rustc suggest adding an import of compiler_builtins

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

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.

@tgross35

cc @tgross35

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

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.

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.

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

This has happened in the past...

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:

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)

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.
Write a test that reproduces the error without the fix (see RFC3516)

Medium-hard?

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

@rustbot

@rustbot claim

@cyrgani

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

Looking at the issue @cyrgani linked, the diagnostic has been an issue back to 1.56 https://rust.godbolt.org/z/cE9YeoqYr. The relevant bit here is just that the relevant trait isn't gated at the AST level starting with that PR. It's still gated by a feature, but apparently being in a private crate behind a disabled cfg is no match for our enthusiastic diagnostics.

Linking some zulip discussion: #t-compiler/help > compiler_builtins isn't a private dependency

Rollup merge of rust-lang#143038 - Qelxiros:142676-private-dependency…

Auto merge of #143038 - Qelxiros:142676-private-dependency-traits, r=…

Rollup merge of rust-lang#143038 - Qelxiros:142676-private-dependency…

Unrolled build for #143038

Rollup merge of #143038 - Qelxiros:142676-private-dependency-traits, …

Rollup merge of rust-lang#143038 - Qelxiros:142676-private-dependency…

Can someone reopen? I get now similar issue.
If i add in random place of my crate f64::PI, it suggests me:

error[E0599]: no associated item named `PI` found for type `f64` in the current scope
  --> src/num.rs:77:14
   |
77 |         f64::PI;
   |              ^^ associated item not found in `f64`
   |
   = help: items from traits can only be used if the trait is in scope
help: trait `Float` which provides `PI` is implemented but not in scope; perhaps you want to import it
   |
 1 + use compiler_builtins::math::libm_math::support::float_traits::Float;
   |

No one of my crates use compiler builtins, and my Cargo.lock dont have compiler_builtins too.
My crate: https://github.com/suprohub/cos

@suprohub

@suprohub could you open a new issue for this? Please include the rustc version you used and steps/code to reproduce as well.

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rustc suggest adding an import of `compiler_builtins` #142676

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

1 remaining item

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	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")
	}
	);

rustc suggest adding an import of compiler_builtins #142676

Description

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

Activity

ChrisDenton commented on Jun 19, 2025

workingjubilee commented on Jun 19, 2025

ChrisDenton commented on Jun 19, 2025

tgross35 commented on Jun 19, 2025

tgross35 commented on Jun 19, 2025

estebank commented on Jun 20, 2025

tgross35 commented on Jun 24, 2025

cyrgani commented on Jun 25, 2025

Qelxiros commented on Jun 25, 2025

1 remaining item

tgross35 commented on Jun 25, 2025

suprohub commented on Oct 17, 2025

tgross35 commented on Oct 20, 2025

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rustc suggest adding an import of `compiler_builtins` #142676