Turbopack: improve sectioned source maps (#76627)

### What?

* avoid single entry sectioned source maps
* avoid empty generated source map sections

Author: sokra

turbopack/crates/turbopack-core/src/code_builder.rs

@@ -187,9 +187,15 @@ impl Code {
             }
             last_byte_pos = *byte_pos;
 
-            sections.push((pos, map.clone().unwrap_or_else(SourceMap::empty_rope)))
+            if pos.column != 0 || map.is_some() {
+                sections.push((pos, map.clone().unwrap_or_else(SourceMap::empty_rope)))
+            }
         }
 
-        SourceMap::sections_to_rope(sections)
+        if sections.len() == 1 && sections[0].0.line == 0 && sections[0].0.column == 0 {
+            Ok(sections.into_iter().next().unwrap().1)
+        } else {
+            SourceMap::sections_to_rope(sections)
+        }
     }
 }

turbopack/crates/turbopack-ecmascript/src/minify.rs

@@ -118,7 +118,7 @@ pub fn minify(path: &FileSystemPath, code: &Code, source_maps: bool, mangle: boo
             builder,
             // findSourceMapURL assumes this co-located sourceMappingURL,
             // and needs to be adjusted in case this is ever changed.
-            "\n\n//# sourceMappingURL={}.map",
+            "\n//# sourceMappingURL={}.map",
             urlencoding::encode(path.file_name())
         )?;
     } else {
@@ -156,6 +156,10 @@ fn print_program(
                 .emit_program(&program)
                 .context("failed to emit module")?;
         }
+        if source_maps {
+            // end with a new line when we have a source map comment
+            buf.push(b'\n');
+        }
         // Invalid utf8 is valid in javascript world.
         // SAFETY: SWC generates valid utf8.
         unsafe { String::from_utf8_unchecked(buf) }