Rollup merge of #132675 - Zalathar:empty-spans, r=jieyouxu

coverage: Restrict empty-span expansion to only cover `{` and `}`

Coverage instrumentation has some tricky code for converting a coverage-relevant `Span` into a set of start/end line/byte-column coordinates that will be embedded in the CGU's coverage metadata.

A big part of this complexity is special code for handling empty spans, which are expanded into non-empty spans (if possible) because LLVM's coverage reporter does not handle empty spans well.

This PR simplifies that code by restricting it to only apply in two specific situations: when the character after the empty span is `{`, or the character before the empty span is `}`.

(As an added benefit, this means that the expanded spans no longer extend awkwardly beyond the end of a physical line, which was common under the previous implementation.)

Along the way, this PR also removes some unhelpful code for dealing with function source code spread across multiple files. Functions currently can't have coverage spans in multiple files, and if that ever changes (e.g. to properly support expansion regions) then this code will need to be completely overhauled anyway.

2 files changed, 7 insertions, 8 deletions

diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
index 4a876dc1228..11a4e7f89a7 100644
--- a/compiler/rustc_middle/src/mir/coverage.rs
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -4,7 +4,7 @@ use std::fmt::{self, Debug, Formatter};
 
 use rustc_index::IndexVec;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
-use rustc_span::{Span, Symbol};
+use rustc_span::Span;
 
 rustc_index::newtype_index! {
     /// Used by [`CoverageKind::BlockMarker`] to mark blocks during THIR-to-MIR
@@ -158,7 +158,6 @@ impl Debug for CoverageKind {
 #[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, Eq, PartialOrd, Ord)]
 #[derive(TypeFoldable, TypeVisitable)]
 pub struct SourceRegion {
-    pub file_name: Symbol,
     pub start_line: u32,
     pub start_col: u32,
     pub end_line: u32,
@@ -167,11 +166,8 @@ pub struct SourceRegion {
 
 impl Debug for SourceRegion {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
-        write!(
-            fmt,
-            "{}:{}:{} - {}:{}",
-            self.file_name, self.start_line, self.start_col, self.end_line, self.end_col
-        )
+        let &Self { start_line, start_col, end_line, end_col } = self;
+        write!(fmt, "{start_line}:{start_col} - {end_line}:{end_col}")
     }
 }
 
@@ -246,6 +242,7 @@ pub struct Mapping {
 #[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct FunctionCoverageInfo {
     pub function_source_hash: u64,
+    pub body_span: Span,
     pub num_counters: usize,
     pub mcdc_bitmap_bits: usize,
     pub expressions: IndexVec<ExpressionId, Expression>,
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index d0f93c19e44..f0e2b7a376c 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -596,8 +596,10 @@ fn write_function_coverage_info(
     function_coverage_info: &coverage::FunctionCoverageInfo,
     w: &mut dyn io::Write,
 ) -> io::Result<()> {
-    let coverage::FunctionCoverageInfo { expressions, mappings, .. } = function_coverage_info;
+    let coverage::FunctionCoverageInfo { body_span, expressions, mappings, .. } =
+        function_coverage_info;
 
+    writeln!(w, "{INDENT}coverage body span: {body_span:?}")?;
     for (id, expression) in expressions.iter_enumerated() {
         writeln!(w, "{INDENT}coverage {id:?} => {expression:?};")?;
     }