Rollup merge of #124425 - saethlin:ceci-nest-pas-une-ice, r=compiler-errors

Do not ICE on invalid consts when walking mono-reachable blocks

The `bug!` here was written under the logic of "this condition is impossible, right?" except that of course, if the compiler is given code that results in an compile error, then the situation is possible.

So now we just direct errors into the already-existing path for when we can't do a mono-time optimization.

compiler/rustc_middle/src/mir/mod.rs

@@ -701,10 +701,7 @@ impl<'tcx> Body<'tcx> {
                 env,
                 crate::ty::EarlyBinder::bind(constant.const_),
             );
-            let Some(bits) = mono_literal.try_eval_bits(tcx, env) else {
-                bug!("Couldn't evaluate constant {:?} in mono {:?}", constant, instance);
-            };
-            bits
+            mono_literal.try_eval_bits(tcx, env)
         };
 
         let TerminatorKind::SwitchInt { discr, targets } = &block.terminator().kind else {
@@ -714,7 +711,7 @@ impl<'tcx> Body<'tcx> {
         // If this is a SwitchInt(const _), then we can just evaluate the constant and return.
         let discr = match discr {
             Operand::Constant(constant) => {
-                let bits = eval_mono_const(constant);
+                let bits = eval_mono_const(constant)?;
                 return Some((bits, targets));
             }
             Operand::Move(place) | Operand::Copy(place) => place,
@@ -748,7 +745,7 @@ impl<'tcx> Body<'tcx> {
         match rvalue {
             Rvalue::NullaryOp(NullOp::UbChecks, _) => Some((tcx.sess.ub_checks() as u128, targets)),
             Rvalue::Use(Operand::Constant(constant)) => {
-                let bits = eval_mono_const(constant);
+                let bits = eval_mono_const(constant)?;
                 Some((bits, targets))
             }
             _ => None,

tests/ui/consts/mono-reachable-invalid-const.rs

@@ -0,0 +1,23 @@
+//@ build-fail
+
+struct Bar<const BITS: usize>;
+
+impl<const BITS: usize> Bar<BITS> {
+    const ASSERT: bool = {
+        let b = std::convert::identity(1);
+        ["oops"][b]; //~ ERROR evaluation of `Bar::<0>::ASSERT` failed
+        true
+    };
+
+    fn assert() {
+        let val = Self::ASSERT;
+        if val {
+            std::convert::identity(val);
+        }
+    }
+}
+
+
+fn main() {
+    Bar::<0>::assert();
+}

tests/ui/consts/mono-reachable-invalid-const.stderr

@@ -0,0 +1,29 @@
+error[E0080]: evaluation of `Bar::<0>::ASSERT` failed
+  --> $DIR/mono-reachable-invalid-const.rs:8:9
+   |
+LL |         ["oops"][b];
+   |         ^^^^^^^^^^^ index out of bounds: the length is 1 but the index is 1
+
+note: erroneous constant encountered
+  --> $DIR/mono-reachable-invalid-const.rs:13:19
+   |
+LL |         let val = Self::ASSERT;
+   |                   ^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/mono-reachable-invalid-const.rs:13:19
+   |
+LL |         let val = Self::ASSERT;
+   |                   ^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+note: the above error was encountered while instantiating `fn Bar::<0>::assert`
+  --> $DIR/mono-reachable-invalid-const.rs:22:5
+   |
+LL |     Bar::<0>::assert();
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.