From 7baae03c8f145262692a4154a9ae64e3ad637d9e Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon.sapin@exyr.org>
Date: Wed, 20 Apr 2016 16:39:39 +0200
Subject: [PATCH] impl<T> From<T> for ()
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I use the `try!` macro a lot, usually with `()` as the error type
since I don’t care that much about tracking data about errors.
When using some existing function or method that returns `Result`
with a different error type, I find myself using code like this:

```rust
try!(foo.map_err(|_| ()))
```

… which is more verbose than I’d like.

The `try!` macro already uses the `From` trait to convert errors,
some implementations like `impl From<std::num::ParseIntError> for ()`
would help. But rather than adding them one by one to many error types,
a generic solution would be nice.
---
 src/libcore/convert.rs   |  7 +++++++
 src/libcoretest/tuple.rs | 10 ++++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index 2d999868f71ec..d9501de7fe8e2 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -216,6 +216,13 @@ impl<T> From<T> for T {
     fn from(t: T) -> T { t }
 }
 
+// A (the) value of the unit type can be created from any value of any type.
+// This is useful for use with the `try!` macro or the `?` operator.
+#[stable(feature = "unit_from_anything", since = "1.10.0")]
+impl<T> From<T> for () {
+    fn from(t: T) -> () {}
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // CONCRETE IMPLS
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/src/libcoretest/tuple.rs b/src/libcoretest/tuple.rs
index 4fe5e0a740bf7..7bcd0863183ed 100644
--- a/src/libcoretest/tuple.rs
+++ b/src/libcoretest/tuple.rs
@@ -66,3 +66,13 @@ fn test_show() {
     let s = format!("{:?}", (1, "hi", true));
     assert_eq!(s, "(1, \"hi\", true)");
 }
+
+#[test]
+fn test_convert_unit_from_any() {
+    fn io() -> Result<u32, ::std::io::Error> { Ok(1) }
+    fn fmt() -> Result<u32, ::std::fmt::Error> { Ok(2) }
+    fn sum() -> Result<u32, ()> {
+        Ok(io()? + try!(fmt()))
+    }
+    assert_eq!(sum(), Ok(3))
+}