn priv_fn() { let unused_struct = PrivStruct; } ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /home/ktt3ja/test.rs:10:0: 10:20 warning: code is never used: `PrivStruct`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:10 struct PrivStruct(); ^~~~~~~~~~~~~~~~~~~~ /home/ktt3ja/test.rs:16:0: 16:29 warning: code is never used: `priv_enum`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:16 enum priv_enum { foo2, bar2 } ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /home/ktt3ja/test.rs:19:0: 22:1 warning: code is never used: `foo`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:19 fn foo() { /home/ktt3ja/test.rs:20 bar(); /home/ktt3ja/test.rs:21 let unused_enum = foo2; /home/ktt3ja/test.rs:22 } /home/ktt3ja/test.rs:24:0: 26:1 warning: code is never used: `bar`, #[warn(dead_code)] on by default /home/ktt3ja/test.rs:24 fn bar() { /home/ktt3ja/test.rs:25 foo(); /home/ktt3ja/test.rs:26 } ``` Furthermore, I would like to solicit some test cases since I haven't tested extensively and I'm still unclear about some of the things in here. For example, I'm not sure how reexports would affect this and just assumed that LiveContext (which is a copy of reachable::ReachableContext) does enough work to handle it. Also, the test case above doesn't include any impl or methods, etc.
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Can be closed? |
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It can indeed, thanks @ktt3ja! |
Add a pass that warns about non-exported functions that are never called.
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