Rollup merge of rust-lang#86165 - m-ou-se:proc-macro-span-shrink, r=dtolnay

Add proc_macro::Span::{before, after}.

This adds `proc_macro::Span::before()` and `proc_macro::Span::after()` to get a zero width span at the start or end of the span.

These are equivalent to rustc's `Span::shrink_to_lo()` and `Span::shrink_to_hi()` but with a less cryptic name. They are useful when generating diagnostlics like "missing \<thing\> after \<thing\>".

E.g.

```rust
syn::Error::new(ident.span().after(), "missing `:` after field name").into_compile_error()
```

Author: Manishearth

compiler/rustc_expand/src/proc_macro_server.rs

@@ -758,6 +758,12 @@ impl server::Span for Rustc<'_> {
         let loc = self.sess.source_map().lookup_char_pos(span.hi());
         LineColumn { line: loc.line, column: loc.col.to_usize() }
     }
+    fn before(&mut self, span: Self::Span) -> Self::Span {
+        span.shrink_to_lo()
+    }
+    fn after(&mut self, span: Self::Span) -> Self::Span {
+        span.shrink_to_hi()
+    }
     fn join(&mut self, first: Self::Span, second: Self::Span) -> Option<Self::Span> {
         let self_loc = self.sess.source_map().lookup_char_pos(first.lo());
         let other_loc = self.sess.source_map().lookup_char_pos(second.lo());

library/proc_macro/src/bridge/mod.rs

@@ -162,6 +162,8 @@ macro_rules! with_api {
                 fn source($self: $S::Span) -> $S::Span;
                 fn start($self: $S::Span) -> LineColumn;
                 fn end($self: $S::Span) -> LineColumn;
+                fn before($self: $S::Span) -> $S::Span;
+                fn after($self: $S::Span) -> $S::Span;
                 fn join($self: $S::Span, other: $S::Span) -> Option<$S::Span>;
                 fn resolved_at($self: $S::Span, at: $S::Span) -> $S::Span;
                 fn source_text($self: $S::Span) -> Option<String>;

library/proc_macro/src/lib.rs

@@ -357,6 +357,18 @@ impl Span {
         self.0.end().add_1_to_column()
     }
 
+    /// Creates an empty span pointing to directly before this span.
+    #[unstable(feature = "proc_macro_span_shrink", issue = "87552")]
+    pub fn before(&self) -> Span {
+        Span(self.0.before())
+    }
+
+    /// Creates an empty span pointing to directly after this span.
+    #[unstable(feature = "proc_macro_span_shrink", issue = "87552")]
+    pub fn after(&self) -> Span {
+        Span(self.0.after())
+    }
+
     /// Creates a new span encompassing `self` and `other`.
     ///
     /// Returns `None` if `self` and `other` are from different files.