Fix ICE during type layout when there's a [type error]

Based on estebank's [comment], except I used `delay_span_bug` because it
should work in more cases, and I think it expresses its intent more
clearly.

[comment]: https://github.com/rust-lang/rust/issues/84108#issuecomment-818916848

compiler/rustc_middle/src/ty/layout.rs

@@ -367,7 +367,14 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
         for &i in &inverse_memory_index {
             let field = fields[i as usize];
             if !sized {
-                bug!("univariant: field #{} of `{}` comes after unsized field", offsets.len(), ty);
+                self.tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    &format!(
+                        "univariant: field #{} of `{}` comes after unsized field",
+                        offsets.len(),
+                        ty
+                    ),
+                );
             }
 
             if field.is_unsized() {

src/test/ui/layout/issue-84108.rs

@@ -0,0 +1,14 @@
+// See issue #84108 -- this is a test to ensure we do not ICE
+// on this invalid code.
+
+#![crate_type = "lib"]
+
+static FOO: (dyn AsRef<OsStr>, u8) = ("hello", 42);
+//~^ ERROR cannot find type `OsStr` in this scope
+
+const BAR: (&Path, [u8], usize) = ("hello", [], 42);
+//~^ ERROR cannot find type `Path` in this scope
+//~| ERROR the size for values of type `[u8]` cannot be known at compilation time
+
+static BAZ: ([u8], usize) = ([], 0);
+//~^ ERROR the size for values of type `[u8]` cannot be known at compilation time

src/test/ui/layout/issue-84108.stderr

@@ -0,0 +1,44 @@
+error[E0412]: cannot find type `OsStr` in this scope
+  --> $DIR/issue-84108.rs:6:24
+   |
+LL | static FOO: (dyn AsRef<OsStr>, u8) = ("hello", 42);
+   |                        ^^^^^ not found in this scope
+   |
+help: consider importing this struct
+   |
+LL | use std::ffi::OsStr;
+   |
+
+error[E0412]: cannot find type `Path` in this scope
+  --> $DIR/issue-84108.rs:9:14
+   |
+LL | const BAR: (&Path, [u8], usize) = ("hello", [], 42);
+   |              ^^^^ not found in this scope
+   |
+help: consider importing this struct
+   |
+LL | use std::path::Path;
+   |
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/issue-84108.rs:9:12
+   |
+LL | const BAR: (&Path, [u8], usize) = ("hello", [], 42);
+   |            ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: only the last element of a tuple may have a dynamically sized type
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/issue-84108.rs:13:13
+   |
+LL | static BAZ: ([u8], usize) = ([], 0);
+   |             ^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: only the last element of a tuple may have a dynamically sized type
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0277, E0412.
+For more information about an error, try `rustc --explain E0277`.