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`.

1 files changed, 52 insertions, 0 deletions

diff --git a/src/test/ui/proc-macro/quote-debug.stdout b/src/test/ui/proc-macro/quote-debug.stdout
new file mode 100644
index 00000000000..4bdc04b9ac4
--- /dev/null
+++ b/src/test/ui/proc-macro/quote-debug.stdout
@@ -0,0 +1,52 @@
+#![feature(prelude_import)]
+#![no_std]
+// 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"]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+
+extern crate proc_macro;
+
+fn main() {
+    [crate::TokenStream::from(crate::TokenTree::Ident(crate::Ident::new("let",
+                                                                        crate::Span::recover_proc_macro_span(0)))),
+     crate::TokenStream::from(crate::TokenTree::Ident(crate::Ident::new("hello",
+                                                                        crate::Span::recover_proc_macro_span(1)))),
+     crate::TokenStream::from(crate::TokenTree::Punct(crate::Punct::new('\u{3d}',
+                                                                        crate::Spacing::Alone))),
+     crate::TokenStream::from(crate::TokenTree::Literal({
+                                                            let mut iter =
+                                                                "\"world\"".parse::<crate::TokenStream>().unwrap().into_iter();
+                                                            if let (Some(crate::TokenTree::Literal(mut lit)),
+                                                                    None) =
+                                                                   (iter.next(),
+                                                                    iter.next())
+                                                               {
+                                                                lit.set_span(crate::Span::recover_proc_macro_span(2));
+                                                                lit
+                                                            } else {
+                                                                {
+                                                                    ::core::panicking::panic("internal error: entered unreachable code")
+                                                                }
+                                                            }
+                                                        })),
+     crate::TokenStream::from(crate::TokenTree::Punct(crate::Punct::new('\u{3b}',
+                                                                        crate::Spacing::Alone)))].iter().cloned().collect::<crate::TokenStream>()
+}
+const _: () =
+    {
+        extern crate proc_macro;
+        #[rustc_proc_macro_decls]
+        #[allow(deprecated)]
+        static _DECLS: &[proc_macro::bridge::client::ProcMacro] = &[];
+    };