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Add error description for E0406 #34230

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2255948
Add error description for E0406
nikhilshagri Jun 11, 2016
27325c9
Error desc: Minor fixes
nikhilshagri Jun 12, 2016
19892f4
Error desc: minor fixes II
nikhilshagri Jun 12, 2016
45eec30
Error desc: minor fixes III
nikhilshagri Jun 13, 2016
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47 changes: 46 additions & 1 deletion src/librustc_resolve/diagnostics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -444,6 +444,52 @@ impl SomeTrait for Foo { // ok!
```
"##,

E0406: r##"
A function is referring to an associated type which hasn't been declared in
the trait. Example of erroneous code:

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You forgot "Example of erroneous code:". Take a look at the RFC for more information.

```compile_fail
trait Foo {
type Bar;

// error: function signature contains a reference to `Self::Baz`, but
// `Baz` hasn't been declared as an associated type of the trait
fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}
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When I try to compile this code, I get E0220, not E0406:

$ cat scratch.rs 
trait Foo {
    type Bar;

    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}

fn main() {}
$ rustc scratch.rs 
scratch.rs:4:40: 4:49 error: associated type `Baz` not found for `Self` [E0220]
scratch.rs:4     fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
                                                    ^~~~~~~~~
scratch.rs:4:40: 4:49 help: run `rustc --explain E0220` to see a detailed explanation
error: aborting due to previous error
$ rustc --version
rustc 1.11.0-nightly (5c2a5d449 2016-06-11)

I've found that greping around in the codebase to see where an error code is being generated can provide a clue as to what kind of examples would trigger that error (although usually not a very good clue, given the vastness of my ignorance of the compiler internals); in this case, it looks like E406 will get emitted during rustc_resolve::Resolver.resolve_generics (the mapping of error to error-code happens elsewhere in the file) ... but I don't know what that means or when it happens.

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I tried searching the codebase, hoping to find answers, but like you, my knowledge of compilers isn't exactly top-notch. I suspect the compiler is showing E0220 until this lands. I asked on the IRC regrading this as well, and it seems that E0406 should be thrown here.

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Ah damn, I tested your code on release. Didn't think it'd change on nightly. :-/

```

The use of 'associated types' allows us declare variables which might be used
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No, that's not what I meant. Don't explain what the user tried to do but in which he/she did wrong.

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Well.... I can't understand what you are trying to say. 😕

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@GuillaumeGomez GuillaumeGomez Jun 12, 2016 •
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Sorry, I'll try to explain more clearly. So in here, what's expected is something which helps the user understands the error. Easy example:

let mut x: u32 = 0;
x = "salut !";

It'll fail because the expected type doesn't correspond to the one we put in x. So instead of explaining what is a variable declaration, it'd be nice to point out that the two types must be the same. In this case, I'd write:

"The type of a variable cannot change once declared. If you want the type of x to change, you must redeclare it but you'll lose its previous content. Otherwise you can just store the other data into a new variable."

Maybe add a link to the variable chapter in the book in case the user really doesn't know how it works.

And then you put examples which show it. Did it help?

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yep, that was useful. I will push a new commit tomorrow, amending the explanation.

in any of the function signatures in a particular trait. This helps in
eliminating redundancies and improves readability of code. It works by using
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"helps eliminate redundancies and improve readability" would have more parallelism (eliminate and improve being bare infinitives complementing helps).

the `type` keyword to declare a variable (Eg: `type A;`), and then using it
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"e.g.," is more standard.

in the function signature as `&Self::A`. `Self` refers to the object which
has invoked the method, and this object can be of any type for which trait
`Foo` has been implemented.

One solution might be to declare the associated type in the trait:

```
trait Foo {
type Bar;
type Baz; // declare `Baz`

fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}
```

Alternatively, you could remove the input variable
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Missing an empty line before this block.

corresponding to the associated type from the function signature:

```
trait Foo {
type Bar;

// `&Self::Baz` has been removed
fn return_bool(&self, &Self::Bar) -> bool;
}
```
"##,

E0407: r##"
A definition of a method not in the implemented trait was given in a trait
implementation. Example of erroneous code:
Expand Down Expand Up @@ -1105,7 +1151,6 @@ register_diagnostics! {
// E0257,
// E0258,
E0402, // cannot use an outer type parameter in this context
E0406, // undeclared associated type
// E0410, merged into 408
// E0413, merged into 530
// E0414, merged into 530
Expand Down