Rollup merge of #100229 - RalfJung:extra-const-ub-checks, r=lcnr

add -Zextra-const-ub-checks to enable more UB checking in const-eval

Cc https://github.com/rust-lang/rust/issues/99923
r? `@oli-obk`

3 files changed, 117 insertions, 0 deletions

diff --git a/src/test/rustdoc-ui/z-help.stdout b/src/test/rustdoc-ui/z-help.stdout
index 6dc41231559..236469ce979 100644
--- a/src/test/rustdoc-ui/z-help.stdout
+++ b/src/test/rustdoc-ui/z-help.stdout
@@ -38,6 +38,7 @@
     -Z                        emit-stack-sizes=val -- emit a section containing stack size metadata (default: no)
     -Z                           emit-thin-lto=val -- emit the bc module with thin LTO info (default: yes)
     -Z               export-executable-symbols=val -- export symbols from executables, as if they were dynamic libraries
+    -Z                   extra-const-ub-checks=val -- turns on more checks to detect const UB, which can be slow (default: no)
     -Z                             fewer-names=val -- reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) (default: no)
     -Z              force-unstable-if-unmarked=val -- force all crates to be `rustc_private` unstable (default: no)
     -Z                                    fuel=val -- set the optimization fuel quota for a crate
diff --git a/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs b/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs
new file mode 100644
index 00000000000..97c9e150519
--- /dev/null
+++ b/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs
@@ -0,0 +1,45 @@
+// revisions: no_flag with_flag
+// [no_flag] check-pass
+// [with_flag] compile-flags: -Zextra-const-ub-checks
+#![feature(const_ptr_read)]
+
+use std::mem::transmute;
+
+const INVALID_BOOL: () = unsafe {
+    let _x: bool = transmute(3u8);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const INVALID_PTR_IN_INT: () = unsafe {
+    let _x: usize = transmute(&3u8);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const INVALID_SLICE_TO_USIZE_TRANSMUTE: () = unsafe {
+    let x: &[u8] = &[0; 32];
+    let _x: (usize, usize) = transmute(x);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const UNALIGNED_PTR: () = unsafe {
+    let _x: &u32 = transmute(&[0u8; 4]);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const UNALIGNED_READ: () = {
+    INNER; //[with_flag]~ERROR any use of this value will cause an error
+    //[with_flag]~| previously accepted
+    // There is an error here but its span is in the standard library so we cannot match it...
+    // so we have this in a *nested* const, such that the *outer* const fails to use it.
+    const INNER: () = unsafe {
+        let x = &[0u8; 4];
+        let ptr = x.as_ptr().cast::<u32>();
+        ptr.read();
+    };
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr b/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr
new file mode 100644
index 00000000000..1706db7ac43
--- /dev/null
+++ b/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr
@@ -0,0 +1,71 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:9:20
+   |
+LL |     let _x: bool = transmute(3u8);
+   |                    ^^^^^^^^^^^^^^ constructing invalid value: encountered 0x03, but expected a boolean
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:15:21
+   |
+LL |     let _x: usize = transmute(&3u8);
+   |                     ^^^^^^^^^^^^^^^ constructing invalid value: encountered (potentially part of) a pointer, but expected plain (non-pointer) bytes
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:22:30
+   |
+LL |     let _x: (usize, usize) = transmute(x);
+   |                              ^^^^^^^^^^^^ constructing invalid value at .0: encountered (potentially part of) a pointer, but expected plain (non-pointer) bytes
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:28:20
+   |
+LL |     let _x: &u32 = transmute(&[0u8; 4]);
+   |                    ^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered an unaligned reference (required 4 byte alignment but found 1)
+
+error[E0080]: evaluation of constant value failed
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |
+LL |         copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         accessing memory with alignment 1, but alignment 4 is required
+   |         inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |
+  ::: $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+LL |         unsafe { read(self) }
+   |                  ---------- inside `ptr::const_ptr::<impl *const u32>::read` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+  ::: $DIR/detect-extra-ub.rs:41:9
+   |
+LL |         ptr.read();
+   |         ---------- inside `INNER` at $DIR/detect-extra-ub.rs:41:9
+
+error: any use of this value will cause an error
+  --> $DIR/detect-extra-ub.rs:34:5
+   |
+LL | const UNALIGNED_READ: () = {
+   | ------------------------
+LL |     INNER;
+   |     ^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0080`.
+Future incompatibility report: Future breakage diagnostic:
+error: any use of this value will cause an error
+  --> $DIR/detect-extra-ub.rs:34:5
+   |
+LL | const UNALIGNED_READ: () = {
+   | ------------------------
+LL |     INNER;
+   |     ^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+