rust/tests/ui/proc-macro/quote-debug.rs

// check-pass
// force-host
// no-prefer-dynamic
// compile-flags: -Z unpretty=expanded
//
// This file is not actually used as a proc-macro - instead,
// it's just used to show the output of the `quote!` macro

#![feature(proc_macro_quote)]
#![crate_type = "proc-macro"]

extern crate proc_macro;

fn main() {
    proc_macro::quote! {
        let hello = "world";
    }
}
Implement span quoting for proc-macros This PR implements span quoting, allowing proc-macros to produce spans pointing into their own crate. This is used by the unstable `proc_macro::quote!` macro, allowing us to get error messages like this: ``` error[E0412]: cannot find type `MissingType` in this scope --> $DIR/auxiliary/span-from-proc-macro.rs:37:20 \| LL \| pub fn error_from_attribute(_args: TokenStream, _input: TokenStream) -> TokenStream { \| ----------------------------------------------------------------------------------- in this expansion of procedural macro `#[error_from_attribute]` ... LL \| field: MissingType \| ^^^^^^^^^^^ not found in this scope \| ::: $DIR/span-from-proc-macro.rs:8:1 \| LL \| #[error_from_attribute] \| ----------------------- in this macro invocation ``` Here, `MissingType` occurs inside the implementation of the proc-macro `#[error_from_attribute]`. Previosuly, this would always result in a span pointing at `#[error_from_attribute]` This will make many proc-macro-related error message much more useful - when a proc-macro generates code containing an error, users will get an error message pointing directly at that code (within the macro definition), instead of always getting a span pointing at the macro invocation site. This is implemented as follows: * When a proc-macro crate is being compiled, it causes the `quote!` macro to get run. This saves all of the sapns in the input to `quote!` into the metadata of the proc-macro-crate (which we are currently compiling). The `quote!` macro then expands to a call to `proc_macro::Span::recover_proc_macro_span(id)`, where `id` is an opaque identifier for the span in the crate metadata. * When the same proc-macro crate is run (e.g. it is loaded from disk and invoked by some consumer crate), the call to `proc_macro::Span::recover_proc_macro_span` causes us to load the span from the proc-macro crate's metadata. The proc-macro then produces a `TokenStream` containing a `Span` pointing into the proc-macro crate itself. The recursive nature of 'quote!' can be difficult to understand at first. The file `src/test/ui/proc-macro/quote-debug.stdout` shows the output of the `quote!` macro, which should make this eaier to understand. This PR also supports custom quoting spans in custom quote macros (e.g. the `quote` crate). All span quoting goes through the `proc_macro::quote_span` method, which can be called by a custom quote macro to perform span quoting. An example of this usage is provided in `src/test/ui/proc-macro/auxiliary/custom-quote.rs` Custom quoting currently has a few limitations: In order to quote a span, we need to generate a call to `proc_macro::Span::recover_proc_macro_span`. However, proc-macros support renaming the `proc_macro` crate, so we can't simply hardcode this path. Previously, the `quote_span` method used the path `crate::Span` - however, this only works when it is called by the builtin `quote!` macro in the same crate. To support being called from arbitrary crates, we need access to the name of the `proc_macro` crate to generate a path. This PR adds an additional argument to `quote_span` to specify the name of the `proc_macro` crate. Howver, this feels kind of hacky, and we may want to change this before stabilizing anything quote-related. Additionally, using `quote_span` currently requires enabling the `proc_macro_internals` feature. The builtin `quote!` macro has an `#[allow_internal_unstable]` attribute, but this won't work for custom quote implementations. This will likely require some additional tricks to apply `allow_internal_unstable` to the span of `proc_macro::Span::recover_proc_macro_span`. 2020-08-02 23:52:16 +00:00			`// check-pass`
			`// force-host`
			`// no-prefer-dynamic`
			`// compile-flags: -Z unpretty=expanded`
			`//`
			`// This file is not actually used as a proc-macro - instead,`
			// it's just used to show the output of the `quote!` macro

			`#![feature(proc_macro_quote)]`
			`#![crate_type = "proc-macro"]`

			`extern crate proc_macro;`

			`fn main() {`
			`proc_macro::quote! {`
			`let hello = "world";`
			`}`
			`}`