Import proc-macro-error-1.0.2
Change-Id: Iebb634ed3b9e204e58ffc8c1035d9e982748ea62
diff --git a/src/dummy.rs b/src/dummy.rs
new file mode 100644
index 0000000..571a595
--- /dev/null
+++ b/src/dummy.rs
@@ -0,0 +1,150 @@
+//! Facility to emit dummy implementations (or whatever) in case
+//! an error happen.
+//!
+//! `compile_error!` does not abort a compilation right away. This means
+//! `rustc` doesn't just show you the error and abort, it carries on the
+//! compilation process looking for other errors to report.
+//!
+//! Let's consider an example:
+//!
+//! ```rust,ignore
+//! use proc_macro::TokenStream;
+//! use proc_macro_error::*;
+//!
+//! trait MyTrait {
+//! fn do_thing();
+//! }
+//!
+//! // this proc macro is supposed to generate MyTrait impl
+//! #[proc_macro_derive(MyTrait)]
+//! #[proc_macro_error]
+//! fn example(input: TokenStream) -> TokenStream {
+//! // somewhere deep inside
+//! abort!(span, "something's wrong");
+//!
+//! // this implementation will be generated if no error happened
+//! quote! {
+//! impl MyTrait for #name {
+//! fn do_thing() {/* whatever */}
+//! }
+//! }
+//! }
+//!
+//! // ================
+//! // in main.rs
+//!
+//! // this derive triggers an error
+//! #[derive(MyTrait)] // first BOOM!
+//! struct Foo;
+//!
+//! fn main() {
+//! Foo::do_thing(); // second BOOM!
+//! }
+//! ```
+//!
+//! The problem is: the generated token stream contains only `compile_error!`
+//! invocation, the impl was not generated. That means user will see two compilation
+//! errors:
+//!
+//! ```text
+//! error: something's wrong
+//! --> $DIR/probe.rs:9:10
+//! |
+//! 9 |#[proc_macro_derive(MyTrait)]
+//! | ^^^^^^^
+//!
+//! error[E0599]: no function or associated item named `do_thing` found for type `Foo` in the current scope
+//! --> src\main.rs:3:10
+//! |
+//! 1 | struct Foo;
+//! | ----------- function or associated item `do_thing` not found for this
+//! 2 | fn main() {
+//! 3 | Foo::do_thing(); // second BOOM!
+//! | ^^^^^^^^ function or associated item not found in `Foo`
+//! ```
+//!
+//! But the second error is meaningless! We definitely need to fix this.
+//!
+//! Most used approach in cases like this is "dummy implementation" -
+//! omit `impl MyTrait for #name` and fill functions bodies with `unimplemented!()`.
+//!
+//! This is how you do it:
+//!
+//! ```rust,ignore
+//! use proc_macro::TokenStream;
+//! use proc_macro_error::*;
+//!
+//! trait MyTrait {
+//! fn do_thing();
+//! }
+//!
+//! // this proc macro is supposed to generate MyTrait impl
+//! #[proc_macro_derive(MyTrait)]
+//! #[proc_macro_error]
+//! fn example(input: TokenStream) -> TokenStream {
+//! // first of all - we set a dummy impl which will be appended to
+//! // `compile_error!` invocations in case a trigger does happen
+//! set_dummy(quote! {
+//! impl MyTrait for #name {
+//! fn do_thing() { unimplemented!() }
+//! }
+//! });
+//!
+//! // somewhere deep inside
+//! abort!(span, "something's wrong");
+//!
+//! // this implementation will be generated if no error happened
+//! quote! {
+//! impl MyTrait for #name {
+//! fn do_thing() {/* whatever */}
+//! }
+//! }
+//! }
+//!
+//! // ================
+//! // in main.rs
+//!
+//! // this derive triggers an error
+//! #[derive(MyTrait)] // first BOOM!
+//! struct Foo;
+//!
+//! fn main() {
+//! Foo::do_thing(); // no more errors!
+//! }
+//! ```
+
+use proc_macro2::TokenStream;
+use std::cell::RefCell;
+
+use crate::check_correctness;
+
+thread_local! {
+ static DUMMY_IMPL: RefCell<Option<TokenStream>> = RefCell::new(None);
+}
+
+/// Sets dummy token stream which will be appended to `compile_error!(msg);...`
+/// invocations in case you'll emit any errors.
+///
+/// See [guide](../index.html#guide).
+pub fn set_dummy(dummy: TokenStream) -> Option<TokenStream> {
+ check_correctness();
+ DUMMY_IMPL.with(|old_dummy| old_dummy.replace(Some(dummy)))
+}
+
+/// Same as [`set_dummy`] but, instead of resetting, appends tokens to the
+/// existing dummy (if any). Behaves as `set_dummy` if no dummy is present.
+pub fn append_dummy(dummy: TokenStream) {
+ check_correctness();
+ DUMMY_IMPL.with(|old_dummy| {
+ let mut cell = old_dummy.borrow_mut();
+ if let Some(ts) = cell.as_mut() {
+ ts.extend(dummy);
+ } else {
+ *cell = Some(dummy);
+ }
+ });
+}
+
+pub(crate) fn cleanup() -> Option<TokenStream> {
+ DUMMY_IMPL.with(|old_dummy| old_dummy.replace(None))
+}
