upgrading to rustc v1.47 has broken builds on master branch

[coriolinus]

$ git rev-parse --short HEAD
ba0d1a32

$ cargo check --all
<snip>
error: failed to run custom build command for `kusama-runtime v0.8.25 (/home/coriolinus/Documents/Projects/paritytech/polkadot/runtime/kusama)`

Caused by:
  process didn't exit successfully: `/home/coriolinus/Documents/Projects/paritytech/polkadot/target/debug/build/kusama-runtime-56e21cfea350959f/build-script-build` (exit code: 1)
  --- stdout
  Executing build command: "rustup" "run" "nightly" "cargo" "rustc" "--target=wasm32-unknown-unknown" "--manifest-path=/home/coriolinus/Documents/Projects/paritytech/polkadot/target/debug/wbuild/kusama-runtime/Cargo.toml" "--color=always" "--release"

  --- stderr
     Compiling wasm-build-runner-impl v1.0.0 (/home/coriolinus/Documents/Projects/paritytech/polkadot/target/debug/wbuild-runner/kusama-runtime15573142716318317940)
      Finished dev [unoptimized + debuginfo] target(s) in 0.79s
       Running `/home/coriolinus/Documents/Projects/paritytech/polkadot/target/debug/wbuild-runner/kusama-runtime15573142716318317940/target/x86_64-unknown-linux-gnu/debug/wasm-build-runner-impl`
     Compiling sp-arithmetic v2.0.0 (https://github.com/paritytech/substrate?branch=master#81f3e7ca)
  error[E0282]: type annotations needed
      --> /home/coriolinus/.cargo/git/checkouts/substrate-7e08433d4c370a21/81f3e7c/primitives/arithmetic/src/fixed_point.rs:541:9
       |
  541  |                   let accuracy = P::ACCURACY.saturated_into();
       |                       ^^^^^^^^ consider giving `accuracy` a type
  ...
  1595 | / implement_fixed!(
  1596 | |     FixedI64,
  1597 | |     test_fixed_i64,
  1598 | |     i64,
  ...    |
  1601 | |     "_Fixed Point 64 bits signed, range = [-9223372036.854775808, 9223372036.854775807]_",
  1602 | | );
       | |__- in this macro invocation
       |
       = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

  error[E0282]: type annotations needed
      --> /home/coriolinus/.cargo/git/checkouts/substrate-7e08433d4c370a21/81f3e7c/primitives/arithmetic/src/fixed_point.rs:541:9
       |
  541  |                   let accuracy = P::ACCURACY.saturated_into();
       |                       ^^^^^^^^ consider giving `accuracy` a type
  ...
  1604 | / implement_fixed!(
  1605 | |     FixedI128,
  1606 | |     test_fixed_i128,
  1607 | |     i128,
  ...    |
  1611 | |         [-170141183460469231731.687303715884105728, 170141183460469231731.687303715884105727]_",
  1612 | | );
       | |__- in this macro invocation
       |
       = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

  error[E0282]: type annotations needed
      --> /home/coriolinus/.cargo/git/checkouts/substrate-7e08433d4c370a21/81f3e7c/primitives/arithmetic/src/fixed_point.rs:541:9
       |
  541  |                   let accuracy = P::ACCURACY.saturated_into();
       |                       ^^^^^^^^ consider giving `accuracy` a type
  ...
  1614 | / implement_fixed!(
  1615 | |     FixedU128,
  1616 | |     test_fixed_u128,
  1617 | |     u128,
  ...    |
  1621 | |         [0.000000000000000000, 340282366920938463463.374607431768211455]_",
  1622 | | );
       | |__- in this macro invocation
       |
       = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

  error: aborting due to 3 previous errors

  For more information about this error, try `rustc --explain E0282`.
  error: could not compile `sp-arithmetic`

  To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

[bkchr]

This is an rustc bug: rust-lang/rust#77653 rust-lang/rust#77638

[coriolinus]

In case it bites anyone else, the solution is to, in the project root, do

rustup override set nightly-2020-10-05 && rustup target add wasm32-unknown-unknown --toolchain nightly-2020-10-05