E0277 highlights really long types for no reason, and doesn't highlight useful help

[jyn514]

Code

#!/usr/bin/env cargo

//! ```cargo
//! [dependencies]
//! chumsky = "1.0.0-alpha.7"
//! ```

use chumsky::{primitive::any, Parser};

pub(crate) trait CSTParser<'a, O = ()>: Parser<'a, &'a str, O> {}

impl<'a, O, T> CSTParser<'a, O> for T where T: Parser<'a, &'a str, O> {}

fn leaf<'a, O>(parser: impl CSTParser<'a, O>) -> impl CSTParser<'a, ()> {
    let ws = any()
        .filter(|c: &char| *c != '\n' && c.is_whitespace())
        .ignored();
    ws().then(parser.map(|_| ()))
}

fn main() {}

Current output

error[E0277]: the trait bound `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>: CSTParser<'a>` is not satisfied
  --> repro.rs:14:50
   |
14 | fn leaf<'a, O>(parser: impl CSTParser<'a, O>) -> impl CSTParser<'a, ()> {
   |                                                  ^^^^^^^^^^^^^^^^^^^^^^ the trait `chumsky::private::ParserSealed<'_, &str, (), chumsky::extra::Full<EmptyErr, (), ()>>` is not implemented for `Then<Ignored<Filter<..., ...>, ...>, ..., ..., ..., ...>`, which is required by `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>: CSTParser<'a>`
   |
   = help: the trait `chumsky::private::ParserSealed<'_, &'a str, ((), ()), chumsky::extra::Full<EmptyErr, (), ()>>` is implemented for `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>`
   = help: for that trait implementation, expected `((), ())`, found `()`
   = note: required for `Then<Ignored<Filter<..., ...>, ...>, ..., ..., ..., ...>` to implement `Parser<'_, &str, ()>`
note: required for `Then<Ignored<Filter<..., ...>, ...>, ..., ..., ..., ...>` to implement `CSTParser<'a>`
  --> repro.rs:12:16
   |
12 | impl<'a, O, T> CSTParser<'a, O> for T where T: Parser<'a, &'a str, O> {}
   |                ^^^^^^^^^^^^^^^^     ^          ---------------------- unsatisfied trait bound introduced here
   = note: the full name for the type has been written to '/home/jyn/.local/lib/cargo/target/debug/deps/foo-f0b1a0054d2a8996.long-type-7449552674356047294.txt'
   = note: consider using `--verbose` to print the full type name to the console
   = note: the full name for the type has been written to '/home/jyn/.local/lib/cargo/target/debug/deps/foo-f0b1a0054d2a8996.long-type-7449552674356047294.txt'
   = note: consider using `--verbose` to print the full type name to the console

For more information about this error, try `rustc --explain E0277`.

Desired output

• expected `((), ())`, found `()` should be highlighted instead of is implemented for `Then<...>`
• the difference between ParserSealed<((), ())> and ParserSealed<()> should be highlighted.
• "the full name for the type has been written ..." should not be printed multiple times
• "trait is not implemented for T" followed by "trait is implemented for T" is really confusing. rustc shouldn't do that; or at least word it differently.
• especially since T is the same in both cases, it should not highlight T in purple.
• this should show the return value of the function somewhere; right now it's not even clear that the ws().then() line is related (from the spans, it looks like this is an error at the level of the function signature, not the function implementation)

Rationale and extra context

Other cases

No response

Rust Version

rustc 1.84.0-nightly (662180b 2024-10-20)
binary: rustc
commit-hash: 662180b
commit-date: 2024-10-20
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

Anything else?

No response

[jyn514]

also this is a bit of a larger change, but it would be nice if rustc never broke up a diagnostic across a type name. it means that it's harder to visually parse, it has bad degenerate behavior when the types are long, and it doesn't really seem more clear than, e.g., "a trait bound was not satisfied: Then<...>: CSTParser"

[jyn514]

i think the the whole ", which is required by ..." clause shouldn't be there either tbh - it's already in a note below, and it's right in the middle of the span making it harder to see the rest of the diagnostic.

[estebank]

but it would be nice if rustc never broke up a diagnostic across a type name

We changed this some time back so it doesn't do that: now it always replaces generic parameters as a whole, the trimming is not textual anymore.

this should show the return value of the function somewhere; right now it's not even clear that the ws().then() line is related (from the spans, it looks like this is an error at the level of the function signature, not the function implementation)

I think that we fail to highlight the tail or return expression for all cases involving impl Trait... We should fix that.

