rust-lang/libm as a whole is available for use under the MIT license:

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Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
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This Rust library contains the following copyrights:

    Copyright (c) 2018 Jorge Aparicio

Portions of this software are derived from third-party works licensed under
terms compatible with the above MIT license:

* musl libc https://www.musl-libc.org/. This library contains the following
  copyright:

      Copyright © 2005-2020 Rich Felker, et al.

* The CORE-MATH project https://core-math.gitlabpages.inria.fr/. CORE-MATH
  routines are available under the MIT license on a per-file basis.

The musl libc COPYRIGHT file also includes the following notice relevant to
math portions of the library:

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Much of the math library code (src/math/* and src/complex/*) is
Copyright © 1993,2004 Sun Microsystems or
Copyright © 2003-2011 David Schultz or
Copyright © 2003-2009 Steven G. Kargl or
Copyright © 2003-2009 Bruce D. Evans or
Copyright © 2008 Stephen L. Moshier or
Copyright © 2017-2018 Arm Limited
and labelled as such in comments in the individual source files. All
have been licensed under extremely permissive terms.
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