Make the UTF-8 statement more explicit and explicitly test for it

Author: est31

library/proc_macro/src/lib.rs

@@ -432,8 +432,8 @@ pub struct LineColumn {
     /// The 1-indexed line in the source file on which the span starts or ends (inclusive).
     #[unstable(feature = "proc_macro_span", issue = "54725")]
     pub line: usize,
-    /// The 1-indexed column (in UTF-8 characters) in the source file on which
-    /// the span starts or ends (inclusive).
+    /// The 1-indexed column (number of bytes in UTF-8 encoding) in the source
+    /// file on which the span starts or ends (inclusive).
     #[unstable(feature = "proc_macro_span", issue = "54725")]
     pub column: usize,
 }

src/test/ui/proc-macro/span-absolute-posititions.rs

@@ -9,8 +9,16 @@ assert_span_pos::assert_span_pos!(5, 35);
 // Test tab indentation
 	assert_span_pos::assert_span_pos!(10, 36);
 
+// Two tests to ensure the promise of the docs that the column is the number
+// of UTF-8 bytes instead of some other number like number of code points.
+
+// Test that multi byte UTF-8 characters indeed count as multiple bytes
+/*🌈*/assert_span_pos::assert_span_pos!(16, 40);
+// Test with a complete grapheme cluster
+/*🏳️‍🌈*/assert_span_pos::assert_span_pos!(18, 43);
+
 // Test that the macro actually emits an error on a mismatch:
-assert_span_pos::assert_span_pos!(0, 35); //~ ERROR line/column mismatch: (0, 35) != (13, 35)
-assert_span_pos::assert_span_pos!(14, 0); //~ ERROR line/column mismatch: (14, 0) != (14, 35)
+assert_span_pos::assert_span_pos!(0, 35); //~ ERROR line/column mismatch: (0, 35) != (21, 35)
+assert_span_pos::assert_span_pos!(22, 0); //~ ERROR line/column mismatch: (22, 0) != (22, 35)
 
 fn main() {}

src/test/ui/proc-macro/span-absolute-posititions.stderr

@@ -1,13 +1,13 @@
-error: line/column mismatch: (0, 35) != (13, 35)
-  --> $DIR/span-absolute-posititions.rs:13:35
+error: line/column mismatch: (0, 35) != (21, 35)
+  --> $DIR/span-absolute-posititions.rs:21:35
    |
 LL | assert_span_pos::assert_span_pos!(0, 35);
    |                                   ^
 
-error: line/column mismatch: (14, 0) != (14, 35)
-  --> $DIR/span-absolute-posititions.rs:14:35
+error: line/column mismatch: (22, 0) != (22, 35)
+  --> $DIR/span-absolute-posititions.rs:22:35
    |
-LL | assert_span_pos::assert_span_pos!(14, 0);
+LL | assert_span_pos::assert_span_pos!(22, 0);
    |                                   ^^
 
 error: aborting due to 2 previous errors