ci: Pin nightly rust version (#1124)

Nightly rust tests are failing, seemingly due to
[rust-lang/rust#125474](rust-lang/rust#125474).

This PR fixes the toolchain to the last working nightly until the
problem is resolved.


CI fail:
https://github.com/CQCL/hugr/actions/runs/9281876942/job/25538729650#step:6:856

.github/workflows/ci-rs.yml

@@ -122,7 +122,9 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        rust: ['1.75', beta, nightly]
+        # Pinned nightly version until this gets resolved:
+        # https://github.com/rust-lang/rust/issues/125474
+        rust: ['1.75', beta, 'nightly-2024-05-22']
     name: tests (Rust ${{ matrix.rust }})
     steps:
       - uses: actions/checkout@v4
@@ -176,8 +178,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: mozilla-actions/[email protected]
-      - uses: dtolnay/rust-toolchain@nightly
+      - uses: dtolnay/rust-toolchain@master
         with:
+          # Nightly is required to count doctests coverage
+          #
+          # Pinned nightly version until this gets resolved
+          # https://github.com/rust-lang/rust/issues/125474
+          toolchain: 'nightly-2024-05-22'
           components: llvm-tools-preview
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov

hugr-core/src/hugr/hugrmut.rs

@@ -222,7 +222,7 @@ pub trait HugrMut: HugrMutInternals {
 }
 
 /// Records the result of inserting a Hugr or view
-/// via [HugrMut::insert_hugr] or [HugrMut::insert_from_view]
+/// via [HugrMut::insert_hugr] or [HugrMut::insert_from_view].
 pub struct InsertionResult {
     /// The node, after insertion, that was the root of the inserted Hugr.
     ///