Fix how nursery GCs are triggered (#727)

Closes #723.

.github/workflows/api-check.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           # We need nightly for cargo-public-api to get the API output.
-          toolchain: nightly
+          toolchain: nightly-2023-01-04
           profile: minimal
           # It is not necessary to use nightly as default (which is used to install cargo-public-api and compile our code).
           # However, our current toolchain is 1.59.0, and cargo-public-api requires 1.60 at least. To make it simple,
@@ -32,6 +32,6 @@ jobs:
 
       # Install cargo-public-api
       - name: Install cargo-public-api
-        run: cargo install cargo-public-api
+        run: cargo install cargo-public-api --version 0.26.0
       - name: API Diff
-        run: cargo public-api --diff-git-checkouts origin/${GITHUB_BASE_REF} ${{ github.event.pull_request.head.sha }} --deny=all
+        run: cargo public-api diff origin/${GITHUB_BASE_REF}..${{ github.event.pull_request.head.sha }} --deny=all

src/plan/generational/global.rs

@@ -149,7 +149,7 @@ impl<VM: VMBinding> Gen<VM> {
     pub fn requires_full_heap_collection<P: Plan>(&self, plan: &P) -> bool {
         // Allow the same 'true' block for if-else.
         // The conditions are complex, and it is easier to read if we put them to separate if blocks.
-        #[allow(clippy::if_same_then_else)]
+        #[allow(clippy::if_same_then_else, clippy::needless_bool)]
         let is_full_heap = if crate::plan::generational::FULL_NURSERY_GC {
             // For barrier overhead measurements, we always do full gc in nursery collections.
             true
@@ -175,7 +175,12 @@ impl<VM: VMBinding> Gen<VM> {
         } else if self.virtual_memory_exhausted(plan) {
             true
         } else {
-            plan.get_total_pages() <= plan.get_reserved_pages()
+            // We use an Appel-style nursery. The default GC (even for a "heap-full" collection)
+            // for generational GCs should be a nursery GC. A full-heap GC should only happen if
+            // there is not enough memory available for allocating into the nursery (i.e. the
+            // available pages in the nursery are less than the minimum nursery pages), if the
+            // virtual memory has been exhausted, or if it is an emergency GC.
+            false
         };
 
         self.gc_full_heap.store(is_full_heap, Ordering::SeqCst);