core::num::f16b Rust's 16bit Brain Float

[Jamesbarford]

Implements the RFC: f16b type. Best reviewed commit by commit, happy to split into separate PRs if that is deemed easier to review. However the line count and surface area is, in my opinion, reasonably small.

Adds;

• ABI plumbing for the f16b along with bfloat lang item to work with LLVM, GCC is explicitly unimplemented!(...)
• f16b feature gate, page for f16b on libruscdoc and a struct bf16 in core::num
• Tests
• Treat f16b as a scalar primitive for scalable vectors

Issues;

• Tracking Issue
• RFC

[rustbot]

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

[rustbot]

r? @jieyouxu

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
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[jieyouxu]

@rustbot reroll

[antoyo]

I believe GCC actually supports this type: https://github.com/rust-lang/gccjit.rs/blob/master/src/context.rs#L1482

View changes since the review

[rustbot]

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

[bjorn3]

You probably also want to add a check that you can't take a reference to the inner field just like for #[repr(simd)].

View changes since the review

[bjorn3]

What is the calling convention of other targets?

View changes since the review

[rust-log-analyzer]

The job x86_64-gnu-gcc failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [ui] tests/ui/float/f16b.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/float/f16b.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/float/f16b/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
##[error]error: internal compiler error: compiler/rustc_codegen_gcc/src/type_.rs:160:9: f16b is not supported by the GCC codegen backend


thread 'rustc' (83853) panicked at compiler/rustc_codegen_gcc/src/type_.rs:160:9:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   3: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   4: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   5: rustc_middle::util::bug::bug_fmt
   6: <rustc_codegen_gcc::context::CodegenCx as rustc_codegen_ssa::traits::type_::BaseTypeCodegenMethods>::type_f16b
   7: <rustc_codegen_gcc::context::CodegenCx as rustc_codegen_ssa::traits::type_::DerivedTypeCodegenMethods>::type_from_float
   8: <rustc_abi::layout::ty::TyAndLayout<rustc_middle::ty::Ty> as rustc_codegen_gcc::type_of::LayoutGccExt>::gcc_type
   9: <rustc_codegen_ssa::mir::operand::OperandRef<gccjit::rvalue::RValue>>::from_const::<rustc_codegen_gcc::builder::Builder>
  10: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::eval_mir_constant_to_operand
  11: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_operand
  12: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_block
  13: rustc_codegen_ssa::mir::codegen_mir::<rustc_codegen_gcc::builder::Builder>
  14: rustc_codegen_ssa::base::codegen_instance::<rustc_codegen_gcc::builder::Builder>
  15: <rustc_middle::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define::<rustc_codegen_gcc::builder::Builder>
  16: rustc_codegen_gcc::base::compile_codegen_unit::module_codegen
  17: rustc_codegen_gcc::base::compile_codegen_unit
  18: <rustc_codegen_gcc::GccCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  19: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  20: rustc_interface::passes::start_codegen
  21: <rustc_interface::queries::Linker>::codegen_and_build_linker
  22: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}>::{closure#2}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>
  23: <rustc_middle::ty::context::TyCtxt>::create_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}>::{closure#2}>
  24: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}>
  25: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
  26: rustc_span::create_session_globals_then::<(), rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.99.0-nightly (12f17464f 2026-08-10) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z simulate-remapped-rust-src-base=/rustc/FAKE_PREFIX -Z translate-remapped-path-to-local-path=no -Z ignore-directory-in-diagnostics-source-blocks=/cargo -Z ignore-directory-in-diagnostics-source-blocks=/checkout/vendor -Z codegen-backend=gcc -C codegen-units=1 -Z ui-testing -Z deduplicate-diagnostics=no -Z write-long-types-to-disk=no -C strip=debuginfo -C prefer-dynamic -C rpath -C debuginfo=0

query stack during panic:
end of query stack
error: aborting due to 1 previous error
------------------------------------------
---
diff of stdout:

15     /*
16     0#0
17     */ {
-     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0 */>)
-     =>
+     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0
+     */>) =>
20     { fn /* 0#0 */ $f /* 0#0 */<$a /* 0#0 */, 'a /* 0#0 */>() {} }
21 }
22 fn f /* 0#0 */<'a /* 0#0 */, 'a /* 0#1 */>() {}

Note: some mismatched output was normalized before being compared
-     ($f /* 2354#0 */:ident /* 1000#0 */<$a /* 2349#0 */:lifetime /* 1097#0
+     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0
+     */>) =>


The actual stdout differed from the expected stdout
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args hygiene/unpretty-debug-lifetimes.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/hygiene/unpretty-debug-lifetimes.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "-Zcodegen-backend=gcc" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/hygiene/unpretty-debug-lifetimes" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-Zunpretty=expanded,hygiene"
--- stdout -------------------------------
//@ check-pass
//@ compile-flags: -Zunpretty=expanded,hygiene

// Regression test for lifetime hygiene annotations in -Zunpretty=expanded,hygiene
// Previously, lifetimes were missing the #N syntax context suffix.

// Don't break whenever Symbol numbering changes
//@ normalize-stdout: "\d+#" -> "0#"

#![feature /* 860#0 */(decl_macro /* 694#0 */)]
#![feature /* 860#0 */(no_core /* 1333#0 */)]
#![no_core /* 1333#0 */]

macro lifetime_hygiene
    /*
    2941#0
    */ {
    ($f /* 2354#0 */:ident /* 1000#0 */<$a /* 2349#0 */:lifetime /* 1097#0
    */>) =>
    { fn /* 12#0 */ $f /* 2354#0 */<$a /* 2349#0 */, 'a /* 2942#0 */>() {} }
}
fn f /* 2354#0 */<'a /* 2942#0 */, 'a /* 2942#1 */>() {}


/*
Expansions:
crate0::{{expn0}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Root
crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "lifetime_hygiene")

SyntaxContexts:
#0: parent: #0, outer_mark: (crate0::{{expn0}}, Opaque)
#1: parent: #0, outer_mark: (crate0::{{expn1}}, Opaque)
*/
------------------------------------------
stderr: none

---- [ui] tests/ui/hygiene/unpretty-debug-lifetimes.rs stdout end ----

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