diff options
5 files changed, 49 insertions, 1 deletions
diff --git a/src/librustc/error_codes.rs b/src/librustc/error_codes.rs index a1bfd417566..6243e911bd5 100644 --- a/src/librustc/error_codes.rs +++ b/src/librustc/error_codes.rs @@ -1207,6 +1207,51 @@ fn main() { ``` "##, +E0284: r##" +This error occurs when the compiler is unable to unambiguously infer the +return type of a function or method which is generic on return type, such +as the `collect` method for `Iterator`s. + +For example: + +```compile_fail,E0284 +fn foo() -> Result<bool, ()> { + let results = [Ok(true), Ok(false), Err(())].iter().cloned(); + let v: Vec<bool> = results.collect()?; + // Do things with v... + Ok(true) +} +``` + +Here we have an iterator `results` over `Result<bool, ()>`. +Hence, `results.collect()` can return any type implementing +`FromIterator<Result<bool, ()>>`. On the other hand, the +`?` operator can accept any type implementing `Try`. + +The author of this code probably wants `collect()` to return a +`Result<Vec<bool>, ()>`, but the compiler can't be sure +that there isn't another type `T` implementing both `Try` and +`FromIterator<Result<bool, ()>>` in scope such that +`T::Ok == Vec<bool>`. Hence, this code is ambiguous and an error +is returned. + +To resolve this error, use a concrete type for the intermediate expression: + +``` +fn foo() -> Result<bool, ()> { + let results = [Ok(true), Ok(false), Err(())].iter().cloned(); + let v = { + let temp: Result<Vec<bool>, ()> = results.collect(); + temp? + }; + // Do things with v... + Ok(true) +} +``` + +Note that the type of `v` can now be inferred from the type of `temp`. +"##, + E0308: r##" This error occurs when the compiler was unable to infer the concrete type of a variable. It can occur for several cases, the most common of which is a @@ -2158,7 +2203,6 @@ register_diagnostics! { E0278, // requirement is not satisfied E0279, // requirement is not satisfied E0280, // requirement is not satisfied - E0284, // cannot resolve type // E0285, // overflow evaluation builtin bounds // E0296, // replaced with a generic attribute input check // E0300, // unexpanded macro diff --git a/src/test/ui/associated-types/associated-types-overridden-binding.stderr b/src/test/ui/associated-types/associated-types-overridden-binding.stderr index fced38caaba..a26ee23894f 100644 --- a/src/test/ui/associated-types/associated-types-overridden-binding.stderr +++ b/src/test/ui/associated-types/associated-types-overridden-binding.stderr @@ -12,3 +12,4 @@ LL | trait Foo: Iterator<Item = i32> {} error: aborting due to previous error +For more information about this error, try `rustc --explain E0284`. diff --git a/src/test/ui/associated-types/associated-types-unconstrained.stderr b/src/test/ui/associated-types/associated-types-unconstrained.stderr index 26e5a6a503c..da14a69ae30 100644 --- a/src/test/ui/associated-types/associated-types-unconstrained.stderr +++ b/src/test/ui/associated-types/associated-types-unconstrained.stderr @@ -6,3 +6,4 @@ LL | let x: isize = Foo::bar(); error: aborting due to previous error +For more information about this error, try `rustc --explain E0284`. diff --git a/src/test/ui/issues/issue-12028.stderr b/src/test/ui/issues/issue-12028.stderr index b9e2e80492b..64694c7a8d0 100644 --- a/src/test/ui/issues/issue-12028.stderr +++ b/src/test/ui/issues/issue-12028.stderr @@ -6,3 +6,4 @@ LL | self.input_stream(&mut stream); error: aborting due to previous error +For more information about this error, try `rustc --explain E0284`. diff --git a/src/test/ui/question-mark-type-infer.stderr b/src/test/ui/question-mark-type-infer.stderr index 2a1bdf57a88..f62a540572c 100644 --- a/src/test/ui/question-mark-type-infer.stderr +++ b/src/test/ui/question-mark-type-infer.stderr @@ -6,3 +6,4 @@ LL | l.iter().map(f).collect()? error: aborting due to previous error +For more information about this error, try `rustc --explain E0284`. |