Auto merge of #84278 - Aaron1011:feature/new-proc-macro-meta-span, r=estebank

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, 18 insertions, 3 deletions

diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index a58caf2667b..61870eede1d 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -309,7 +309,9 @@ pub trait Emitter {
                     // are some which do actually involve macros.
                     ExpnKind::Inlined | ExpnKind::Desugaring(..) | ExpnKind::AstPass(..) => None,
 
-                    ExpnKind::Macro(macro_kind, _) => Some(macro_kind),
+                    ExpnKind::Macro { kind: macro_kind, name: _, proc_macro: _ } => {
+                        Some(macro_kind)
+                    }
                 }
             });
 
@@ -371,10 +373,19 @@ pub trait Emitter {
                     new_labels
                         .push((trace.call_site, "in the inlined copy of this code".to_string()));
                 } else if always_backtrace {
+                    let proc_macro = if let ExpnKind::Macro { kind: _, name: _, proc_macro: true } =
+                        trace.kind
+                    {
+                        "procedural macro "
+                    } else {
+                        ""
+                    };
+
                     new_labels.push((
                         trace.def_site,
                         format!(
-                            "in this expansion of `{}`{}",
+                            "in this expansion of {}`{}`{}",
+                            proc_macro,
                             trace.kind.descr(),
                             if macro_backtrace.len() > 1 {
                                 // if macro_backtrace.len() == 1 it'll be
@@ -400,7 +411,11 @@ pub trait Emitter {
                 // and it needs an "in this macro invocation" label to match that.
                 let redundant_span = trace.call_site.contains(sp);
 
-                if !redundant_span && matches!(trace.kind, ExpnKind::Macro(MacroKind::Bang, _))
+                if !redundant_span
+                    && matches!(
+                        trace.kind,
+                        ExpnKind::Macro { kind: MacroKind::Bang, name: _, proc_macro: _ }
+                    )
                     || always_backtrace
                 {
                     new_labels.push((