From f89ba5da4a7f9a6446b91cb621ed000d28529d82 Mon Sep 17 00:00:00 2001 From: Matt Brubeck Date: Tue, 18 Oct 2016 10:56:12 -0700 Subject: [PATCH] Fix some mistakes in HRTB docs The example code for higher-ranked trait bounds on closures had an unnecessary `mut` which was confusing, and the text referred to an mutable reference which does not exist in the code (and isn't needed). Removed the `mut`s and fixed the text to better describe the actual error for the failing example. --- src/doc/book/closures.md | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-) diff --git a/src/doc/book/closures.md b/src/doc/book/closures.md index 3ed85c1a90b..60b99c08825 100644 --- a/src/doc/book/closures.md +++ b/src/doc/book/closures.md @@ -327,7 +327,7 @@ that takes a reference like so: fn call_with_ref(some_closure:F) -> i32 where F: Fn(&i32) -> i32 { - let mut value = 0; + let value = 0; some_closure(&value) } ``` @@ -340,14 +340,15 @@ fn call_with_ref<'a, F>(some_closure:F) -> i32 where F: Fn(&'a i32) -> i32 { ``` -However this presents a problem in our case. When you specify the explicit -lifetime on a function it binds that lifetime to the *entire* scope of the function -instead of just the invocation scope of our closure. This means that the borrow checker -will see a mutable reference in the same lifetime as our immutable reference and fail -to compile. +However, this presents a problem in our case. When a function has an explicit +lifetime parameter, that lifetime must be at least as long as the *entire* +call to that function. The borrow checker will complain that `value` doesn't +live long enough, because it is only in scope after its declaration inside the +function body. -In order to say that we only need the lifetime to be valid for the invocation scope -of the closure we can use Higher-Ranked Trait Bounds with the `for<...>` syntax: +What we need is a closure that can borrow its argument only for its own +invocation scope, not for the outer function's scope. In order to say that, +we can use Higher-Ranked Trait Bounds with the `for<...>` syntax: ```ignore fn call_with_ref(some_closure:F) -> i32 @@ -362,7 +363,7 @@ expect. fn call_with_ref(some_closure:F) -> i32 where F: for<'a> Fn(&'a i32) -> i32 { - let mut value = 0; + let value = 0; some_closure(&value) } ```