core: implement float conversion methods

[valentynkit]

Implements the accepted ACP rust-lang/libs-team#810, adding float conversion methods on f16, f32, f64, and f128:

• cast converts to the target float type (self as Flt, IEEE rounding).
• to_int_saturating converts to an integer with as semantics: saturating, and NaN to 0.
• to_int_checked truncates toward zero and returns None on NaN, infinity, or out of range.
• to_int_strict is to_int_checked().unwrap().

Per the libs-api meetings on the ACP, to_float is named cast and to_int_saturating maps NaN to 0 rather than panicking (which the team noted makes rust-lang/libs-team#831 redundant).

The integer-to-float cast from the ACP is left out: it shares the cast method name on integer types with the integer-to-integer cast in rust-lang/libs-team#811 (being unified with the other integer cast methods in rust-lang/libs-team#833), so it needs a decision on how those fit together. Happy to follow up once that settles.

Tracking issue: #159913

r? libs

[valentynkit]

The Codegen Cranelift failure was a pre-existing cg_clif bug in f128 as {i8,i16,u8,u16}, unrelated to this PR.
Reported in #1680 fixed by rust-lang/rustc_codegen_cranelift@29b920534.
Probably need time to sync the fix from latest changes in rustc codegen cranelift?..

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] corebenches test:true 9.119
[RUSTC-TIMING] allocbenches test:true 8.023
[RUSTC-TIMING] alloctests test:true 18.376
[RUSTC-TIMING] alloctests test:true 19.983
error: VerifierErrors([VerifierError { location: inst22, context: Some("v19 = select.i32 v18, v16, v14  ; v16 = 255"), message: "arg 2 (v14) has type i8, expected i32" }])

error: cranelift verify error:
       function u0:2537(f128) -> i8 uext, i8 system_v {
           ss0 = explicit_slot 2
           gv0 = symbol colocated userextname5
           sig0 = (f128, f128) -> f128 system_v
           sig1 = (f128, f128) -> i32 system_v
           sig2 = (f128, f128) -> i32 system_v
           sig3 = (f128) -> i8 system_v
           sig4 = (f128, f128) -> i32 system_v
           fn0 = u0:6734 sig0
           fn1 = u0:6735 sig1
           fn2 = u0:6736 sig2
           fn3 = u0:6737 sig3
           fn4 = u0:6738 sig4
           const0 = 0x00000000000000000000000000000000
           const1 = 0xbfff0000000000000000000000000000
           const2 = 0x40070000000000000000000000000000
       
                                       block0(v0: f128):
                                           v1 -> v0
                                           v8 -> v0
                                           v13 -> v0
                                           nop 
                                           jump block1
       
                                       block1:
                                           nop 
       @0000                               v2 = f128const const0
       @0000                               v3 = call fn0(v1, v2)  ; v2 = 0.0
       @0000                               v4 = f128const const1
       @0000                               v5 = call fn1(v3, v4)  ; v4 = -0x1.0000000000000000000000000000p0
       @0000                               v6 = iconst.i32 0
       @0000                               v7 = icmp sgt v5, v6  ; v6 = 0
       @0000                               brif v7, block2, block4
       
                                       block2:
       @0000                               nop 
       @0002                               v9 = f128const const2
       @0002                               v10 = call fn2(v8, v9)  ; v9 = 0x1.0000000000000000000000000000p8
       @0002                               v11 = iconst.i32 0
       @0002                               v12 = icmp slt v10, v11  ; v11 = 0
       @0002                               brif v12, block3, block5
       
                                       block3:
       @0002                               nop 
       @0003                               v14 = call fn3(v13)
       @0003                               v15 = iconst.i32 0
       @0003                               v16 = iconst.i32 255
       @0003                               v17 = iconst.i8 -1
       @0003                               v18 = icmp ugt v14, v17  ; v17 = -1
       @0003                               v19 = select v18, v16, v14  ; v16 = 255
       ;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       ; error: inst22 (v19 = select.i32 v18, v16, v14  ; v16 = 255): arg 2 (v14) has type i8, expected i32
       
       @0003                               v20 = ireduce.i8 v19
       @0003                               v21 = call fn4(v13, v13)
       @0003                               v22 = iconst.i32 0
       @0003                               v23 = icmp eq v21, v22  ; v22 = 0
       @0003                               v24 = iconst.i8 0
       @0003                               v25 = select v23, v20, v24  ; v24 = 0
       @0004                               v26 = stack_addr.i64 ss0+1
       @0004                               store notrap v25, v26
       @0004                               v27 = iconst.i8 1
       @0004                               v28 = stack_addr.i64 ss0
       @0004                               store notrap v27, v28  ; v27 = 1
       @0006                               v29 = stack_addr.i64 ss0
       @0006                               v30 = load.i8 notrap v29
       @0006                               v31 = stack_addr.i64 ss0+1
       @0006                               v32 = load.i8 notrap v31
       @0006                               return v30, v32
       
