Switch some fallbacks to debug asserts

Author: rtfeldman

src/canonicalize/NodeStore.zig

@@ -2626,14 +2626,9 @@ pub fn clearScratchDefsFrom(store: *NodeStore, start: u32) void {
 }
 
 /// Creates a slice corresponding to a span.
-/// Returns an empty slice if the span is out of bounds (can happen with corrupted malformed code).
 pub fn sliceFromSpan(store: *const NodeStore, comptime T: type, span: base.DataSpan) []T {
     const extra_data_len = store.extra_data.items.items.len;
-    // Bounds check: ensure the span doesn't exceed the extra_data length
-    if (span.start >= extra_data_len or span.start + span.len > extra_data_len) {
-        // Return empty slice for corrupted spans
-        return &[_]T{};
-    }
+    std.debug.assert(span.start + span.len <= extra_data_len);
     return @ptrCast(store.extra_data.items.items[span.start..][0..span.len]);
 }
 

src/check/Check.zig

@@ -524,15 +524,7 @@ fn instantiateVarHelp(
     env: *Env,
     region_behavior: InstantiateRegionBehavior,
 ) std.mem.Allocator.Error!Var {
-    // Guard against corrupted vars from malformed code (e.g., uninitialized memory)
-    const var_idx: u64 = @intFromEnum(var_to_instantiate);
-    const type_store_len = self.types.len();
-    if (var_idx >= type_store_len) {
-        // Return a fresh error var for corrupted input - this properly maintains sync
-        const fresh_error_var = try self.types.freshFromContentWithRank(.err, env.rank());
-        try self.fillInRegionsThrough(fresh_error_var);
-        return fresh_error_var;
-    }
+    std.debug.assert(@intFromEnum(var_to_instantiate) < self.types.len());
 
     // First, reset state
     instantiator.var_map.clearRetainingCapacity();

src/types/instantiate.zig

@@ -73,14 +73,7 @@ pub const Instantiator = struct {
         self: *Self,
         initial_var: Var,
     ) std.mem.Allocator.Error!Var {
-        // Guard against corrupted vars from malformed code (e.g., uninitialized memory)
-        // We return var 0 (which always exists) rather than creating a new var,
-        // since we can't create new vars here without causing array sync issues
-        const var_idx: u64 = @intFromEnum(initial_var);
-        const store_len = self.store.len();
-        if (var_idx >= store_len) {
-            return @enumFromInt(0);
-        }
+        std.debug.assert(@intFromEnum(initial_var) < self.store.len());
 
         const resolved = self.store.resolveVar(initial_var);
         const resolved_var = resolved.var_;