rustc suggest adding an import of `compiler_builtins`

[jendrikw]

Code

fn f() {
    u8::cast_from_lossy(9);
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0599]: no function or associated item named `cast_from_lossy` found for type `u8` in the current scope
 --> src/lib.rs:2:9
  |
2 |     u8::cast_from_lossy(9);
  |         ^^^^^^^^^^^^^^^ function or associated item not found in `u8`
  |
  = help: items from traits can only be used if the trait is in scope
help: trait `CastFrom` which provides `cast_from_lossy` is implemented but not in scope; perhaps you want to import it
  |
1 + use compiler_builtins::math::libm::support::int_traits::CastFrom;
  |

For more information about this error, try `rustc --explain E0599`.
error: could not compile `playground` (lib) due to 1 previous error

Desired output

Compiling playground v0.0.1 (/playground)
error[E0599]: no function or associated item named `cast_from_lossy` found for type `u8` in the current scope
 --> src/lib.rs:2:9
  |
2 |     u8::cast_from_lossy(9);
  |         ^^^^^^^^^^^^^^^ function or associated item not found in `u8`
  |

For more information about this error, try `rustc --explain E0599`.
error: could not compile `playground` (lib) due to 1 previous error

Rationale and extra context

The suggested import does not work. Additionally, the import will be suggested even if it's already there.

Other cases

A previously reported issue #71090 was supposed to be fixed, but maybe it regressed?

Rust Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Anything else?

No response

[ChrisDenton]

This regressed in version 1.86.

F:\scratch> cat test.rs
fn f() {
    u8::cast_from(9);
}

F:\scratch> rustc +1.86 test.rs --crate-type lib
error[E0599]: no function or associated item named `cast_from` found for type `u8` in the current scope
 --> test.rs:2:9
  |
2 |     u8::cast_from(9);
  |         ^^^^^^^^^ function or associated item not found in `u8`
  |
  = help: items from traits can only be used if the trait is in scope
help: trait `CastFrom` which provides `cast_from` is implemented but not in scope; perhaps you want to import it
  |
1 + use compiler_builtins::math::libm::support::int_traits::CastFrom;
  |

error: aborting due to 1 previous error

F:\scratch> rustc +1.85 test.rs --crate-type lib
error[E0599]: no function or associated item named `cast_from` found for type `u8` in the current scope
 --> test.rs:2:9
  |
2 |     u8::cast_from(9);
  |         ^^^^^^^^^ function or associated item not found in `u8`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0599`.

As noted in the OP, this error was previously seen in #71090 but had been fixed years ago.

[workingjubilee]

cc @tgross35

[ChrisDenton]

Bisection:

searched nightlies: from nightly-2025-01-27 to nightly-2025-06-19
regressed nightly: nightly-2025-02-26
searched commit range: 617aad8...85abb27
regressed commit: 4ecd70d

Most obvious candidate is #137515 which leads to rust-lang/compiler-builtins#766

[tgross35]

You're kidding me, still? This was supposed to be gone forever with #135501 🙃. Either compiler_builtins is getting marked public somehow, or diagnostics are suggesting from private crates when they shouldn't be.

[tgross35]

With RUSTC_LOG=rustc_metadata=info https://rust.godbolt.org/z/qcG85vvr3 I see:

   name: compiler_builtins
   cnum: 4
   hash: 9c1a86ef5a39ee579d9f9d59f9e50b8b
   reqd: Explicit
   priv: true
    rlib: /opt/compiler-explorer/rust-1.87.0/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-b91655ac28a1a56c.rlib

So it is still private at the end of resolution, meaning that the diagnostic is making suggestions from crates it shouldn't. At least resolution doesn't seem borked.

[estebank]

or diagnostics are suggesting from private crates when they shouldn't be.

This has happened in the past...

[tgross35]

I probably won't be able to look at this for a while and this isn't the highest priority, so I'll label for someone to pick it up if they get the chance. The steps are most likely:

1. The error comes from here

   rust/compiler/rustc_hir_typeck/src/method/suggest.rs

   Lines 3476 to 3508 in 3129d37

   	fn suggest_valid_traits(
   	&self,
   	err: &mut Diag<'_>,
   	item_name: Ident,
   	valid_out_of_scope_traits: Vec<DefId>,
   	explain: bool,
   	) -> bool {
   	if !valid_out_of_scope_traits.is_empty() {
   	let mut candidates = valid_out_of_scope_traits;
   	candidates.sort_by_key(|id| self.tcx.def_path_str(id));
   	candidates.dedup();
   	// `TryFrom` and `FromIterator` have no methods
   	let edition_fix = candidates
   	.iter()
   	.find(|did| self.tcx.is_diagnostic_item(sym::TryInto, **did))
   	.copied();
   	if explain {
   	err.help("items from traits can only be used if the trait is in scope");
   	}
   	let msg = format!(
   	"{this_trait_is} implemented but not in scope",
   	this_trait_is = if candidates.len() == 1 {
   	format!(
   	"trait `{}` which provides `{item_name}` is",
   	self.tcx.item_name(candidates[0]),
   	)
   	} else {
   	format!("the following traits which provide `{item_name}` are")
   	}
   	);

   . Figure out which time it is called is relevant (probably just with info! statements and RUSTC_LOG https://rustc-dev-guide.rust-lang.org/tracing.html)
2. Figure out where that valid_out_of_scope_traits comes from and why it's including private dependencies. Or maybe just filter it within suggest_valid_traits.
3. Write a test that reproduces the error without the fix (see RFC3516)

Medium-hard?

[cyrgani]

A very similar issue with object instead of compiler_builtins is #138191.

[Qelxiros]

@rustbot claim