`useless_let_if_seq` ignores pattern if any expr is between variable declaration and conditional assignment.

Summary

Lint is ignored if any expression (pure or not) is between a variable declaration and the conditional assignment.

Lint Name

useless_let_if_seq

Reproducer

I tried this code:

let foo;
let _ = bar();

if bar() {
    foo = 42;
} else {
    foo = 0;
}

let mut baz = None;

if bar() {
    baz = Some(42);
}

I expected to see this happen:

warning: `if _ { .. } else { .. }` is an expression
  --> src/lib.rs:5:5
   |
5  | /     let foo;
6  | |     let _ = bar();
7  | |
8  | |     if bar() {
...  |
11 | |         foo = 0;
12 | |     }
   | |_____^ help: it is more idiomatic to write: `let foo = if bar() { 42 } else { 0 };`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_let_if_seq
note: the lint level is defined here
  --> src/lib.rs:1:9
   |
1  | #![warn(clippy::useless_let_if_seq)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `if _ { .. } else { .. }` is an expression
  --> src/lib.rs:14:5
   |
14 | /     let mut baz = None;
15 | |
16 | |     if bar() {
17 | |         baz = Some(42);
18 | |     }
   | |_____^ help: it is more idiomatic to write: `let <mut> baz = if bar() { Some(42) } else { None };`
   |
   = note: you might not need `mut` at all
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_let_if_seq

warning: `playground` (lib) generated 2 warnings

Instead, this happened:
Only the second warning appeared, due to the statement between the declaration and assignment.

Version

rustc 1.75.0-nightly (bf9a1c8a1 2023-10-08)
binary: rustc
commit-hash: bf9a1c8a193fc373897196321215794c8bebbeec
commit-date: 2023-10-08
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.2

Start by locating the implementation and regression tests for the useless_let_if_seq lint, then reproduce the issue with the provided Rust example. Trace why an intervening expression prevents the first warning, and add coverage so the lint reports both cases while preserving the existing behavior for the second conditional assignment.