dead_code lint does not trigger for tuple structs with unread fields

@FabianWolff

This code:

fn main() {
    let _ = Example { u: 0 };
}

struct Example {
    u: u8,
}

Produces this warning:

warning: field is never read: `u`
 --> src/main.rs:6:5
  |
6 |     u: u8,
  |     ^^^^^
  |

However, switching to a tuple struct:

fn main() {
    let _ = Example(0);
}

struct Example(u8);

Does not report a warning.

Original report misleadingly focusing on Debug

fn main() {
    dbg!(Example { a: 0 });
}

#[derive(Debug)]
struct Example {
    a: u8,
}

produces

warning: field is never read: `a`
 --> src/main.rs:7:5
  |
7 |     a: u8,
  |     ^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

However, switching to a tuple struct:

fn main() {
    dbg!(Example(0));
}

#[derive(Debug)]
struct Example(u8);

does not produce the warning.

/cc @FabianWolff
/cc #85200; #84647; #88900

Meta

Rustc 1.57.0 and 1.60.0-nightly (2022-01-10 89b9f7b)

This has nothing to do with the #[derive(Debug)]:

fn main() {
    let _ = Example { u: 0 };
}

struct Example {
    u: u8,
}

warning: field is never read: `u`
 --> src/main.rs:6:5
  |
6 |     u: u8,
  |     ^^^^^
  |

but

fn main() {
    let _ = Example(0);
}

struct Example(u8);

gives no warning.

@FabianWolff

@FabianWolff oh dear. Thanks! I've updated the title and original post; sorry for the false ping.

Not a regression AFAIK, but seems like worth prioritizing.

Not a regression AFAIK

I agree. I did a quick test across Rust 1.{0,10,20,30,40,50}.0 and none of them reported the lint.

Thanks for checking!

@rustbot

This behavior does indeed date back to the very beginning of the detection of dead struct fields (commit 0271224 in 2014). The culprit is line 621 in dead.rs:

rust/compiler/rustc_passes/src/dead.rs

Lines 618 to 625 in 72e74d7

    
           fn should_warn_about_field(&mut self, field: &hir::FieldDef<'_>) -> bool { 
        
               let def_id = self.tcx.hir().local_def_id(field.hir_id); 
        
               let field_type = self.tcx.type_of(def_id); 
        
               !field.is_positional() 
        
                   && !self.symbol_is_live(def_id) 
        
                   && !field_type.is_phantom_data() 
        
                   && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id) 
        
           }

Commenting out this line with the above tuple struct example gives:

warning: field is never read: `0`
 --> s2.rs:6:16
  |
6 | struct Example(u8);
  |                ^^
  |

I can open a PR for this, or is there a reason anyone here can think of why dead tuple struct fields shouldn't get a warning?

@rustbot claim

I can't think of one, so opening a PR seems good.

FWIW, I find the example with dbg! to be surprising: dbg! is debug-printing a struct, and thus reading its fields

@danielhenrymantilla

@danielhenrymantilla see #85200; #84647; #88900

@rustbot

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium

is there a reason anyone here can think of why dead tuple struct fields shouldn't get a warning?

One thought is that deleting a dead tuple field may be non-trivial, if it is not the last field. E.g., imagine if someone got:

struct DeadLive(u8,u8);
...
warning: field is never read: `0`

You can't simply delete the dead field. You have to also renumber all the uses of .1, which might be a non-trivial and error prone task. So the cost v. benefit tradeoff of having a warning is different to the named-field case.

Rollup merge of rust-lang#95977 - FabianWolff:issue-92790-dead-tuple,…

Auto merge of rust-lang#95977 - FabianWolff:issue-92790-dead-tuple, r…

@FabianWolff

Thank you @FabianWolff !

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dead_code lint does not trigger for tuple structs with unread fields #92790

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	fn should_warn_about_field(&mut self, field: &hir::FieldDef<'_>) -> bool {
	let def_id = self.tcx.hir().local_def_id(field.hir_id);
	let field_type = self.tcx.type_of(def_id);
	!field.is_positional()
	&& !self.symbol_is_live(def_id)
	&& !field_type.is_phantom_data()
	&& !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id)
	}

dead_code lint does not trigger for tuple structs with unread fields #92790

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FabianWolff commented on Jan 11, 2022

shepmaster commented on Jan 11, 2022

camelid commented on Jan 12, 2022

shepmaster commented on Jan 12, 2022

camelid commented on Jan 12, 2022

FabianWolff commented on Jan 12, 2022

camelid commented on Jan 12, 2022

danielhenrymantilla commented on Jan 12, 2022

shepmaster commented on Jan 12, 2022

apiraino commented on Jan 13, 2022

rcls commented on Feb 21, 2022

shepmaster commented on Aug 5, 2022

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