coverage: `llvm-cov` expects column numbers to be bytes, not code points

2 files changed, 59 insertions, 14 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 98de9d829d2..dcd7014f4fc 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -23,7 +23,7 @@ use rustc_middle::mir::{
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::source_map::SourceMap;
-use rustc_span::{Span, Symbol};
+use rustc_span::{BytePos, Pos, RelativeBytePos, Span, Symbol};
 
 /// Inserts `StatementKind::Coverage` statements that either instrument the binary with injected
 /// counters, via intrinsic `llvm.instrprof.increment`, and/or inject metadata used during codegen
@@ -258,7 +258,16 @@ fn inject_statement(mir_body: &mut mir::Body<'_>, counter_kind: CoverageKind, bb
     data.statements.insert(0, statement);
 }
 
-/// Convert the Span into its file name, start line and column, and end line and column
+/// Convert the Span into its file name, start line and column, and end line and column.
+///
+/// Line numbers and column numbers are 1-based. Unlike most column numbers emitted by
+/// the compiler, these column numbers are denoted in **bytes**, because that's what
+/// LLVM's `llvm-cov` tool expects to see in coverage maps.
+///
+/// Returns `None` if the conversion failed for some reason. This shouldn't happen,
+/// but it's hard to rule out entirely (especially in the presence of complex macros
+/// or other expansions), and if it does happen then skipping a span or function is
+/// better than an ICE or `llvm-cov` failure that the user might have no way to avoid.
 fn make_code_region(
     source_map: &SourceMap,
     file_name: Symbol,
@@ -272,20 +281,55 @@ fn make_code_region(
         source_map.span_to_diagnostic_string(body_span)
     );
 
-    let (file, mut start_line, mut start_col, mut end_line, mut end_col) =
-        source_map.span_to_location_info(span);
-    if span.hi() == span.lo() {
-        // Extend an empty span by one character so the region will be counted.
-        if span.hi() == body_span.hi() {
-            start_col = start_col.saturating_sub(1);
-        } else {
-            end_col = start_col + 1;
-        }
+    let lo = span.lo();
+    let hi = span.hi();
+
+    let file = source_map.lookup_source_file(lo);
+    if !file.contains(hi) {
+        debug!(?span, ?file, ?lo, ?hi, "span crosses multiple files; skipping");
+        return None;
+    }
+
+    // Column numbers need to be in bytes, so we can't use the more convenient
+    // `SourceMap` methods for looking up file coordinates.
+    let rpos_and_line_and_byte_column = |pos: BytePos| -> Option<(RelativeBytePos, usize, usize)> {
+        let rpos = file.relative_position(pos);
+        let line_index = file.lookup_line(rpos)?;
+        let line_start = file.lines()[line_index];
+        // Line numbers and column numbers are 1-based, so add 1 to each.
+        Some((rpos, line_index + 1, (rpos - line_start).to_usize() + 1))
     };
-    if let Some(file) = file {
-        start_line = source_map.doctest_offset_line(&file.name, start_line);
-        end_line = source_map.doctest_offset_line(&file.name, end_line);
+
+    let (lo_rpos, mut start_line, mut start_col) = rpos_and_line_and_byte_column(lo)?;
+    let (hi_rpos, mut end_line, mut end_col) = rpos_and_line_and_byte_column(hi)?;
+
+    // If the span is empty, try to expand it horizontally by one character's
+    // worth of bytes, so that it is more visible in `llvm-cov` reports.
+    // We do this after resolving line/column numbers, so that empty spans at the
+    // end of a line get an extra column instead of wrapping to the next line.
+    if span.is_empty()
+        && body_span.contains(span)
+        && let Some(src) = &file.src
+    {
+        // Prefer to expand the end position, if it won't go outside the body span.
+        if hi < body_span.hi() {
+            let hi_rpos = hi_rpos.to_usize();
+            let nudge_bytes = src.ceil_char_boundary(hi_rpos + 1) - hi_rpos;
+            end_col += nudge_bytes;
+        } else if lo > body_span.lo() {
+            let lo_rpos = lo_rpos.to_usize();
+            let nudge_bytes = lo_rpos - src.floor_char_boundary(lo_rpos - 1);
+            // Subtract the nudge, but don't go below column 1.
+            start_col = start_col.saturating_sub(nudge_bytes).max(1);
+        }
+        // If neither nudge could be applied, stick with the empty span coordinates.
     }
+
+    // Apply an offset so that code in doctests has correct line numbers.
+    // FIXME(#79417): Currently we have no way to offset doctest _columns_.
+    start_line = source_map.doctest_offset_line(&file.name, start_line);
+    end_line = source_map.doctest_offset_line(&file.name, end_line);
+
     Some(CodeRegion {
         file_name,
         start_line: start_line as u32,
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index f5f51c0ec8a..2c1602dadc1 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -9,6 +9,7 @@
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(option_get_or_insert_default)]
+#![feature(round_char_boundary)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]