Remove destructuring assignment for compatibility with Rust 1.58.1 (#21)

* Avoid destructuring assignment for old rustc compatibility.

This syntax was stablized in Rust 1.59.0. Avoid it for now to
support projects with an older minimum-supported rust version.

* github actions: Test against rust 1.58.1 as MSRV

Build against an old release so there's notification when new
code requires a more recent toolchain. This helps enforce a
Minimum Supported Rust Version policy.

.github/workflows/rust.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [stable, beta, nightly]
+        rust: [stable, beta, nightly, 1.58.1]
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1

dotenv/src/iter.rs

@@ -114,7 +114,9 @@ impl<B: BufRead> Iterator for QuotedLines<B> {
                     if buf.trim_start().starts_with('#') {
                         return Some(Ok(String::with_capacity(0)));
                     }
-                    (cur_pos, cur_state) = eval_end_state(cur_state, &buf[buf_pos..]);
+                    let result = eval_end_state(cur_state, &buf[buf_pos..]);
+                    cur_pos = result.0;
+                    cur_state = result.1;
 
                     match cur_state {
                         ParseState::Complete => {