TRPL: (assignment == empty tuple), please elaborate

[mcast]

Hi,

I really like TRPL and have found it answers most of my questions in the footnotes.
Looking at https://github.com/rust-lang/rust/blame/stable/src/doc/trpl/functions.md#L144-L153

Note that assigning to an already-bound variable (e.g. y = 5) is still an expression,
although its value is not particularly useful. Unlike other languages where an assignment
evaluates to the assigned value (e.g. 5 in the previous example), in Rust the value of an
assignment is an empty tuple ():

let mut y = 5;
 
let x = (y = 6);  // x has the value `()`, not `6`

I guessed this difference would be to discourage "clever" coding..? It would be nice to have an explanatory link to read.

Thanks,

Matthew

---

ps. research done... I could find only https://internals.rust-lang.org/t/allow-loops-to-return-values-other-than/567 on loop return values.

[tbu-]

Since you can't return the new value of y (it would need to be cloned), which value could you return? The only possibility would be the old value of y, but that would probably be too surprising.

[bluss]

mcast, this is the impact of ownership semantics. For general types, you can't give the same value to two different owners (both y and x).

[mcast]

Thanks @tbu- @bluss, that makes sense now. Would it be right to add a forward reference?

diff --git a/src/doc/trpl/functions.md b/src/doc/trpl/functions.md
--- a/src/doc/trpl/functions.md
+++ b/src/doc/trpl/functions.md
@@ -146,3 +146,5 @@
  expression, although its value is not particularly useful. Unlike other
  languages where an assignment evaluates to the assigned value (e.g. `5` in the
- previous example), in Rust the value of an assignment is an empty tuple `()`:
+ previous example), in Rust the value of an assignment is an empty tuple `()`
+ because the value can have (just one owner)[ownership.html], and any other
+ value would be too surprising:

(diff cooked manually, hope it fits)

[steveklabnik]

@mcast yes, I would accept that as a PR. :)

[mcast]

Thanks for the merge, and sorry about the mistakes.