Why is a type annotation needed on a function that declares more generic types than it uses?
fn lifetime_tester<A, B, C>(a: Box<A>, b: Box<B>, c: &i32) -> &i32 {
c
}
fn main() {
let a = Box::new(String::from("Test1"));
let b = Box::new(55 as i32);
let c: i32;
{
c = 34 as i32;
}
println!("{}", lifetime_tester(a, b, &c));
}
Error:
error[E0282]: type annotations needed
--> src/main.rs:12:20
|
12 | println!("{}", lifetime_tester(a, b, &c));
| ^^^^^^^^^^^^^^^ cannot infer type for `C`
I am baffled about where a "type annotation" needs to go. I've specified a return type as &i32, I've specified an argument type of &i32 and I've also specified that c is an i32.
rust
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
add a comment |
fn lifetime_tester<A, B, C>(a: Box<A>, b: Box<B>, c: &i32) -> &i32 {
c
}
fn main() {
let a = Box::new(String::from("Test1"));
let b = Box::new(55 as i32);
let c: i32;
{
c = 34 as i32;
}
println!("{}", lifetime_tester(a, b, &c));
}
Error:
error[E0282]: type annotations needed
--> src/main.rs:12:20
|
12 | println!("{}", lifetime_tester(a, b, &c));
| ^^^^^^^^^^^^^^^ cannot infer type for `C`
I am baffled about where a "type annotation" needs to go. I've specified a return type as &i32, I've specified an argument type of &i32 and I've also specified that c is an i32.
rust
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
8
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
4
There is no link betweenCandc, your Minimal, Complete, and Verifiable example don't make much sense.
– Stargateur
Nov 13 '18 at 5:31
add a comment |
fn lifetime_tester<A, B, C>(a: Box<A>, b: Box<B>, c: &i32) -> &i32 {
c
}
fn main() {
let a = Box::new(String::from("Test1"));
let b = Box::new(55 as i32);
let c: i32;
{
c = 34 as i32;
}
println!("{}", lifetime_tester(a, b, &c));
}
Error:
error[E0282]: type annotations needed
--> src/main.rs:12:20
|
12 | println!("{}", lifetime_tester(a, b, &c));
| ^^^^^^^^^^^^^^^ cannot infer type for `C`
I am baffled about where a "type annotation" needs to go. I've specified a return type as &i32, I've specified an argument type of &i32 and I've also specified that c is an i32.
rust
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
fn lifetime_tester<A, B, C>(a: Box<A>, b: Box<B>, c: &i32) -> &i32 {
c
}
fn main() {
let a = Box::new(String::from("Test1"));
let b = Box::new(55 as i32);
let c: i32;
{
c = 34 as i32;
}
println!("{}", lifetime_tester(a, b, &c));
}
Error:
error[E0282]: type annotations needed
--> src/main.rs:12:20
|
12 | println!("{}", lifetime_tester(a, b, &c));
| ^^^^^^^^^^^^^^^ cannot infer type for `C`
I am baffled about where a "type annotation" needs to go. I've specified a return type as &i32, I've specified an argument type of &i32 and I've also specified that c is an i32.
rust
rust
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
edited Nov 15 '18 at 15:20
Shepmaster
151k13293432
151k13293432
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
asked Nov 13 '18 at 3:51
the_endianthe_endian
545621
545621
8
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
4
There is no link betweenCandc, your Minimal, Complete, and Verifiable example don't make much sense.
– Stargateur
Nov 13 '18 at 5:31
add a comment |
8
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
4
There is no link betweenCandc, your Minimal, Complete, and Verifiable example don't make much sense.
– Stargateur
Nov 13 '18 at 5:31
8
8
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
4
4
There is no link between
C and c, your Minimal, Complete, and Verifiable example don't make much sense.– Stargateur
Nov 13 '18 at 5:31
There is no link between
C and c, your Minimal, Complete, and Verifiable example don't make much sense.– Stargateur
Nov 13 '18 at 5:31
add a comment |
1 Answer
1
active
oldest
votes
Since there is no link between the arguments to your function and the generic type C, the compiler has no idea which type you mean to replace C in the function call. In this case your code doesn't even use the type C so it doesn't matter, but Rust is big about "local reasoning" - i.e. it won't peek inside your function to determine you're not using C at the syntax checking stage.
You can explicitly tell it what your types are using the "turbofish" operator, ::<>, like this
println!("{}", lifetime_tester::<_, _, i32>(a, b, &c));
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
add a comment |
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Since there is no link between the arguments to your function and the generic type C, the compiler has no idea which type you mean to replace C in the function call. In this case your code doesn't even use the type C so it doesn't matter, but Rust is big about "local reasoning" - i.e. it won't peek inside your function to determine you're not using C at the syntax checking stage.
You can explicitly tell it what your types are using the "turbofish" operator, ::<>, like this
println!("{}", lifetime_tester::<_, _, i32>(a, b, &c));
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
add a comment |
Since there is no link between the arguments to your function and the generic type C, the compiler has no idea which type you mean to replace C in the function call. In this case your code doesn't even use the type C so it doesn't matter, but Rust is big about "local reasoning" - i.e. it won't peek inside your function to determine you're not using C at the syntax checking stage.
You can explicitly tell it what your types are using the "turbofish" operator, ::<>, like this
println!("{}", lifetime_tester::<_, _, i32>(a, b, &c));
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
add a comment |
Since there is no link between the arguments to your function and the generic type C, the compiler has no idea which type you mean to replace C in the function call. In this case your code doesn't even use the type C so it doesn't matter, but Rust is big about "local reasoning" - i.e. it won't peek inside your function to determine you're not using C at the syntax checking stage.
You can explicitly tell it what your types are using the "turbofish" operator, ::<>, like this
println!("{}", lifetime_tester::<_, _, i32>(a, b, &c));
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
Since there is no link between the arguments to your function and the generic type C, the compiler has no idea which type you mean to replace C in the function call. In this case your code doesn't even use the type C so it doesn't matter, but Rust is big about "local reasoning" - i.e. it won't peek inside your function to determine you're not using C at the syntax checking stage.
You can explicitly tell it what your types are using the "turbofish" operator, ::<>, like this
println!("{}", lifetime_tester::<_, _, i32>(a, b, &c));
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
edited Nov 15 '18 at 15:21
Shepmaster
151k13293432
151k13293432
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
answered Nov 13 '18 at 8:27
Michael AndersonMichael Anderson
45k693148
45k693148
add a comment |
add a comment |
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8
I'm no rust expert, but it seems like the problem is that you never mention the generic parameter (uppercase) C in the function, so it can't infer its type from the arguments. So you probably need to drop C or explicitly supply the types when calling lifetime_tester
– happydave
Nov 13 '18 at 3:55
4
There is no link between
Candc, your Minimal, Complete, and Verifiable example don't make much sense.– Stargateur
Nov 13 '18 at 5:31