Include type of missing trait methods in error

[estebank]

For a given file foo.rs:

use std::str::FromStr;

struct A {}

trait X<T> {
    type Foo;
    const BAR: u32 = 128;

    fn foo() -> T;
    fn bar();
    fn    bay<
        'lifetime,    TypeParameterA
            >(  a   : usize,
                b: u8  );
}

impl std::fmt::Display for A {
}
impl FromStr for A{}

impl X<usize> for A {
}

Provide the following output:

error: main function not found

error[E0046]: not all trait items implemented, missing: `fmt`
  --> file2.rs:18:1
   |
18 | impl std::fmt::Display for A {
   | ^ missing `fmt` in implementation
   |
   = note: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;

error[E0046]: not all trait items implemented, missing: `Err`, `from_str`
  --> file2.rs:20:1
   |
20 | impl FromStr for A{}
   | ^^^^^^^^^^^^^^^^^^^^ missing `Err`, `from_str` in implementation
   |
   = note: type Err;
   = note: fn from_str(&str) -> std::result::Result<Self, <Self as std::str::FromStr>::Err>;

error[E0046]: not all trait items implemented, missing: `Foo`, `foo`, `bar`, `bay`
  --> file2.rs:22:1
   |
22 | impl X<usize> for A {
   | ^ missing `Foo`, `foo`, `bar`, `bay` in implementation
   |
   = note: type Foo;
   = note: fn foo() -> T;
   = note: fn bar();
   = note: fn bay<'lifetime, TypeParameterA>(a: usize, b: u8);

error: aborting due to 3 previous errors

Fixes #24626

For a given file foo.rs:

struct A {}

impl std::fmt::Display for A {
    fn fmt() -> () {}
}

provide the expected method signature:

error: main function not found

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
 --> foo.rs:4:5
  |
4 |     fn fmt() -> () {}
  |     ^^^^^^^^^^^^^^^^^ expected `&self` in impl
  |
  = note: Expected signature: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;
  = note:    Found signature: fn fmt();

error: aborting due to previous error

Fixes #28011

[rust-highfive]

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[arielb1]

Add spans to everything?

[nikomatsakis]

Hmm, while it is nice to see the type, I'm not sure if this change is quite handling it right. The error messages themselves look awfully busy to me now, but maybe colors would help there. But moreover the types are using our internal type-printing, which is probably not what you would want to use when defining the method.

To pick one example:

= note: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;

you would probably want:

fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error>

but this will also depend on what imports you have around (e.g., is std::fmt imported?).

I'm not sure what's the best way to handle this, unfortunately. :(

[nikomatsakis]

@estebank can you maybe give the current messages too, just for comparison purposes?

[estebank]

For the first case:

error: main function not found

error[E0046]: not all trait items implemented, missing: `fmt`
  --> file4.rs:17:1
   |
17 | impl std::fmt::Display for A {
   | ^ missing `fmt` in implementation

error[E0046]: not all trait items implemented, missing: `Err`, `from_str`
  --> file4.rs:19:1
   |
19 | impl FromStr for A{}
   | ^^^^^^^^^^^^^^^^^^^^ missing `Err`, `from_str` in implementation

error[E0046]: not all trait items implemented, missing: `Foo`, `foo`, `bar`, `bay`
  --> file4.rs:21:1
   |
21 | impl X<usize> for A {
   | ^ missing `Foo`, `foo`, `bar`, `bay` in implementation

error: aborting due to 3 previous errors

and for the second case:

error: main function not found

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
 --> file5.rs:4:5
  |
4 |     fn fmt() -> () {}
  |     ^^^^^^^^^^^^^^^^^ expected `&self` in impl

error: aborting due to previous error

I agree with the original tickets that showing the signature for the missing implementations is very much needed, otherwise you must go around fishing around the docs.

= note: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;

you would probably want:

fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error>

but this will also depend on what imports you have around (e.g., is std::fmt imported?).

I'm not sure what's the best way to handle this.

Neither am I. Looking at the current context to identify that for example std::result::Result<(), std::fmt::Error> is known in this context as Result<(), fmt::Error> sounds non trivial.

I could rework this so as to only show the signature for methods for which we have spans for now.

[sophiajt]

@estebank - there has been some recent work on E0186 which I think helps some. Here's one example:

error[E0186]: method `foo` has a `&self` declaration in the trait, but not in the impl
  --> src/test/compile-fail/E0186.rs:18:5
   |
12 |     fn foo(&self);
   |     -------------- `&self` used in trait
...
18 |     fn foo() {}
   |     ^^^^^^^^^^^ expected `&self` in impl

It's similar in spirit, I think, to what @nikomatsakis is saying about using the simpler type if we can.

