bad diagnostic for trait impl requirement

[Mark-Simulacrum]

Noticed in Crater, seems like it's probably a regression but just in quality (this is entirely new AFAICT):

[INFO] [stderr] note: the requirement `_ <: _` appears on the `impl`'s method `try_from` but not on the corresponding trait's method
[INFO] [stderr]  --> $RUST/core/src/convert/mod.rs
[INFO] [stderr]   |
[INFO] [stderr]   = note: in this trait
[INFO] [stderr]  ::: $RUST/core/src/convert/mod.rs
[INFO] [stderr]   |
[INFO] [stderr]   = note: this trait's method doesn't have the requirement `_ <: _`

• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/reg/enumorph-0.1.2/log.txt

Saying that "_ <: _" is probably useless to 99% of readers, and <: in general feels a bit internal-y to me?

[fmease]

Inlining the reproducer (not minimized yet):

// requires dependency `enumorph`, version 0.1.2
#[derive(enumorph::Enumorph)]
enum Enum {
    A(u8),
    B { c: u8 },
}

fn main() {}

Compiler output diff (stable vs. nightly)

  error[E0119]: conflicting implementations of trait `TryFrom<Enum>` for type `u8`
   --> src/main.rs:4:12
    |
  3 |     A(u8),
    |       -- first implementation here
  4 |     B { c: u8 },
    |            ^^ conflicting implementation for `u8`
  
+ error[E0282]: type annotations needed
+    --> src/main.rs:3:7
+     |
+ 3   |     A(u8),
+     |       ^^ cannot infer type
+     |
+ note: the requirement `_ <: _` appears on the `impl`'s method `try_from` but not on the corresponding trait's method
+    --> /home/fmease/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:687:8
+     |
+ 679 | pub trait TryFrom<T>: Sized {
+     |           ------- in this trait
+ ...
+ 687 |     fn try_from(value: T) -> Result<Self, Self::Error>;
+     |        ^^^^^^^^ this trait's method doesn't have the requirement `_ <: _`
+ 
  error[E0119]: conflicting implementations of trait `From<u8>` for type `Enum`
   --> src/main.rs:4:12
    |
  3 |     A(u8),
    |       -- first implementation here
  4 |     B { c: u8 },
    |            ^^ conflicting implementation for `Enum`
  
+ error[E0282]: type annotations needed
+    --> src/main.rs:4:12
+     |
+ 4   |     B { c: u8 },
+     |            ^^ cannot infer type
+     |
+ note: the requirement `_ <: _` appears on the `impl`'s method `try_from` but not on the corresponding trait's method
+    --> /home/fmease/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:687:8
+     |
+ 679 | pub trait TryFrom<T>: Sized {
+     |           ------- in this trait
+ ...
+ 687 |     fn try_from(value: T) -> Result<Self, Self::Error>;
+     |        ^^^^^^^^ this trait's method doesn't have the requirement `_ <: _`
+ 

Meaning we previously suppressed those useless type annotations needed diagnostics. This smells like a regression caused by one of @oli-obk's PRs that make rustc continue analysis even if there were errors in some previous passes / stages.

[fmease]

Minimized:

//@ edition: 2021

struct Ty;

impl TryFrom<Ty> for u8 {
    type Error = Ty;
    fn try_from(_: Ty) -> Result<Self, Self::Error> {
        loop {}
    }
}

impl TryFrom<Ty> for u8 {
    type Error = Ty;
    fn try_from(_: Ty) -> Result<Self, Self::Error> {
        loop {}
    }
}

fn main() {}