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> You can still have use-after-free bugs in a GCed language. A trivial example is calling methods on a closed/disposed/??? socket, file, or other handle. What

by Rochus 5y ago

> You can still have use-after-free bugs in a GCed language. A trivial example is calling methods on a closed/disposed/??? socket, file, or other handle. What prevents you from doing such things in Rust?

MaulingMonkey 5y ago
Such operations often consume `self` in Rust, resulting in compile time errors: use std::io::{self, *}; fn main() -> io::Result<()> { let mut f = std::fs::File::create("asdf.txt")?; writeln!(f, "asdf")?; drop(f); // closes asdf.txt writeln!(f, "asdf")?; // error[E0382]: borrow of moved value: `f` Ok(()) } Full diagnostics: error[E0382]: borrow of moved value: `f` --> src/main.rs:7:5 | 4 | let mut f = std::fs::File::create("asdf")?; | ----- move occurs because `f` has type `File`, which does not implement the `Copy` trait 5 | writeln!(f, "asdf")?; 6 | drop(f); | - value moved here 7 | writeln!(f, "asdf")?; | ^^^^^^^^^^^^^^^^^^^ value borrowed here after move | = note: this error originates in the macro `writeln` (in Nightly builds, run with -Z macro-backtrace for more info) For more information about this error, try `rustc --explain E0382`. Of course, a socket timing out or a drive getting unmounted could still yank a handle out from under these APIs, and e.g. C++ still allows for using such zombie objects. There are also ways to help catch such bugs in GCed languages as well (be it ObjectDisposedException making such bugs shallower in C#, or using lambdas/blocks for scopes to turn continued object use into a syntax error:) # Ruby File.open("asdf.txt", "w") do |f| f.write("asdf") end f.write("asdf") # f wasn't in scope Ruby's dynamic nature means this will still fail at runtime, but similar patterns in C# can force compile time syntax errors for use after close. EDIT: You even see this pattern in Rust, occasionally, to limit reference lifetimes to avoid soundness issues (e.g. all the annoying `.with(...)` blocks you see when using thread_local! : https://doc.rust-lang.org/std/thread/struct.LocalKey.html https://doc.rust-lang.org/std/thread/struct.LocalKey.html ).
- Rochus 5y ago
  That's interesting, thanks; with a GC language one would raise a runtime exception instead.

by Rochus 5y ago

MaulingMonkey 5y ago

Such operations often consume `self` in Rust, resulting in compile time errors: use std::io::{self, *}; fn main() -> io::Result<()> { let mut f = std::fs::File::create("asdf.txt")?; writeln!(f, "asdf")?; drop(f); // closes asdf.txt writeln!(f, "asdf")?; // error[E0382]: borrow of moved value: `f` Ok(()) } Full diagnostics: error[E0382]: borrow of moved value: `f` --> src/main.rs:7:5 | 4 | let mut f = std::fs::File::create("asdf")?; | ----- move occurs because `f` has type `File`, which does not implement the `Copy` trait 5 | writeln!(f, "asdf")?; 6 | drop(f); | - value moved here 7 | writeln!(f, "asdf")?; | ^^^^^^^^^^^^^^^^^^^ value borrowed here after move | = note: this error originates in the macro `writeln` (in Nightly builds, run with -Z macro-backtrace for more info) For more information about this error, try `rustc --explain E0382`. Of course, a socket timing out or a drive getting unmounted could still yank a handle out from under these APIs, and e.g. C++ still allows for using such zombie objects. There are also ways to help catch such bugs in GCed languages as well (be it ObjectDisposedException making such bugs shallower in C#, or using lambdas/blocks for scopes to turn continued object use into a syntax error:) # Ruby File.open("asdf.txt", "w") do |f| f.write("asdf") end f.write("asdf") # f wasn't in scope Ruby's dynamic nature means this will still fail at runtime, but similar patterns in C# can force compile time syntax errors for use after close. EDIT: You even see this pattern in Rust, occasionally, to limit reference lifetimes to avoid soundness issues (e.g. all the annoying `.with(...)` blocks you see when using thread_local! : https://doc.rust-lang.org/std/thread/struct.LocalKey.html https://doc.rust-lang.org/std/thread/struct.LocalKey.html ).

Rochus 5y ago
That's interesting, thanks; with a GC language one would raise a runtime exception instead.