We may want to do something similar for E0046 as well. For example, instead of:

error[E0046]: not all trait items implemented, missing: `foo`
  --> src/test/compile-fail/E0046.rs:17:1
   |
17 | impl Foo for Bar {}
   | ^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation

have:

error[E0046]: not all trait items implemented, missing: `foo`
  --> src/test/compile-fail/E0046.rs:17:1
   |
12 |     fn foo();
   |     -------- `foo` from trait
...
17 | impl Foo for Bar {}
   | ^^^^^^^^^^^^^^^^^^^ expected `foo` in implementation

[nikomatsakis]

error[E0046]: not all trait items implemented, missing: `foo`
  --> src/test/compile-fail/E0046.rs:17:1
   |
12 |     fn foo();
   |     -------- `foo` from trait
...
17 | impl Foo for Bar {}
   | ^^^^^^^^^^^^^^^^^^^ expected `foo` in implementation

Ooh, I like this! It gives you the stuff to copy-and-paste while making it clear where it comes from.

(Of course, it won't work across crates.)

[sophiajt]

@nikomatsakis - interesting point about working across crates. I'm curious what we might able to print instead. I'm guessing we have the trait information sitting in the rlib since any consumer of a crate can implement a trait with their own types. Maybe there's a way to print it back out again.

@alexcrichton - do you know if ^ is possible?

[alexcrichton]

@jonathandturner I know rustdoc at least reconstructs information like trait signatures from metadata. I don't believe it's lossless though as I think there's at least a bug with lifetime parameters. For simple cases though if rustdoc can recreate a trait signature then surely the compiler error messages can!

[estebank]

I like the idea of showing the annotated span for the definitions that are not from a different crate, but I'd like to show a reconstructed signature for other crates' definitions.

I took a look the code that formats method declarations for rustdoc, and it can't be used directly in the compiler, as it works on rustdoc's own AST and is tied to HTML output only.

I feel like I could move the presentation logic up onto the compiler, but as clean will give you an element from the rustdoc AST that has no connection to the compiler's AST, I see only two options: move the Clean construct from rustdoc to the compiler or pretty much copy the logic on the compiler (which is pretty much what I did in this PR, even though I hadn't seen that code already).

[estebank]

I've been looking at the Span rendering code when an error is emited and it seems like the display of multiline Spans is not supported.

Is there any way at this time to display multiline spans? If not, do we want to add it? Otherwise we end up with a pretty substandard note:

note: missing trait item definition
  --> missing-impls.rs:10:5
   |
10 |     fn bar();
   |     ^^^^^^^^^ `bar` from trait
note: missing trait item definition
  --> missing-impls.rs:11:5
   |
11 |     fn    bay<
   |     ^ `bay` from trait

[sophiajt]

@estebank - there currently isn't a way to display multispans as a "single thing" in the error message, though it does use multispans to draw the error display.

What specifically are you trying to do?

[estebank]

@jonathandturner I feel that showing the complete original span for missing items would help particularly well for the most problematic of the method definitions, those too long to be readable without formatting. By showing the entire multiline span you're taking advantage of the original definition's formatting, which we can assume to be readable most of the time:

note: missing trait item definition
  --> missing-impls.rs:10:5
   |
10 |     fn bar();
   |     ^^^^^^^^^ `bar` from trait
note: missing trait item definition
  --> missing-impls.rs:11:5
   |
11 |     fn    bay<
12 |        'lifetime,    TypeParameterA
13 |            >(  a   : usize,
14 |                b: u8  );
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `bay` from trait

Also, if adding a multiline span message is desirable, a presentation like the following might be desirable:

  --> missing-impls.rs:11:5
   |
11 |     fn    bay<                      ╮ `bay` from trait
12 |        'lifetime,    TypeParameterA |
13 |            >(  a   : usize,         |
14 |                b: u8  );            ╯

[nikomatsakis]

@estebank

Also, if adding a multiline span message is desirable, a presentation like the following might be desirable

I agree that this looks good in this case. I'm not sure how well it composes with errors that have other spans in them as well though... we've been striving for just one snippet, with labels collected together, versus the way your examples are formatted, where there are two independent snippets, right? Maybe that goal doesn't make sense here though.

[sophiajt]

@nikomatsakis - do you know if there is a way to pretty print this so that we don't have all the whitespace and linebreaks?

[estebank]

I've been messing around to see what it would look like and although it is far from perfect, I think it is in the right direction:

Here there're three different impls with missing items, each on its own error section (briefly looked at merging all errors of the same type of the same file into the same code preview, and it seems like a much bigger change than I'd like to do at this point).

The full multiline support was relatively easy to add, while the addition for them will take some extra work. I'm mildly worried about this change since every place that expects multiline spans to be shown as just the first line will now show the entire span (which could be huge) and would need to be changed to check wether it is multiline and in that case only use the first char of the first line.

When I looked a couple of weeks back for user facing type printing, I couldn't find any, but I'll look again.

