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Document that assert! format arguments are evaluated lazily #82169

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Feb 17, 2021
Merged

Document that assert! format arguments are evaluated lazily #82169

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cb653b1
Document that `assert!` format arguments are evaluated lazily
not-an-aardvark Feb 16, 2021
15197cb
Ensure debug_assert! tests get run
not-an-aardvark Feb 17, 2021
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3 changes: 2 additions & 1 deletion library/core/src/macros/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1208,7 +1208,8 @@ pub(crate) mod builtin {
///
/// This macro has a second form, where a custom panic message can
/// be provided with or without arguments for formatting. See [`std::fmt`]
/// for syntax for this form.
/// for syntax for this form. Expressions used as format arguments will only
/// be evaluated if the assertion fails.
///
/// [`std::fmt`]: ../std/fmt/index.html
///
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11 changes: 11 additions & 0 deletions src/test/ui/macros/assert-format-lazy.rs
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@@ -0,0 +1,11 @@
// run-pass

#[allow(unreachable_code)]
fn main() {
assert!(true, "Failed: {:?}", panic!("assert! evaluated format expressions"));
debug_assert!(true, "Failed: {:?}", panic!("debug_assert! evaluated format expressions"));
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Are debug assertions disabled for tests? If so, this doesn't test what is intended.

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I believe it does right?

rust/src/tools/clippy/tests/ui/debug_assert_with_mut_call.rs

Line 31 in 0d97f7a

debug_assert!(bool_ref(&3));

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From trying it out, it looks like the debug assertions were indeed disabled for this test, so I updated it to ensure they're enabled. (I verified that the test now fails when debug_assert!(true, ...) is changed to debug_assert!(false, ...).

assert_eq!(1, 1, "Failed: {:?}", panic!("assert_eq! evaluated format expressions"));
debug_assert_eq!(1, 1, "Failed: {:?}", panic!("debug_assert_eq! evaluated format expressions"));
assert_ne!(1, 2, "Failed: {:?}", panic!("assert_ne! evaluated format expressions"));
debug_assert_ne!(1, 2, "Failed: {:?}", panic!("debug_assert_ne! evaluated format expressions"));
}