Debug implementation: Don't use format! or String when core is enabled

As --use-core is typically given when the wrapped library is to be used
in a no_std environment, format! and String can not be used.

This is a quick fix that will cause regressions in the quality of the
debug output to users that use core but are not no_std, but enables the
use of bindgen with implemented Debug traits to no_std users.

Closes: #1100

Author: chrysn

src/codegen/impl_debug.rs

@@ -186,28 +186,44 @@ impl<'a> ImplDebug<'a> for Item {
                     // The simple case
                     debug_print(name, quote! { #name_ident })
                 } else {
-                    // Let's implement our own print function
+                    if ctx.options().use_core {
+                        // There is no String in core; reducing field visibility to avoid breaking
+                        // no_std setups.
+                        Some((
+                            format!("{}: [...]", name), vec![]
+                        ))
+                    } else {
+                        // Let's implement our own print function
+                        Some((
+                            format!("{}: [{{}}]", name),
+                            vec![quote! {
+                                self.#name_ident
+                                    .iter()
+                                    .enumerate()
+                                    .map(|(i, v)| format!("{}{:?}", if i > 0 { ", " } else { "" }, v))
+                                    .collect::<String>()
+                            }],
+                        ))
+                    }
+                }
+            }
+            TypeKind::Vector(_, len) => {
+                if ctx.options().use_core {
+                    // There is no format! in core; reducing field visibility to avoid breaking
+                    // no_std setups.
                     Some((
-                        format!("{}: [{{}}]", name),
+                        format!("{}(...)", name), vec![]
+                    ))
+                } else {
+                    let self_ids = 0..len;
+                    Some((
+                        format!("{}({{}})", name),
                         vec![quote! {
-                            self.#name_ident
-                                .iter()
-                                .enumerate()
-                                .map(|(i, v)| format!("{}{:?}", if i > 0 { ", " } else { "" }, v))
-                                .collect::<String>()
-                        }],
+                            #(format!("{:?}", self.#self_ids)),*
+                        }]
                     ))
                 }
             }
-            TypeKind::Vector(_, len) => {
-                let self_ids = 0..len;
-                Some((
-                    format!("{}({{}})", name),
-                    vec![quote! {
-                        #(format!("{:?}", self.#self_ids)),*
-                    }]
-                ))
-            }
 
             TypeKind::ResolvedTypeRef(t) |
             TypeKind::TemplateAlias(t, _) |