no method named "fract" found for type "f64" in the current scope when in no_std

[hukasu]

I tried this code:

#![no_std]

fn get_fract(f: f64) -> f64 {
    f.fract()
}

I expected to see this happen: Compiles successfully

Instead, this happened: Fails to compile with E0599 "no method named fract found for type f64 in the current scope"

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: x86_64-unknown-linux-gnu
release: 1.83.0
LLVM version: 19.1.1

[hukasu]

error[E0599]: no method named `fract` found for reference `&f64` in the current scope
  --> src/ext/float.rs:12:23
   |
12 |         self.borrow().fract().classify() == FpCategory::Zero
   |                       ^^^^^ method not found in `&f64`

error[E0599]: no method named `powf` found for type `f64` in the current scope
   --> src/program/binops/mod.rs:184:49
    |
184 |             Ok(ExpDesc::Float({ *lhs_i as f64 }.powf(*rhs_i as f64)))
    |                                                 ^^^^ method not found in `f64`

error[E0599]: no method named `powf` found for reference `&f64` in the current scope
   --> src/program/binops/mod.rs:186:83
    |
186 |         (ExpDesc::Float(lhs_f), ExpDesc::Float(rhs_f)) => Ok(ExpDesc::Float(lhs_f.powf(*rhs_f))),
    |                                                                                   ^^^^ method not found in `&f64`

error[E0599]: no method named `powf` found for type `f64` in the current scope
   --> src/program/binops/mod.rs:188:47
    |
188 |             Ok(ExpDesc::Float((*lhs_i as f64).powf(*rhs_f)))
    |                                               ^^^^ method not found in `f64`

error[E0599]: no method named `powf` found for reference `&f64` in the current scope
   --> src/program/binops/mod.rs:191:37
    |
191 |             Ok(ExpDesc::Float(lhs_f.powf(*rhs_i as f64)))
    |                                     ^^^^ method not found in `&f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/program/binops/mod.rs:256:47
    |
256 |             Ok(ExpDesc::Float((lhs_f / rhs_f).trunc()))
    |                                               ^^^^^ method not found in `f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/program/binops/mod.rs:259:55
    |
259 |             Ok(ExpDesc::Float((*lhs_i as f64 / rhs_f).trunc()))
    |                                                       ^^^^^ method not found in `f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/program/binops/mod.rs:262:55
    |
262 |             Ok(ExpDesc::Float((lhs_f / *rhs_i as f64).trunc()))
    |                                                       ^^^^^ method not found in `f64`

error[E0599]: no method named `powf` found for type `f64` in the current scope
   --> src/lib.rs:305:54
    |
305 | ...                   Value::Float((*l as f64).powf(*r as f64))
    |                                                ^^^^ method not found in `f64`

error[E0599]: no method named `powf` found for reference `&f64` in the current scope
   --> src/lib.rs:307:78
    |
307 |                         (Value::Float(l), Value::Float(r)) => Value::Float(l.powf(*r)),
    |                                                                              ^^^^ method not found in `&f64`

error[E0599]: no method named `powf` found for type `f64` in the current scope
   --> src/lib.rs:308:90
    |
308 |                         (Value::Integer(l), Value::Float(r)) => Value::Float((*l as f64).powf(*r)),
    |                                                                                          ^^^^ method not found in `f64`

error[E0599]: no method named `powf` found for reference `&f64` in the current scope
   --> src/lib.rs:309:80
    |
309 |                         (Value::Float(l), Value::Integer(r)) => Value::Float(l.powf(*r as f64)),
    |                                                                                ^^^^ method not found in `&f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/lib.rs:355:84
    |
355 |                         (Value::Float(l), Value::Float(r)) => Value::Float((l / r).trunc()),
    |                                                                                    ^^^^^ method not found in `f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/lib.rs:357:58
    |
357 | ...                   Value::Float((*l as f64 / r).trunc())
    |                                                    ^^^^^ method not found in `f64`

error[E0599]: no method named `trunc` found for type `f64` in the current scope
   --> src/lib.rs:360:58
    |
360 | ...                   Value::Float((l / *r as f64).trunc())
    |                                                    ^^^^^ method not found in `f64`

[hukasu]

This is when running cargo check and cargo build on a lib, when running cargo test it compiles just fine.

[jieyouxu]

Triage: I think f64::fract is in std but not core. Tagging T-libs/T-libs-api. Someone more familiar on the libs side of things will know better.

[FractalFir]

EDIT: Irrelevant, I tought I found something possibly related, but I was wrong.

[hukasu]

i saw a change in behavior between 1.82 and 1.83 though

[zachs18]

The example in the OP also fails on Rust 1.82.0: godbolt link. Can you provide an example of the behavior change you saw between 1.82.0 and 1.83.0?

[hukasu]

weird, i reverted back to 1.82 and i does error. did i never run cargo check on that code while on 1.82 to notice? and when compiling the tests, since the tests are not no_std they do not fail.

what are the alternatives for those methods in no_std?

[hukasu]

running a example that has #![no_std] still compiles and runs normally

[hukasu]

Command	No arguments	--examples	--example hello
cargo check	❌	✅	✅
cargo build	❌	✅	✅
cargo run	Can't because it is a lib	Can't run all examples at once	✅
cargo test	✅	✅	✅

the examples don't have tests, but they compile normally, the hello example has #![no_std]

[zachs18]

Given this project source (created using cargo new --lib f64-thing and modified from there), I get "E0599: no method named fract found for type f64 in the current scope" on all relevant invocations. If you have a different observation for your project, can you please provide your project's source code?

project source

f64-thing/
├── Cargo.lock
├── Cargo.toml
├── examples
│   └── hello.rs
└── src
    └── lib.rs

// lib.rs
#![no_std]

pub fn fract(f: f64) -> f64 {
    f.fract()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = fract(2.5);
        assert_eq!(result, 0.5);
    }
}

// examples/hello.rs
fn main() {}

Command	No args	--examples	--example hello
cargo check	E0599	E0599	E0599
cargo build	E0599	E0599	E0599
cargo run	N/A	N/A	E0599
cargo test	E0599	E0599	E0599

---

what are the alternatives for those methods in no_std?

For some functions, you can use the pure-rust libm crate, but it does not appear to have fract specifically (though it does have trunc, so you can do x - x.trunc()).

[saethlin]

Isn't this thread just rediscovering #50145?

[jieyouxu]

Triage: AFAICT this is intentional, marking as C-discussion for now.