Sema: fix `is_non_null_ptr` handling for runtime-known pointers

We can still often determine a comptime result based on the type, even
if the pointer is runtime-known.

Also, we previously used load -> is non null instead of AIR
`is_non_null_ptr` if the pointer is comptime-known, but that's a bad
heuristic. Instead, we should check for the pointer to be
comptime-known, *and* for the load to be comptime-known, and only in
that case should we call `Sema.analyzeIsNonNull`.

Resolves: #22556

1 files changed, 10 insertions, 0 deletions

diff --git a/src/Type.zig b/src/Type.zig
index c6c4334270..b628e0c60e 100644
--- a/src/Type.zig
+++ b/src/Type.zig
@@ -4114,6 +4114,16 @@ pub fn containerTypeName(ty: Type, ip: *const InternPool) InternPool.NullTermina
     };
 }
 
+/// Returns `true` if a value of this type is always `null`.
+/// Returns `false` if a value of this type is neve `null`.
+/// Returns `null` otherwise.
+pub fn isNullFromType(ty: Type, zcu: *const Zcu) ?bool {
+    if (ty.zigTypeTag(zcu) != .optional and !ty.isCPtr(zcu)) return false;
+    const child = ty.optionalChild(zcu);
+    if (child.zigTypeTag(zcu) == .noreturn) return true; // `?noreturn` is always null
+    return null;
+}
+
 pub const @"u1": Type = .{ .ip_index = .u1_type };
 pub const @"u8": Type = .{ .ip_index = .u8_type };
 pub const @"u16": Type = .{ .ip_index = .u16_type };