From f916b0474a0443c8ce9915efb59b7465b42e03f8 Mon Sep 17 00:00:00 2001 From: Aaron Hill Date: Sun, 2 Aug 2020 19:52:16 -0400 Subject: 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`. --- compiler/rustc_session/src/parse.rs | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'compiler/rustc_session/src') diff --git a/compiler/rustc_session/src/parse.rs b/compiler/rustc_session/src/parse.rs index 65d5d96aba1..7b7878e9c7f 100644 --- a/compiler/rustc_session/src/parse.rs +++ b/compiler/rustc_session/src/parse.rs @@ -137,6 +137,9 @@ pub struct ParseSess { pub type_ascription_path_suggestions: Lock>, /// Whether cfg(version) should treat the current release as incomplete pub assume_incomplete_release: bool, + /// Spans passed to `proc_macro::quote_span`. Each span has a numerical + /// identifier represented by its position in the vector. + pub proc_macro_quoted_spans: Lock>, } impl ParseSess { @@ -164,6 +167,7 @@ impl ParseSess { env_depinfo: Default::default(), type_ascription_path_suggestions: Default::default(), assume_incomplete_release: false, + proc_macro_quoted_spans: Default::default(), } } @@ -236,4 +240,14 @@ impl ParseSess { ); } } + + pub fn save_proc_macro_span(&self, span: Span) -> usize { + let mut spans = self.proc_macro_quoted_spans.lock(); + spans.push(span); + return spans.len() - 1; + } + + pub fn proc_macro_quoted_spans(&self) -> Vec { + self.proc_macro_quoted_spans.lock().clone() + } } -- cgit 1.4.1-3-g733a5