Add an Mmap wrapper to rustc_data_structures

This wrapper implements StableAddress and falls back to directly reading
the file on wasm32

compiler/rustc_data_structures/src/lib.rs

@@ -84,6 +84,7 @@ pub mod snapshot_map;
 pub mod stable_map;
 pub mod svh;
 pub use ena::snapshot_vec;
+pub mod memmap;
 pub mod sorted_map;
 pub mod stable_set;
 #[macro_use]

compiler/rustc_data_structures/src/memmap.rs

@@ -0,0 +1,40 @@
+use std::fs::File;
+use std::io;
+use std::ops::Deref;
+
+use crate::owning_ref::StableAddress;
+
+/// A trivial wrapper for [`memmap2::Mmap`] that implements [`StableAddress`].
+#[cfg(not(target_arch = "wasm32"))]
+pub struct Mmap(memmap2::Mmap);
+
+#[cfg(target_arch = "wasm32")]
+pub struct Mmap(Vec<u8>);
+
+#[cfg(not(target_arch = "wasm32"))]
+impl Mmap {
+    pub unsafe fn map(file: File) -> io::Result<Self> {
+        memmap2::Mmap::map(&file).map(Mmap)
+    }
+}
+
+#[cfg(target_arch = "wasm32")]
+impl Mmap {
+    pub unsafe fn map(mut file: File) -> io::Result<Self> {
+        use std::io::Read;
+
+        let mut data = Vec::new();
+        file.read_to_end(&mut data)?;
+        Ok(Mmap(data))
+    }
+}
+
+impl Deref for Mmap {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        &*self.0
+    }
+}
+
+unsafe impl StableAddress for Mmap {}