                                       block4:
       @0006                               nop 
       @0007                               jump block5
       
                                       block5:
       @0007                               nop 
       @0008                               v33 = symbol_value.i64 gv0
       @0008                               v34 = load.i8 notrap v33
       @0008                               v35 = load.i8 notrap v33+1
       @0008                               v36 = stack_addr.i64 ss0
       @0008                               store notrap v34, v36
       @0008                               v37 = stack_addr.i64 ss0+1
       @0008                               store notrap v35, v37
       @0006                               v38 = stack_addr.i64 ss0
       @0006                               v39 = load.i8 notrap v38
       @0006                               v40 = stack_addr.i64 ss0+1
       @0006                               v41 = load.i8 notrap v40
       @0006                               return v39, v41
       
                                       block6:
       @0006                               nop 
       @000a                               v42 = stack_addr.i64 ss0
       @000a                               v43 = load.i8 notrap v42
       @000a                               v44 = stack_addr.i64 ss0+1
       @000a                               v45 = load.i8 notrap v44
       @000a                               return v43, v45
       }
       
       ; 1 verifier error detected (see above). Compilation aborted.
       

fn <f128 as std::convert::FloatToInt<u8>>::to_int_checked(_1: f128) -> std::option::Option<u8> {
    debug self => _1;
    let mut _0: std::option::Option<u8>;
    let mut _2: bool;
    let mut _3: f128;
    let mut _4: bool;
    let mut _5: u8;

    bb0: {
        StorageLive(_2);
        StorageLive(_3);
        _3 = Sub(copy _1, const 0f128);
        _2 = Gt(move _3, const -1f128);
        switchInt(move _2) -> [0: bb3, otherwise: bb1];
    }

    bb1: {
        StorageDead(_3);
        StorageLive(_4);
        _4 = Lt(copy _1, const 256f128);
        switchInt(move _4) -> [0: bb4, otherwise: bb2];
    }

    bb2: {
        StorageLive(_5);
        _5 = copy _1 as u8 (FloatToInt);
---
        return;
    }
}

alloc156042 (size: 2, align: 1) {
    00 __                                           │ .░
}

[RUSTC-TIMING] coretests test:true 90.314
error: could not compile `coretests` (test "coretests") due to 2 previous errors
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Zannotate-moves\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.99.0-nightly\t(a56f74a57\t2026-08-12)" CARGO_ENCODED_RUSTFLAGS="-Zannotate-moves\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-21/lib\u{1f}-Cllvm-args=-import-instr-limit=10\u{1f}-Clink-args=-Wl,-z,origin\u{1f}-Clink-args=-Wl,-rpath,$ORIGIN/../lib\u{1f}-Alinker-messages\u{1f}--cap-lints=allow\u{1f}--cfg\u{1f}randomized_layouts" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustdoc-clif" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "test" "--manifest-path" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests/Cargo.toml" "--target-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests_target" "--locked" "--target" "aarch64-unknown-linux-gnu" "-p" "coretests" "-p" "alloctests" "--tests" "--" "-q" exited with status ExitStatus(unix_wait_status(25856))
Bootstrap failed while executing `--stage 2 test --skip tidy --skip intrinsic-test --skip src/tools/rust-analyzer --skip tests --skip library --skip tidyselftest`
Currently active steps:
test::CodegenCranelift { compilers: RustcPrivateCompilers { build_compiler: Compiler { stage: 1, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target_compiler: Compiler { stage: 2, host: aarch64-unknown-linux-gnu, forced_compiler: false } }, target: aarch64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:4321
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:4335:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:4380:26

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:21:35
  local time: Wed Aug 12 16:41:47 UTC 2026
  network time: Wed, 12 Aug 2026 16:41:47 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[JohnTitor]

Seems still failed, is it synced yet or any other issues happen?

[valentynkit]

Seems still failed, is it synced yet or any other issues happen?

Not synced yet. The fix is rust-lang/rustc_codegen_cranelift@29b9205 and the last subtree sync was #159712.
So current CI failures, are still because of this bug, when it will be synced it should pass.
I re triggered the CI and expected it to be already synced, but it is not.

[JohnTitor]

Alright, cc @bjorn3 for awareness
marking as blocked until it gets synced.