I need to clean this up, but I managed to reuse the type comparison machinery for traits and got this:

textual output

error[E0277]: the trait bound `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@src/main.rs:9:17: 9:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@src/main.rs:11:24: 11:27}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>: CSTParser<'a>` is not satisfied
 --> src/main.rs:7:50
  |
7 | fn leaf<'a, O>(parser: impl CSTParser<'a, O>) -> impl CSTParser<'a, ()> {
  |                                                  ^^^^^^^^^^^^^^^^^^^^^^ the trait `chumsky::private::ParserSealed<'_, &str, (), chumsky::extra::Full<EmptyErr, (), ()>>` is not implemented for `Then<Ignored<..., ...>, ..., ..., ..., ...>`
  |
  = help: the trait `ParserSealed<'_, &_, (), Full<_, _, _>>` is not implemented for `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@src/main.rs:9:17: 9:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@src/main.rs:11:24: 11:27}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>`
  = help: the trait `ParserSealed<'_, &'a _, ((), ()), Full<_, _, _>>` is implemented for `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@src/main.rs:9:17: 9:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@src/main.rs:11:24: 11:27}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>`
  = help: for that trait implementation, expected `((), ())`, found `()`
  = note: required for `Then<Ignored<..., ...>, ..., ..., ..., ...>` to implement `Parser<'_, &str, ()>`
note: required for `Then<Ignored<..., ...>, ..., ..., ..., ...>` to implement `CSTParser<'a>`
 --> src/main.rs:5:16
  |
5 | impl<'a, O, T> CSTParser<'a, O> for T where T: Parser<'a, &'a str, O> {}
  |                ^^^^^^^^^^^^^^^^     ^          ---------------------- unsatisfied trait bound introduced here
  = note: the full name for the type has been written to '/home/gh-estebank/longlong/target/debug/deps/longlong-df9d52be87eada65.long-type-2878432305708693415.txt'
  = note: consider using `--verbose` to print the full type name to the console

The reason the is/isn't implemented for X type is present is because although in this case the implementer is the same, like in:

error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
  --> $DIR/as_expression.rs:57:15
   |
LL |     SelectInt.check("bar");
   |               ^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str`
   |
   = help: the trait `AsExpression<Integer>` is not implemented for `&str`
   = help: the trait `AsExpression<Text>` is implemented for `&str`
   = help: for that trait implementation, expected `Text`, found `Integer`

there are cases where they are not, like:

error[E0277]: the type `[{integer}]` cannot be indexed by `i32`
  --> $DIR/index-help.rs:3:7
   |
LL |     x[0i32];
   |       ^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`, which is required by `Vec<{integer}>: Index<_>`
   = help: the trait `SliceIndex<_>` is not implemented for `i32`
   = help: the trait `SliceIndex<_>` is implemented for `usize`
   = help: for that trait implementation, expected `usize`, found `i32`
   = note: required for `Vec<{integer}>` to implement `Index<i32>`

We should absolutely account for both cases slightly differently.

[estebank]

@jyn514 what do you think of this?

textual output

error[E0277]: the trait bound `Then<Ignored<chumsky::combinator::Filter<chumsky::primitive::Any<&str, chumsky::extra::Full<EmptyErr, (), ()>>, {closure@src/main.rs:9:17: 9:27}>, char>, chumsky::combinator::Map<impl CSTParser<'a, O>, O, {closure@src/main.rs:11:24: 11:27}>, (), (), chumsky::extra::Full<EmptyErr, (), ()>>: CSTParser<'a>` is not satisfied
  --> src/main.rs:7:50
   |
7  | fn leaf<'a, O>(parser: impl CSTParser<'a, O>) -> impl CSTParser<'a, ()> {
   |                                                  ^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound
...
11 |     ws.then(parser.map(|_| ()))
   |     --------------------------- return type was inferred to be `Then<Ignored<Filter<Any<&str, ...>, ...>, ...>, ..., ..., ..., ...>` here
   |
   = help: the trait `ParserSealed<'_, &_, (), Full<_, _, _>>` is not implemented for `Then<Ignored<Filter<Any<&str, ...>, ...>, ...>, ..., ..., ..., ...>`
           but trait `ParserSealed<'_, &'a _, ((), ()), Full<_, _, _>>` is implemented for it
   = help: for that trait implementation, expected `((), ())`, found `()`
   = note: required for `Then<Ignored<Filter<Any<&str, ...>, ...>, ...>, ..., ..., ..., ...>` to implement `Parser<'_, &str, ()>`
note: required for `Then<Ignored<Filter<Any<&str, ...>, ...>, ...>, ..., ..., ..., ...>` to implement `CSTParser<'a>`
  --> src/main.rs:5:16
   |
5  | impl<'a, O, T> CSTParser<'a, O> for T where T: Parser<'a, &'a str, O> {}
   |                ^^^^^^^^^^^^^^^^     ^          ---------------------- unsatisfied trait bound introduced here
   = note: the full name for the type has been written to '/home/gh-estebank/longlong/target/debug/deps/longlong-df9d52be87eada65.long-type-3075078524015204798.txt'
   = note: consider using `--verbose` to print the full type name to the console