I don't know if this is related to previous bugs with the same name, so filing a new one.
alexbool@alexbool-osx ~> rustup -v update nightly
verbose: read metadata version: '12'
verbose: updating existing install for 'nightly-x86_64-apple-darwin'
verbose: toolchain directory: '/Users/alexbool/.multirust/toolchains/nightly-x86_64-apple-darwin'
info: syncing channel updates for 'nightly-x86_64-apple-darwin'
verbose: creating temp file: /Users/alexbool/.multirust/tmp/xzj52e23ecrqyt5o_file
verbose: downloading file from: 'https://static.rust-lang.org/dist/channel-rust-nightly.toml.sha256'
verbose: deleted temp file: /Users/alexbool/.multirust/tmp/xzj52e23ecrqyt5o_file
verbose: creating temp file: /Users/alexbool/.multirust/tmp/b6yr1ng0mairn39o_file.toml
verbose: downloading file from: 'https://static.rust-lang.org/dist/channel-rust-nightly.toml'
verbose: deleted temp file: /Users/alexbool/.multirust/tmp/b6yr1ng0mairn39o_file.toml
error: checksum failed, expected: '3721ec0a1b3a063da1f4fe594a0f4afdefefcb6fd4c3453b848efd980e4530a9', calculated: 'dc069d0e5b653add4afee522e2f139a9f1479ca5967694e9ef351e1a5648b0b1'