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_expand/src/proc_macro.rs

@@ -9,12 +9,14 @@ use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorReported;
 use rustc_parse::nt_to_tokenstream;
 use rustc_parse::parser::ForceCollect;
+use rustc_span::def_id::CrateNum;
 use rustc_span::{Span, DUMMY_SP};
 
 const EXEC_STRATEGY: pm::bridge::server::SameThread = pm::bridge::server::SameThread;
 
 pub struct BangProcMacro {
     pub client: pm::bridge::client::Client<fn(pm::TokenStream) -> pm::TokenStream>,
+    pub krate: CrateNum,
 }
 
 impl base::ProcMacro for BangProcMacro {
@@ -24,7 +26,7 @@ impl base::ProcMacro for BangProcMacro {
         span: Span,
         input: TokenStream,
     ) -> Result<TokenStream, ErrorReported> {
-        let server = proc_macro_server::Rustc::new(ecx);
+        let server = proc_macro_server::Rustc::new(ecx, self.krate);
         self.client.run(&EXEC_STRATEGY, server, input, ecx.ecfg.proc_macro_backtrace).map_err(|e| {
             let mut err = ecx.struct_span_err(span, "proc macro panicked");
             if let Some(s) = e.as_str() {
@@ -38,6 +40,7 @@ impl base::ProcMacro for BangProcMacro {
 
 pub struct AttrProcMacro {
     pub client: pm::bridge::client::Client<fn(pm::TokenStream, pm::TokenStream) -> pm::TokenStream>,
+    pub krate: CrateNum,
 }
 
 impl base::AttrProcMacro for AttrProcMacro {
@@ -48,7 +51,7 @@ impl base::AttrProcMacro for AttrProcMacro {
         annotation: TokenStream,
         annotated: TokenStream,
     ) -> Result<TokenStream, ErrorReported> {
-        let server = proc_macro_server::Rustc::new(ecx);
+        let server = proc_macro_server::Rustc::new(ecx, self.krate);
         self.client
             .run(&EXEC_STRATEGY, server, annotation, annotated, ecx.ecfg.proc_macro_backtrace)
             .map_err(|e| {
@@ -64,6 +67,7 @@ impl base::AttrProcMacro for AttrProcMacro {
 
 pub struct ProcMacroDerive {
     pub client: pm::bridge::client::Client<fn(pm::TokenStream) -> pm::TokenStream>,
+    pub krate: CrateNum,
 }
 
 impl MultiItemModifier for ProcMacroDerive {
@@ -97,7 +101,7 @@ impl MultiItemModifier for ProcMacroDerive {
             nt_to_tokenstream(&item, &ecx.sess.parse_sess, CanSynthesizeMissingTokens::No)
         };
 
-        let server = proc_macro_server::Rustc::new(ecx);
+        let server = proc_macro_server::Rustc::new(ecx, self.krate);
         let stream =
             match self.client.run(&EXEC_STRATEGY, server, input, ecx.ecfg.proc_macro_backtrace) {
                 Ok(stream) => stream,