diff options
| -rw-r--r-- | library/core/src/intrinsics.rs | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs index eafbaba9ef3..5a9c900f643 100644 --- a/library/core/src/intrinsics.rs +++ b/library/core/src/intrinsics.rs @@ -1130,7 +1130,10 @@ extern "rust-intrinsic" { /// may lead to unexpected and unstable compilation results. This makes `transmute` **incredibly /// unsafe**. `transmute` should be the absolute last resort. /// - /// Transmuting pointers to integers in a `const` context is [undefined behavior][ub]. + /// Transmuting pointers *to* integers in a `const` context is [undefined behavior][ub], + /// unless the pointer was originally created *from* an integer. + /// (That includes this function specifically, integer-to-pointer casts, and helpers like [`invalid`][crate::ptr::invalid], + /// but also semantically-equivalent conversions such as punning through `repr(C)` union fields.) /// Any attempt to use the resulting value for integer operations will abort const-evaluation. /// (And even outside `const`, such transmutation is touching on many unspecified aspects of the /// Rust memory model and should be avoided. See below for alternatives.) |