Expose proc_macro's source_text() on Span

build.rs

@@ -100,6 +100,10 @@ fn main() {
         println!("cargo:rustc-cfg=no_is_available");
     }
 
+    if version.minor < 66 {
+        println!("cargo:rustc-cfg=no_source_text");
+    }
+
     let target = env::var("TARGET").unwrap();
     if !enable_use_proc_macro(&target) {
         return;

src/fallback.rs

@@ -342,6 +342,7 @@ thread_local! {
         files: vec![FileInfo {
             #[cfg(procmacro2_semver_exempt)]
             name: "<unspecified>".to_owned(),
+            source_text: String::new(),
             span: Span { lo: 0, hi: 0 },
             lines: vec![0],
         }],
@@ -352,6 +353,7 @@ thread_local! {
 struct FileInfo {
     #[cfg(procmacro2_semver_exempt)]
     name: String,
+    source_text: String,
     span: Span,
     lines: Vec<usize>,
 }
@@ -379,6 +381,12 @@ impl FileInfo {
     fn span_within(&self, span: Span) -> bool {
         span.lo >= self.span.lo && span.hi <= self.span.hi
     }
+
+    fn source_text(&self, span: Span) -> String {
+        let lo = (span.lo - self.span.lo) as usize;
+        let hi = (span.hi - self.span.lo) as usize;
+        self.source_text[lo..hi].to_owned()
+    }
 }
 
 /// Computes the offsets of each line in the given source string
@@ -425,6 +433,7 @@ impl SourceMap {
         self.files.push(FileInfo {
             #[cfg(procmacro2_semver_exempt)]
             name: name.to_owned(),
+            source_text: src.to_owned(),
             span,
             lines,
         });
@@ -554,6 +563,16 @@ impl Span {
         })
     }
 
+    #[cfg(not(span_locations))]
+    pub fn source_text(&self) -> Option<String> {
+        None
+    }
+
+    #[cfg(span_locations)]
+    pub fn source_text(&self) -> Option<String> {
+        SOURCE_MAP.with(|cm| Some(cm.borrow().fileinfo(*self).source_text(*self)))
+    }
+
     #[cfg(not(span_locations))]
     pub(crate) fn first_byte(self) -> Self {
         self

src/lib.rs

@@ -528,6 +528,17 @@ impl Span {
     pub fn eq(&self, other: &Span) -> bool {
         self.inner.eq(&other.inner)
     }
+
+    /// Returns the source text behind a span. This preserves the original
+    /// source code, including spaces and comments. It only returns a result if
+    /// the span corresponds to real source code.
+    ///
+    /// Note: The observable result of a macro should only rely on the tokens
+    /// and not on this source text. The result of this function is a best
+    /// effort to be used for diagnostics only.
+    pub fn source_text(&self) -> Option<String> {
+        self.inner.source_text()
+    }
 }
 
 /// Prints a span in a form convenient for debugging.

src/wrapper.rs

@@ -530,6 +530,16 @@ impl Span {
         }
     }
 
+    pub fn source_text(&self) -> Option<String> {
+        match self {
+            #[cfg(not(no_source_text))]
+            Span::Compiler(s) => s.source_text(),
+            #[cfg(no_source_text)]
+            Span::Compiler(_) => None,
+            Span::Fallback(s) => s.source_text(),
+        }
+    }
+
     fn unwrap_nightly(self) -> proc_macro::Span {
         match self {
             Span::Compiler(s) => s,