Currently, Cargo.toml specifies Apache-2.0 OR MIT, but LICENSE.txt
describes MIT OR NCSA. compiler-builtins is derived from LLVM's
compiler-rt. LICENSE.txt correctly reflects the state of compiler-rt
prior to relicensing on 2019-01-19, during which time software was
available for use under either MIT or the University of Illinois NCSA
license. After relicensing, however, compiler-rt is available for use
only under Apache-2.0 with the LLVM exception; this is not reflected
anywhere in the repository.
Update the SPDX license identifier to `MIT AND Apache-2.0 WITH
LLVM-exception AND (MIT OR Apache-2.0)`. Each AND portion covers
something specific:
* Apache-2.0 WITH LLVM-exception: this covers work that is derived from
the LLVM repository since after the LLVM relicensing.
* MIT: This covers work that is derived from LLVM before the LLVM
relicensing (under MIT OR NCSA), as well as the vendored `libm`
components.
* MIT AND Apache-2.0: This ensures that any contributions to this
repository, in addition to meeting the above required licenses, is
also released for use under the Rust-standard Apache-2.0 with no LLVM
exception.
See also the parallel license update in rust-lang/libm [1].
Fixes: https://github.com/rust-lang/compiler-builtins/issues/307
Closes: https://github.com/rust-lang/compiler-builtins/pull/511
Link: https://rust-lang.zulipchat.com/#narrow/channel/335408-foundation/topic/Request.20for.20legal.20team.20input.20on.20crate.20licensing
Link: https://github.com/rust-lang/libm/pull/317 [1]
6399bed36c