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Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0193.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0193.md | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0193.md b/compiler/rustc_error_codes/src/error_codes/E0193.md new file mode 100644 index 000000000..e29a949ff --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0193.md @@ -0,0 +1,44 @@ +#### Note: this error code is no longer emitted by the compiler. + +`where` clauses must use generic type parameters: it does not make sense to use +them otherwise. An example causing this error: + +``` +trait Foo { + fn bar(&self); +} + +#[derive(Copy,Clone)] +struct Wrapper<T> { + Wrapped: T +} + +impl Foo for Wrapper<u32> where Wrapper<u32>: Clone { + fn bar(&self) { } +} +``` + +This use of a `where` clause is strange - a more common usage would look +something like the following: + +``` +trait Foo { + fn bar(&self); +} + +#[derive(Copy,Clone)] +struct Wrapper<T> { + Wrapped: T +} +impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone { + fn bar(&self) { } +} +``` + +Here, we're saying that the implementation exists on Wrapper only when the +wrapped type `T` implements `Clone`. The `where` clause is important because +some types will not implement `Clone`, and thus will not get this method. + +In our erroneous example, however, we're referencing a single concrete type. +Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no +reason to also specify it in a `where` clause. |