[estebank]

@nikomatsakis I've been thinking about how it would be best to present annotated multiline spans in a composable way, and visually I arrived on:

error[E0046]: not all trait items implemented, missing: `bar`, `bay`
  --> file.rs:27:1
   |
15 |       fn bar();
   |       --------- `bar` from trait
...
16 | ┌     fn    bay<'lifetime,
   | ╎           ^^^ showing off an error
17 | ╎                                      TypeParameterA
18 | ╎
19 | ╎ ┌             >(  a   : usize,
   | ╎ ╎                 ----------- error
20 | ╎ ╎                 b: u8  );
   | └╴┼╴╴╴╴^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ some error
   |   └╴╴╴╴╴╴╴╴╴╴╴╴╴---------------- overlapped errors to show off how it could look like

...    
27 |     impl X<usize> for A {
   |     ^ missing `bar`, `bay` in implementation

If you think that there're other places where it'd be beneficial to show labeled multiline spans, I'll go ahead and implement this.

The other concern I have is who should be deciding wether to show the span. The way I see it there're two options:

• see if the span is short enough (maybe less than 9 lines?) and just show it in its entirety, otherwise continue doing what it's doing now
• let the caller decide wether they'd accept a full multiline span or not, which requires people adding messages to be able to predict wether a multiline span is even possible, as well as adding the potential to get it wrong and showing a huge amount of code

Thoughts?

[sophiajt]

Since we're recreating the type signature, instead of printing this:

  --> missing-impls.rs:11:5
   |
11 |     fn    bay<                      ╮ `bay` from trait
12 |        'lifetime,    TypeParameterA |
13 |            >(  a   : usize,         |
14 |                b: u8  );            ╯

Can we print this?

     fn bay<'lifetime, TypeParameterA>(a: usize, b: u8);
        --- `bay` from trait

[nikomatsakis]

@jonathandturner

Since we're recreating the type signature, instead of printing this:

I'm confused, I thought the point was that we wouldn't be recreating the signature? But rather highlighting the original text? (In general, I agree that if we can print original text, it's a better thing to do.)

[nikomatsakis]

The full multiline support was relatively easy to add, while the addition for them will take some extra work. I'm mildly worried about this change since every place that expects multiline spans to be shown as just the first line will now show the entire span (which could be huge) and would need to be changed to check wether it is multiline and in that case only use the first char of the first line.

I think that if we get some attractive way to highlight multiline spans, that's probably ok, though we do (I think) in general want to be avoiding those in favor of highlighting some representative thing. The use of the first character was seen as a reasonable stopgap while we went and tried to improve the code not to give multiline spans: but there hasn't been much progress on that and it's hard in general. :(

FWIW, the idea of moving into a "bracket" on the RHS seems visually nice, though it of course means we can't clearly indicate where the bracket begins/ends in the line. For a method definition, that's fine of course. For other things, it's less good. Consider something like:

foo(1 + bar(x,
            y),
    z);

where we want to highlight the bar(x, y) expression. Today we would print:

foo(1 + bar(x,
        ^ wrong type
            y),
    z);

This can of course be ambiguous if the extent of the expression is unclear, and the number of complaints makes it clear that this can be confusing in practice.

Putting it on the side might look like this. I am avoiding unicode because many terminals can't handle it, so in the past we've tried to stick to ASCII.

foo(1 + bar(x,  <| wrong type
            y), <|
    z);

It seems clear that this too is ambiguous and potentially confusing, particularly absent colors.

In the past (in LALRPOP) I tried something like:

        +-------+
        v       |
foo(1 + bar(x,  | wrong type
            y), |
            ^   |
            +-+
    z);

but I think that is really confusing too =).

Your idea of adding in the ^ and combining with the brackets seems appealing, but maybe confusing too. I'm not sure how to 'link' the | and ^:

foo(1 + bar(x,  | wrong type
        ^
            y), |
    z);

thoughts?

[bors]

☔ The latest upstream changes (presumably #37269) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

I'm a bit confused as to the status of this PR. It seems like we're still doing a lot of active debate about what it should do -- I'm inclined to say let's close it and we can move this conversation maybe into an issue, or internals, or something? (Right now it shows up each time I sweep my assigned PRs...)

[sophiajt]

I'm okay with the design moving to an issue

[estebank]

Sorry for being unresponsive, I've been away offline for the past two weeks. I've created https://internals.rust-lang.org/t/proposal-for-multiline-span-comments/4242 for continuing this conversation. Feel free to close the PR. I will continue working regardless, and will create a new PR once a consensus arrises in internals.

[estebank]

@nikomatsakis

where we want to highlight the bar(x, y) expression. Today we would print:

X |  foo(1 + bar(x,
  |              ^ wrong type
X |              y),
X |      z);

Just one thing, currently what would be displayed is just

X |    foo(1 + bar(x,
  |            ^ wrong type