From cd4633917d4dd446f9f37efbf750bbd4ed2b46d6 Mon Sep 17 00:00:00 2001 From: Guillaume Gomez Date: Wed, 5 Aug 2020 14:14:21 +0200 Subject: [PATCH] Clean up E0746 explanation --- src/librustc_error_codes/error_codes/E0746.md | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/src/librustc_error_codes/error_codes/E0746.md b/src/librustc_error_codes/error_codes/E0746.md index 305667e58f8..90755d47f67 100644 --- a/src/librustc_error_codes/error_codes/E0746.md +++ b/src/librustc_error_codes/error_codes/E0746.md @@ -1,4 +1,4 @@ -Return types cannot be `dyn Trait`s as they must be `Sized`. +An unboxed trait object was used as a return value. Erroneous code example: @@ -13,11 +13,13 @@ impl T for S { // Having the trait `T` as return type is invalid because // unboxed trait objects do not have a statically known size: -fn foo() -> dyn T { +fn foo() -> dyn T { // error! S(42) } ``` +Return types cannot be `dyn Trait`s as they must be `Sized`. + To avoid the error there are a couple of options. If there is a single type involved, you can use [`impl Trait`]: @@ -32,7 +34,7 @@ If there is a single type involved, you can use [`impl Trait`]: # } // The compiler will select `S(usize)` as the materialized return type of this // function, but callers will only know that the return type implements `T`. -fn foo() -> impl T { +fn foo() -> impl T { // ok! S(42) } ``` @@ -57,7 +59,7 @@ impl T for O { // This now returns a "trait object" and callers are only be able to access // associated items from `T`. -fn foo(x: bool) -> Box { +fn foo(x: bool) -> Box { // ok! if x { Box::new(S(42)) } else { -- 2.44.0