Missing build warning "temporary_string_as_ptr" when String is used with "as_ptr" on one line

[mirao]

Playground

It's rather a suggestion than a bug.

Have this code:

use std::ffi::CString;

fn main() {
    let pointer = CString::new("Hello world!").unwrap().as_ptr();
    unsafe {
        println!("{:?}", *pointer as u8 as char);
    }
}

Rust reports the warning rustc(temporary_cstring_as_ptr) during build. The detail is this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime which is expected 🆗 .
If you run the code, a "random" character will be printed and that's of course unexpected. A solution is an assignment of the string into a standalone variable as explained here.

---

Have the same code, but this time it uses String instead of CString:

fn main() {
    let pointer = String::from("Hello world!").as_ptr();

    unsafe {
        println!("{:?}", *pointer as char);
    }
}

I expected to see this happen:

• I should get similar kind of warning as in case of CString, because also in this case the pointer is dangling and if you run the code, a random character will be printed. A solution is the same as for CString

Instead, this happened:

• Rust doesn't warn

Meta

rustc --version --verbose:

rustc 1.58.0-nightly (e249ce6b2 2021-10-30)
binary: rustc
commit-hash: e249ce6b2345587d6e11052779c86adbad626dff
commit-date: 2021-10-30
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

[fmease]

Note that miri catches this with:

error: Undefined Behavior: memory access failed: alloc859 has been freed, so this pointer is dangling
 --> src/main.rs:5:26
  |
5 |         println!("{:?}", *pointer as char);
  |                          ^^^^^^^^ memory access failed: alloc859 has been freed, so this pointer is dangling
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
help: alloc859 was allocated here:
 --> src/main.rs:2:19
  |
2 |     let pointer = String::from("Hello world!").as_ptr();
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: alloc859 was deallocated here:
 --> src/main.rs:2:56
  |
2 |     let pointer = String::from("Hello world!").as_ptr();
  |                                                        ^
  = note: BACKTRACE (of the first span):
  = note: inside `main` at src/main.rs:5:26: 5:34

[jieyouxu]

Triage: closing in favor of a newer issue since temporary_string_as_ptr was renamed and generalized into dangling_pointers_from_temporaries in #128985.