rust/tests/ui/proc-macro/span-from-proc-macro.rs

// aux-build:custom-quote.rs
// aux-build:span-from-proc-macro.rs
// compile-flags: -Z macro-backtrace

#[macro_use]
extern crate span_from_proc_macro;

#[error_from_attribute] //~ ERROR cannot find type `MissingType`
struct ShouldBeRemoved;

#[derive(ErrorFromDerive)] //~ ERROR cannot find type `OtherMissingType`
struct Kept;

fn main() {
    error_from_bang!(); //~ ERROR mismatched types
    other_error_from_bang!(); //~ ERROR cannot find value `my_ident`
}
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			`// aux-build:custom-quote.rs`
			`// aux-build:span-from-proc-macro.rs`
			`// compile-flags: -Z macro-backtrace`

			`#[macro_use]`
			`extern crate span_from_proc_macro;`

			#[error_from_attribute] //~ ERROR cannot find type `MissingType`
			`struct ShouldBeRemoved;`

			#[derive(ErrorFromDerive)] //~ ERROR cannot find type `OtherMissingType`
			`struct Kept;`

			`fn main() {`
			`error_from_bang!(); //~ ERROR mismatched types`
			other_error_from_bang!(); //~ ERROR cannot find value `my_ident`
			`}`