transpile: use built-in SIMD items from stdlib, not via reexports

these will have `use`s added unconditionally with the stdlib path if the main module uses them directly, so in order to allow deduplication without tracing through reexports, we should have the main module import these from their original (stdlib) location, not the reexported one

Author: fw-immunant

c2rust-transpile/src/translator/mod.rs

@@ -1084,8 +1084,25 @@ fn make_submodule(
             match use_idents(tree) {
                 IdentsOrGlob::Idents(idents) => {
                     for ident in idents {
-                        // Add a `use` for `self::this_module::exported_name`.
-                        use_item_store.add_use(false, use_path(), &ident.to_string());
+                        fn is_simd_type_name(name: &str) -> bool {
+                            const SIMD_TYPE_NAMES: &[&str] = &[
+                                "__m128i", "__m128", "__m128d", "__m64", "__m256", "__m256d",
+                                "__m256i",
+                            ];
+                            SIMD_TYPE_NAMES.contains(&name) || name.starts_with("_mm_")
+                        }
+                        let name = &*ident.to_string();
+                        if is_simd_type_name(name) {
+                            // Import vector type/operation names from the stdlib, as we also generate
+                            // other uses for them from that location and can't easily reason about
+                            // the ultimate target of reexported names when avoiding duplicate imports
+                            // (which are verboten).
+                            simd::add_arch_use(use_item_store, "x86", name);
+                            simd::add_arch_use(use_item_store, "x86_64", name);
+                        } else {
+                            // Add a `use` for `self::this_module::exported_name`.
+                            use_item_store.add_use(false, use_path(), name);
+                        }
                     }
                 }
                 IdentsOrGlob::Glob => {}

c2rust-transpile/src/translator/simd.rs

@@ -67,7 +67,7 @@ static SIMD_X86_64_ONLY: &[&str] = &[
     "_mm_crc32_u64",
 ];
 
-fn add_arch_use(store: &mut ItemStore, arch_name: &str, item_name: &str) {
+pub fn add_arch_use(store: &mut ItemStore, arch_name: &str, item_name: &str) {
     store.add_use_with_attr(
         true,
         vec!["core".into(), "arch".into(), arch_name.into()],