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Warn on assert!(always_false) #73435
Description
It would be useful if rustc emitted a warning on assertions when the condition can be statically proven to always be false.
Example:
pub fn meh() {
let always_false = 2 > 3;
assert!(always_false);
println!("foo");
}Currently compiles cleanly without any warnings (playground) — neither about the always false assertion, nor about the unreachable statement.
The LLVM IR emitted:
; playground::meh
; Function Attrs: noreturn nonlazybind uwtable
define void @_ZN10playground3meh17h3a7dd68d372b246aE() unnamed_addr #3 {
start:
; call std::panicking::begin_panic
tail call fastcc void @_ZN3std9panicking11begin_panic17h982033eda4eb16e4E()
unreachable
}
suggests rustc is able to statically prove the assertion to be always false even before LLVM optimization is run.
I would expect to get something similar to this:
warning: assertion is always false: `always_false`
--> meh.rs:3:15
|
3 | assert!(always_false);
| ^^^^^^^^^^^^ the assertion is always false
|
= help: to unconditionally panic, use `panic!()` or `unreachable!()`
= note: `#[warn(assert_is_always_false)]` on by default
warning: unreachable statement
--> meh.rs:4:5
|
3 | assert!(always_false);
| ---------------------- any code following this expression is unreachable
4 | println!("foo");
| ^^^^^^^^^^^^^^^^ unreachable statement
|
= note: `#[warn(unreachable_code)]` on by default
Meta
This is not about any particular version or platform, but here it goes anyway:
rustc --version --verbose:
rustc 1.44.0 (49cae5576 2020-06-01)
binary: rustc
commit-hash: 49cae55760da0a43428eba73abcb659bb70cf2e4
commit-date: 2020-06-01
host: x86_64-unknown-linux-gnu
release: 1.44.0
LLVM version: 9.0
Reacted by ratijas and DocJade
Activity
matthiaskrgr commentedon Jun 17, 2020
There's already a clippy lint that lints on literal
trueandfalse. rust-lang/rust-clippy#3582It does not warn on constants or variables that can be const-folded though.
See also rust-lang/rust-clippy#3575
bugaevc commentedon Jun 17, 2020
That's clearly related, but my request is about a different use-case, where I write an actual assertion that I think guards something, but due to some stupid mistake it doesn't, in which case I would like rustc to let me know. Explicitly and intentionally writing
assert!(false)is different.