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nagisa opened this issue Jan 6, 2016 · 4 comments
Open

DebugTuple builder needs more options controlling its behaviour #1448

nagisa opened this issue Jan 6, 2016 · 4 comments
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T-libs-api Relevant to the library API team, which will review and decide on the RFC.

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@nagisa
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nagisa commented Jan 6, 2016

DebugTuple should have options controlling how to output tuples with 0 and 1 element.

Currently if 0 fields are provided, only the name passed into debug_tuple is printed. Would be nice if there was option to print {name}().

Currently if only 1 field is provided, trailing comma is not printed. There should be an option to add one: (value,).

These could be added as option setters on the DebugTuple builder. E.g. fn empty_tuple(bool) and fn trailing_comma(bool).

That would make debug_tuple actually usable to print tuples.
@nagisa
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nagisa commented Jan 6, 2016

cc @tsion

@solson
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solson commented Jan 6, 2016

This explanation might be redundant, but I see two separate issues:

  • A named tuple with no elements prints Name rather than Name(). Admittedly, the latter doesn't currently exist in Rust, but it might be added later like struct Name {} was, and the former Name isn't a tuple struct at all. This might be considered an actual bug.
  • An un-named tuple with 0 or 1 elements prints nothing or (elem) (with no trailing comma), respectively. This is basically a feature request to make debug_tuple work for anonymous tuples.

@sfackler
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sfackler commented Jan 6, 2016

Tuples already have debug implementations - what external implementations would exist?

@nagisa
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nagisa commented Jan 6, 2016

@sfackler we’re trying to use in pretty-printing MIR representation for tuple aggregates. That’s at least one use case.

@solson solson mentioned this issue Jan 6, 2016
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bors added a commit to rust-lang/rust that referenced this issue Jan 7, 2016
@bors
Auto merge of #30748 - tsion:mir-tuple-fix, r=eddyb
5c92010 
r? @nikomatsakis

(Related issue about `debug_tuple` at rust-lang/rfcs#1448.)

```rust

fn main() {
    let _x = ();
}
```

```diff
--- empty_tuple-old.mir	2016-01-06 16:04:24.206409186 -0600
+++ empty_tuple-new.mir	2016-01-06 14:26:17.324888585 -0600
@@ -1,13 +1,13 @@
 fn() -> () {
     let var0: (); // _x
     let mut tmp0: ();

     bb0: {
-        var0 = ;
+        var0 = ();
         Some(goto -> bb1);
     }

     bb1: {
         Some(return);
     }
 }
```
@nrc nrc added the T-libs-api Relevant to the library API team, which will review and decide on the RFC. label Aug 18, 2016
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@solson @nagisa @nrc @sfackler