shadow_unrelated is using idents from RHS of assignment

Clippy outputs these warnings:

warning: `state
            .parser
            .as_mut()
            .ok_or(InterpreterExtractError::new())?` is being shadowed
  --> artichoke-backend/src/parser.rs:39:22
   |
39 |           let parser = state
   |  ______________________^
40 | |             .parser
41 | |             .as_mut()
42 | |             .ok_or(InterpreterExtractError::new())?;
   | |___________________________________________________^
   |
note: initialization happens here
  --> artichoke-backend/src/parser.rs:39:22
   |
39 |           let parser = state
   |  ______________________^
40 | |             .parser
41 | |             .as_mut()
42 | |             .ok_or(InterpreterExtractError::new())?;
   | |___________________________________________________^
note: previous binding is here
  --> artichoke-backend/src/parser.rs:38:21
   |
38 |         let state = self.state.as_mut().ok_or(InterpreterExtractError::new())?;
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#shadow_unrelated

warning: `u16::try_from(new)
            .map_err(|_| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?` is being shadowed
  --> artichoke-backend/src/state/parser.rs:68:21
   |
68 |           let store = u16::try_from(new)
   |  _____________________^
69 | |             .map_err(|_| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?;
   | |________________________________________________________________________________^
   |
note: initialization happens here
  --> artichoke-backend/src/state/parser.rs:68:21
   |
68 |           let store = u16::try_from(new)
   |  _____________________^
69 | |             .map_err(|_| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?;
   | |________________________________________________________________________________^
note: previous binding is here
  --> artichoke-backend/src/state/parser.rs:65:19
   |
65 |           let new = old
   |  ___________________^
66 | |             .checked_add(val)
67 | |             .ok_or_else(|| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?;
   | |__________________________________________________________________________________^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#shadow_unrelated

For this code:

    fn add_fetch_lineno(&mut self, val: usize) -> Result<usize, Self::Error> {
        let state = self.state.as_mut().ok_or(InterpreterExtractError::new())?;
        let parser = state
            .parser
            .as_mut()
            .ok_or(InterpreterExtractError::new())?;
        let lineno = parser.add_fetch_lineno(val)?;
        Ok(lineno)
    }

and

    pub fn add_fetch_lineno(&mut self, val: usize) -> Result<usize, IncrementLinenoError> {
        let old = usize::from(unsafe { self.context.as_ref() }.lineno);
        let new = old
            .checked_add(val)
            .ok_or_else(|| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?;
        let store = u16::try_from(new)
            .map_err(|_| IncrementLinenoError::Overflow(usize::from(u16::MAX)))?;
        unsafe {
            self.context.as_mut().lineno = store;
        }
        Ok(new)
    }

I expected to see this happen: No errors, as in Rust 1.46.0

Meta

• cargo clippy -V: clippy 0.0.212 (18bf6b4f0 2020-10-07)
• rustc -Vv:

  rustc 1.47.0 (18bf6b4f0 2020-10-07)
  binary: rustc
  commit-hash: 18bf6b4f01a6feaf7259ba7cdae58031af1b7b39
  commit-date: 2020-10-07
  host: x86_64-apple-darwin
  release: 1.47.0
  LLVM version: 11.0
  

Start by reproducing the shadow_unrelated warnings with the two Rust snippets and compare the behavior between Rust 1.46.0 and 1.47.0. Trace the lint's handling of assignment right-hand sides, then verify that the shown state/parser and old/new/store examples no longer produce false-positive warnings while unrelated shadowing remains diagnosed.