check that first arg to `panic!()` in const is `&str`

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -360,6 +360,18 @@ impl NonConstOp for Panic {
     }
 }
 
+/// A call to a `panic()` lang item where the first argument is _not_ a `&str`.
+#[derive(Debug)]
+pub struct PanicNonStr;
+impl NonConstOp for PanicNonStr {
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        ccx.tcx.sess.struct_span_err(
+            span,
+            "argument to `panic!()` in a const context must have type `&str`",
+        )
+    }
+}
+
 #[derive(Debug)]
 pub struct RawPtrComparison;
 impl NonConstOp for RawPtrComparison {

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -796,7 +796,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
         self.super_terminator(terminator, location);
 
         match &terminator.kind {
-            TerminatorKind::Call { func, .. } => {
+            TerminatorKind::Call { func, args, .. } => {
                 let ConstCx { tcx, body, param_env, .. } = *self.ccx;
                 let caller = self.def_id().to_def_id();
 
@@ -857,9 +857,17 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 }
 
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
-
                 if is_lang_panic_fn(tcx, callee) {
                     self.check_op(ops::Panic);
+
+                    // const-eval of the `begin_panic` fn assumes the argument is `&str`
+                    if Some(callee) == tcx.lang_items().begin_panic_fn() {
+                        match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
+                            ty::Ref(_, ty, _) if ty.is_str() => (),
+                            _ => self.check_op(ops::PanicNonStr),
+                        }
+                    }
+
                     return;
                 }
 

src/test/ui/consts/issue-66693-panic-in-array-len.rs

@@ -0,0 +1,17 @@
+// This is a separate test from `issue-66693.rs` because array lengths are evaluated
+// in a separate stage before `const`s and `statics` and so the error below is hit and
+// the compiler exits before generating errors for the others.
+
+#![feature(const_panic)]
+
+fn main() {
+    let _ = [0i32; panic!(2f32)];
+    //~^ ERROR: argument to `panic!()` in a const context must have type `&str`
+
+    // ensure that conforming panics are handled correctly
+    let _ = [false; panic!()];
+    //~^ ERROR: evaluation of constant value failed
+
+    // typechecking halts before getting to this one
+    let _ = ['a', panic!("panic in array len")];
+}

src/test/ui/consts/issue-66693-panic-in-array-len.stderr

@@ -0,0 +1,19 @@
+error: argument to `panic!()` in a const context must have type `&str`
+  --> $DIR/issue-66693-panic-in-array-len.rs:8:20
+   |
+LL |     let _ = [0i32; panic!(2f32)];
+   |                    ^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/issue-66693-panic-in-array-len.rs:12:21
+   |
+LL |     let _ = [false; panic!()];
+   |                     ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/issue-66693-panic-in-array-len.rs:12:21
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/issue-66693.rs

@@ -0,0 +1,24 @@
+// Tests that the compiler does not ICE when const-evaluating a `panic!()` invocation with a
+// non-`&str` argument.
+
+#![feature(const_panic)]
+
+const _: () = panic!(1);
+//~^ ERROR: argument to `panic!()` in a const context must have type `&str`
+
+static _FOO: () = panic!(true);
+//~^ ERROR: argument to `panic!()` in a const context must have type `&str`
+
+const fn _foo() {
+    panic!(&1); //~ ERROR: argument to `panic!()` in a const context must have type `&str`
+}
+
+// ensure that conforming panics don't cause an error
+const _: () = panic!();
+static _BAR: () = panic!("panic in static");
+
+const fn _bar() {
+    panic!("panic in const fn");
+}
+
+fn main() {}

src/test/ui/consts/issue-66693.stderr

@@ -0,0 +1,26 @@
+error: argument to `panic!()` in a const context must have type `&str`
+  --> $DIR/issue-66693.rs:13:5
+   |
+LL |     panic!(&1);
+   |     ^^^^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: argument to `panic!()` in a const context must have type `&str`
+  --> $DIR/issue-66693.rs:6:15
+   |
+LL | const _: () = panic!(1);
+   |               ^^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: argument to `panic!()` in a const context must have type `&str`
+  --> $DIR/issue-66693.rs:9:19
+   |
+LL | static _FOO: () = panic!(true);
+   |                   ^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+

src/test/ui/consts/issue-76064.rs

@@ -1,3 +1,5 @@
-struct Bug([u8; panic!(1)]); //~ ERROR panicking in constants is unstable
+// Note: non-`&str` panic arguments gained a separate error in PR #80734
+// which is why this doesn't match the issue
+struct Bug([u8; panic!("panic")]); //~ ERROR panicking in constants is unstable
 
 fn main() {}

src/test/ui/consts/issue-76064.stderr

@@ -1,8 +1,8 @@
 error[E0658]: panicking in constants is unstable
-  --> $DIR/issue-76064.rs:1:17
+  --> $DIR/issue-76064.rs:3:17
    |
-LL | struct Bug([u8; panic!(1)]);
-   |                 ^^^^^^^^^
+LL | struct Bug([u8; panic!("panic")]);
+   |                 ^^^^^^^^^^^^^^^
    |
    = note: see issue #51999 <https://github.com/rust-lang/rust/issues/51999> for more information
    = help: add `#![feature(const_panic)]` to the crate attributes to enable