Prevent overflowing enum cast from ICEing

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -281,7 +281,14 @@ impl<'tcx> Cx<'tcx> {
                 .unwrap_or_else(|e| panic!("could not compute layout for {param_env_ty:?}: {e:?}"))
                 .size;
 
-            let lit = ScalarInt::try_from_uint(discr_offset as u128, size).unwrap();
+            let (lit, overflowing) = ScalarInt::truncate_from_uint(discr_offset as u128, size);
+            if overflowing {
+                // An erroneous enum with too many variants for its repr will emit E0081 and E0370
+                self.tcx.dcx().span_delayed_bug(
+                    source.span,
+                    "overflowing enum wasn't rejected by hir analysis",
+                );
+            }
             let kind = ExprKind::NonHirLiteral { lit, user_ty: None };
             let offset = self.thir.exprs.push(Expr { temp_lifetime, ty: discr_ty, span, kind });
 

tests/ui/error-codes/E0081.rs

@@ -50,5 +50,47 @@ enum MultipleDuplicates {
     //~^ NOTE `-2` assigned here
 }
 
+// Test for #131902
+// Ensure that casting an enum with too many variants for its repr
+// does not ICE
+#[repr(u8)]
+enum TooManyVariants {
+    //~^ ERROR discriminant value `0` assigned more than once
+    X000, X001, X002, X003, X004, X005, X006, X007, X008, X009,
+    //~^ NOTE `0` assigned here
+    //~| NOTE discriminant for `X256` incremented from this startpoint
+    X010, X011, X012, X013, X014, X015, X016, X017, X018, X019,
+    X020, X021, X022, X023, X024, X025, X026, X027, X028, X029,
+    X030, X031, X032, X033, X034, X035, X036, X037, X038, X039,
+    X040, X041, X042, X043, X044, X045, X046, X047, X048, X049,
+    X050, X051, X052, X053, X054, X055, X056, X057, X058, X059,
+    X060, X061, X062, X063, X064, X065, X066, X067, X068, X069,
+    X070, X071, X072, X073, X074, X075, X076, X077, X078, X079,
+    X080, X081, X082, X083, X084, X085, X086, X087, X088, X089,
+    X090, X091, X092, X093, X094, X095, X096, X097, X098, X099,
+    X100, X101, X102, X103, X104, X105, X106, X107, X108, X109,
+    X110, X111, X112, X113, X114, X115, X116, X117, X118, X119,
+    X120, X121, X122, X123, X124, X125, X126, X127, X128, X129,
+    X130, X131, X132, X133, X134, X135, X136, X137, X138, X139,
+    X140, X141, X142, X143, X144, X145, X146, X147, X148, X149,
+    X150, X151, X152, X153, X154, X155, X156, X157, X158, X159,
+    X160, X161, X162, X163, X164, X165, X166, X167, X168, X169,
+    X170, X171, X172, X173, X174, X175, X176, X177, X178, X179,
+    X180, X181, X182, X183, X184, X185, X186, X187, X188, X189,
+    X190, X191, X192, X193, X194, X195, X196, X197, X198, X199,
+    X200, X201, X202, X203, X204, X205, X206, X207, X208, X209,
+    X210, X211, X212, X213, X214, X215, X216, X217, X218, X219,
+    X220, X221, X222, X223, X224, X225, X226, X227, X228, X229,
+    X230, X231, X232, X233, X234, X235, X236, X237, X238, X239,
+    X240, X241, X242, X243, X244, X245, X246, X247, X248, X249,
+    X250, X251, X252, X253, X254, X255,
+    X256,
+    //~^ ERROR enum discriminant overflowed
+    //~| NOTE overflowed on value after 255
+    //~| NOTE explicitly set `X256 = 0`
+    //~| NOTE `0` assigned here
+}
+
 fn main() {
+    TooManyVariants::X256 as u8;
 }

tests/ui/error-codes/E0081.stderr

@@ -73,6 +73,30 @@ LL |
 LL |     V9,
    |     -- `-2` assigned here
 
-error: aborting due to 5 previous errors
+error[E0370]: enum discriminant overflowed
+  --> $DIR/E0081.rs:87:5
+   |
+LL |     X256,
+   |     ^^^^ overflowed on value after 255
+   |
+   = note: explicitly set `X256 = 0` if that is desired outcome
+
+error[E0081]: discriminant value `0` assigned more than once
+  --> $DIR/E0081.rs:57:1
+   |
+LL | enum TooManyVariants {
+   | ^^^^^^^^^^^^^^^^^^^^
+LL |
+LL |     X000, X001, X002, X003, X004, X005, X006, X007, X008, X009,
+   |     ----
+   |     |
+   |     `0` assigned here
+   |     discriminant for `X256` incremented from this startpoint (`X000` + 256 variants later => `X256` = 0)
+...
+LL |     X256,
+   |     ---- `0` assigned here
+
+error: aborting due to 7 previous errors
 
-For more information about this error, try `rustc --explain E0081`.
+Some errors have detailed explanations: E0081, E0370.
+For more information about an error, try `rustc --explain E0081`.