[ci] Skip some matrix combinations (#1596)

This commit modifies our CI `build_test` matrix job to skip some matrix
combinations that are unlikely to be useful. This should cut down on the
total number of matrix combinations and buy us runway before we exceed
the limit of 256 total combinations, as happened in [1].

[1] https://github.com/google/zerocopy/actions/runs/10477927587

.github/workflows/ci.yml

@@ -46,7 +46,18 @@ jobs:
       matrix:
         # See `INTERNAL.md` for an explanation of these pinned toolchain
         # versions.
-        toolchain: [ "msrv", "stable", "nightly",  "zerocopy-generic-bounds-in-const-fn", "zerocopy-aarch64-simd", "zerocopy-panic-in-const", ]
+        toolchain: [
+          "msrv",
+          "stable",
+          "nightly",
+
+          # These are the names of specific Rust versions detected in
+          # `build.rs`. Each of these represents the minimum Rust version for
+          # which a particular feature is supported.
+          "zerocopy-generic-bounds-in-const-fn",
+          "zerocopy-aarch64-simd",
+          "zerocopy-panic-in-const"
+        ]
         target: [
           "i686-unknown-linux-gnu",
           "x86_64-unknown-linux-gnu",
@@ -59,7 +70,12 @@ jobs:
           "x86_64-pc-windows-msvc",
           "wasm32-wasi"
         ]
-        features: [ "--no-default-features", "", "--features __internal_use_only_features_that_work_on_stable", "--all-features" ]
+        features: [
+          "--no-default-features",
+          "",
+          "--features __internal_use_only_features_that_work_on_stable",
+          "--all-features"
+        ]
         crate: [ "zerocopy", "zerocopy-derive" ]
         event_name: [ "${{ github.event_name }}" ]
         exclude:
@@ -93,6 +109,46 @@ jobs:
             toolchain: "zerocopy-aarch64-simd"
           - crate: "zerocopy-derive"
             toolchain: "zerocopy-panic-in-const"
+          # Exclude non-aarch64 targets from the `zerocopy-aarch64-simd`
+          # toolchain.
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "i686-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "x86_64-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "arm-unknown-linux-gnueabi"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "powerpc-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "powerpc64-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "riscv64gc-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "s390x-unknown-linux-gnu"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "x86_64-pc-windows-msvc"
+          - toolchain: "zerocopy-aarch64-simd"
+            target: "wasm32-wasi"
+          # Exclude most targets targets from the
+          # `zerocopy-generic-bounds-in-const-fn` toolchain since the
+          # `zerocopy-generic-bounds-in-const-fn` feature is unrelated to
+          # compilation target. This only leaves i686 and x86_64 targets.
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "arm-unknown-linux-gnueabi"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "aarch64-unknown-linux-gnu"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "powerpc-unknown-linux-gnu"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "powerpc64-unknown-linux-gnu"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "riscv64gc-unknown-linux-gnu"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "s390x-unknown-linux-gnu"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "x86_64-pc-windows-msvc"
+          - toolchain: "zerocopy-generic-bounds-in-const-fn"
+            target: "wasm32-wasi"
           # Exclude most targets during PR development, but allow them in the
           # merge queue. This speeds up our development flow, while still
           # ensuring that errors on these targets are caught before a PR is