proc_macro/bridge: send diagnostics over the bridge as a struct

This removes some RPC when creating and emitting diagnostics, and
simplifies the bridge slightly.

After this change, there are no remaining methods which take advantage
of the support for `&mut` references to objects in the store as
arguments, meaning that support for them could technically be removed if
we wanted. The only remaining uses of immutable references into the
store are `TokenStream` and `SourceFile`.

library/proc_macro/src/bridge/client.rs

@@ -176,8 +176,6 @@ define_handles! {
     FreeFunctions,
     TokenStream,
     SourceFile,
-    MultiSpan,
-    Diagnostic,
 
     'interned:
     Span,

library/proc_macro/src/bridge/mod.rs

@@ -57,6 +57,7 @@ macro_rules! with_api {
                 fn track_env_var(var: &str, value: Option<&str>);
                 fn track_path(path: &str);
                 fn literal_from_str(s: &str) -> Result<Literal<$S::Span, $S::Symbol>, ()>;
+                fn emit_diagnostic(diagnostic: Diagnostic<$S::Span>);
             },
             TokenStream {
                 fn drop($self: $S::TokenStream);
@@ -87,22 +88,6 @@ macro_rules! with_api {
                 fn path($self: &$S::SourceFile) -> String;
                 fn is_real($self: &$S::SourceFile) -> bool;
             },
-            MultiSpan {
-                fn drop($self: $S::MultiSpan);
-                fn new() -> $S::MultiSpan;
-                fn push($self: &mut $S::MultiSpan, span: $S::Span);
-            },
-            Diagnostic {
-                fn drop($self: $S::Diagnostic);
-                fn new(level: Level, msg: &str, span: $S::MultiSpan) -> $S::Diagnostic;
-                fn sub(
-                    $self: &mut $S::Diagnostic,
-                    level: Level,
-                    msg: &str,
-                    span: $S::MultiSpan,
-                );
-                fn emit($self: $S::Diagnostic);
-            },
             Span {
                 fn debug($self: $S::Span) -> String;
                 fn source_file($self: $S::Span) -> $S::SourceFile;
@@ -510,6 +495,18 @@ compound_traits!(
     }
 );
 
+#[derive(Clone, Debug)]
+pub struct Diagnostic<Span> {
+    pub level: Level,
+    pub message: String,
+    pub spans: Vec<Span>,
+    pub children: Vec<Diagnostic<Span>>,
+}
+
+compound_traits!(
+    struct Diagnostic<Span> { level, message, spans, children }
+);
+
 /// Globals provided alongside the initial inputs for a macro expansion.
 /// Provides values such as spans which are used frequently to avoid RPC.
 #[derive(Clone)]

library/proc_macro/src/bridge/server.rs

@@ -11,8 +11,6 @@ pub trait Types {
     type FreeFunctions: 'static;
     type TokenStream: 'static + Clone;
     type SourceFile: 'static + Clone;
-    type MultiSpan: 'static;
-    type Diagnostic: 'static;
     type Span: 'static + Copy + Eq + Hash;
     type Symbol: 'static;
 }

library/proc_macro/src/diagnostic.rs

@@ -161,22 +161,15 @@ impl Diagnostic {
     /// Emit the diagnostic.
     #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
     pub fn emit(self) {
-        fn to_internal(spans: Vec<Span>) -> crate::bridge::client::MultiSpan {
-            let mut multi_span = crate::bridge::client::MultiSpan::new();
-            for span in spans {
-                multi_span.push(span.0);
+        fn to_internal(diag: Diagnostic) -> crate::bridge::Diagnostic<crate::bridge::client::Span> {
+            crate::bridge::Diagnostic {
+                level: diag.level,
+                message: diag.message,
+                spans: diag.spans.into_iter().map(|s| s.0).collect(),
+                children: diag.children.into_iter().map(to_internal).collect(),
             }
-            multi_span
         }
 
-        let mut diag = crate::bridge::client::Diagnostic::new(
-            self.level,
-            &self.message[..],
-            to_internal(self.spans),
-        );
-        for c in self.children {
-            diag.sub(c.level, &c.message[..], to_internal(c.spans));
-        }
-        diag.emit();
+        crate::bridge::client::FreeFunctions::emit_diagnostic(to_internal(self));
     }
 }