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`.

4 files changed, 46 insertions, 15 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index d74fef0045f..4a01731b512 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -716,30 +716,37 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .decode((self, sess))
     }
 
-    fn load_proc_macro(&self, id: DefIndex, sess: &Session) -> SyntaxExtension {
-        let (name, kind, helper_attrs) = match *self.raw_proc_macro(id) {
+    fn load_proc_macro(&self, def_id: DefId, sess: &Session) -> SyntaxExtension {
+        let (name, kind, helper_attrs) = match *self.raw_proc_macro(def_id.index) {
             ProcMacro::CustomDerive { trait_name, attributes, client } => {
                 let helper_attrs =
                     attributes.iter().cloned().map(Symbol::intern).collect::<Vec<_>>();
                 (
                     trait_name,
-                    SyntaxExtensionKind::Derive(Box::new(ProcMacroDerive { client })),
+                    SyntaxExtensionKind::Derive(Box::new(ProcMacroDerive {
+                        client,
+                        krate: def_id.krate,
+                    })),
                     helper_attrs,
                 )
             }
-            ProcMacro::Attr { name, client } => {
-                (name, SyntaxExtensionKind::Attr(Box::new(AttrProcMacro { client })), Vec::new())
-            }
-            ProcMacro::Bang { name, client } => {
-                (name, SyntaxExtensionKind::Bang(Box::new(BangProcMacro { client })), Vec::new())
-            }
+            ProcMacro::Attr { name, client } => (
+                name,
+                SyntaxExtensionKind::Attr(Box::new(AttrProcMacro { client, krate: def_id.krate })),
+                Vec::new(),
+            ),
+            ProcMacro::Bang { name, client } => (
+                name,
+                SyntaxExtensionKind::Bang(Box::new(BangProcMacro { client, krate: def_id.krate })),
+                Vec::new(),
+            ),
         };
 
-        let attrs: Vec<_> = self.get_item_attrs(id, sess).collect();
+        let attrs: Vec<_> = self.get_item_attrs(def_id.index, sess).collect();
         SyntaxExtension::new(
             sess,
             kind,
-            self.get_span(id, sess),
+            self.get_span(def_id.index, sess),
             helper_attrs,
             self.root.edition,
             Symbol::intern(name),
@@ -1379,6 +1386,15 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         }
     }
 
+    fn get_proc_macro_quoted_span(&self, index: usize, sess: &Session) -> Span {
+        self.root
+            .tables
+            .proc_macro_quoted_spans
+            .get(self, index)
+            .unwrap_or_else(|| panic!("Missing proc macro quoted span: {:?}", index))
+            .decode((self, sess))
+    }
+
     fn get_foreign_modules(&self, tcx: TyCtxt<'tcx>) -> Lrc<FxHashMap<DefId, ForeignModule>> {
         if self.root.is_proc_macro_crate() {
             // Proc macro crates do not have any *target* foreign modules.
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index b11ad6c7ff8..cda57f48cae 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -411,7 +411,7 @@ impl CStore {
 
         let data = self.get_crate_data(id.krate);
         if data.root.is_proc_macro_crate() {
-            return LoadedMacro::ProcMacro(data.load_proc_macro(id.index, sess));
+            return LoadedMacro::ProcMacro(data.load_proc_macro(id, sess));
         }
 
         let span = data.get_span(id.index, sess);
@@ -461,6 +461,15 @@ impl CStore {
     pub fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
         self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess).collect()
     }
+
+    pub fn get_proc_macro_quoted_span_untracked(
+        &self,
+        cnum: CrateNum,
+        id: usize,
+        sess: &Session,
+    ) -> Span {
+        self.get_crate_data(cnum).get_proc_macro_quoted_span(id, sess)
+    }
 }
 
 impl CrateStore for CStore {
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index e8f02b8e66f..c61417f9998 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1579,6 +1579,11 @@ impl EncodeContext<'a, 'tcx> {
             let proc_macro_decls_static = tcx.proc_macro_decls_static(LOCAL_CRATE).unwrap().index;
             let stability = tcx.lookup_stability(DefId::local(CRATE_DEF_INDEX)).copied();
             let macros = self.lazy(hir.krate().proc_macros.iter().map(|p| p.owner.local_def_index));
+            let spans = self.tcx.sess.parse_sess.proc_macro_quoted_spans();
+            for (i, span) in spans.into_iter().enumerate() {
+                let span = self.lazy(span);
+                self.tables.proc_macro_quoted_spans.set(i, span);
+            }
 
             record!(self.tables.def_kind[LOCAL_CRATE.as_def_id()] <- DefKind::Mod);
             record!(self.tables.span[LOCAL_CRATE.as_def_id()] <- tcx.def_span(LOCAL_CRATE.as_def_id()));
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 9f665d5daaa..6b375045f14 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -258,15 +258,15 @@ crate struct TraitImpls {
 
 /// Define `LazyTables` and `TableBuilders` at the same time.
 macro_rules! define_tables {
-    ($($name:ident: Table<DefIndex, $T:ty>),+ $(,)?) => {
+    ($($name:ident: Table<$IDX:ty, $T:ty>),+ $(,)?) => {
         #[derive(MetadataEncodable, MetadataDecodable)]
         crate struct LazyTables<'tcx> {
-            $($name: Lazy!(Table<DefIndex, $T>)),+
+            $($name: Lazy!(Table<$IDX, $T>)),+
         }
 
         #[derive(Default)]
         struct TableBuilders<'tcx> {
-            $($name: TableBuilder<DefIndex, $T>),+
+            $($name: TableBuilder<$IDX, $T>),+
         }
 
         impl TableBuilders<'tcx> {
@@ -315,6 +315,7 @@ define_tables! {
     // definitions from any given crate.
     def_keys: Table<DefIndex, Lazy<DefKey>>,
     def_path_hashes: Table<DefIndex, Lazy<DefPathHash>>,
+    proc_macro_quoted_spans: Table<usize, Lazy<Span>>,
 }
 
 #[derive(Copy, Clone, MetadataEncodable, MetadataDecodable)]