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>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full>: 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>` is not implemented for `Then, ...>, ..., ..., ..., ...>`, which is required by `Then>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full>: CSTParser<'a>` | = help: the trait `chumsky::private::ParserSealed<'_, &'a str, ((), ()), chumsky::extra::Full>` is implemented for `Then>, {closure@repro.rs:16:17: 16:27}>, char>, chumsky::combinator::Map, O, {closure@repro.rs:18:32: 18:35}>, (), (), chumsky::extra::Full>` = help: for that trait implementation, expected `((), ())`, found `()` = note: required for `Then, ...>, ..., ..., ..., ...>` to implement `Parser<'_, &str, ()>` note: required for `Then, ...>, ..., ..., ..., ...>` 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 (662180b34 2024-10-20) binary: rustc commit-hash: 662180b34d95f72d05b7c467b0baf4d23d36b1e1 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.