Auto merge of #99780 - Nilstrieb:mir-opt-test-line-no, r=oli-obk

Use line numbers relative to the function in mir-opt tests

As shown in #99770, the line numbers can be a big source of needless and confusing diffs. This PR adds a new flag `-Zmir-pretty-relative-line-numbers` to make them relative to the function declaration, which avoids most needless diffs from attribute changes.

`@JakobDegen` told me that there has been a zulip conversation about disabling line numbers with mixed opinions, so I'd like to get some feedback here, for this hopefully better solution.

r? rust-lang/wg-mir-opt

4 files changed, 42 insertions, 6 deletions

diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 21d9eaccf67..57ce4933a3b 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -670,6 +670,7 @@ fn test_unstable_options_tracking_hash() {
     untracked!(ls, true);
     untracked!(macro_backtrace, true);
     untracked!(meta_stats, true);
+    untracked!(mir_pretty_relative_line_numbers, true);
     untracked!(nll_facts, true);
     untracked!(no_analysis, true);
     untracked!(no_interleave_lints, true);
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 78b5131bac6..0ce41337b91 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -360,7 +360,7 @@ where
             "{:A$} // {}{}",
             indented_body,
             if tcx.sess.verbose() { format!("{:?}: ", current_location) } else { String::new() },
-            comment(tcx, statement.source_info),
+            comment(tcx, statement.source_info, body.span),
             A = ALIGN,
         )?;
 
@@ -381,7 +381,7 @@ where
         "{:A$} // {}{}",
         indented_terminator,
         if tcx.sess.verbose() { format!("{:?}: ", current_location) } else { String::new() },
-        comment(tcx, data.terminator().source_info),
+        comment(tcx, data.terminator().source_info, body.span),
         A = ALIGN,
     )?;
 
@@ -518,8 +518,14 @@ impl<'tcx> Visitor<'tcx> for ExtraComments<'tcx> {
     }
 }
 
-fn comment(tcx: TyCtxt<'_>, SourceInfo { span, scope }: SourceInfo) -> String {
-    format!("scope {} at {}", scope.index(), tcx.sess.source_map().span_to_embeddable_string(span))
+fn comment(tcx: TyCtxt<'_>, SourceInfo { span, scope }: SourceInfo, function_span: Span) -> String {
+    let location = if tcx.sess.opts.unstable_opts.mir_pretty_relative_line_numbers {
+        tcx.sess.source_map().span_to_relative_line_string(span, function_span)
+    } else {
+        tcx.sess.source_map().span_to_embeddable_string(span)
+    };
+
+    format!("scope {} at {}", scope.index(), location,)
 }
 
 /// Prints local variables in a scope tree.
@@ -550,7 +556,7 @@ fn write_scope_tree(
             "{0:1$} // in {2}",
             indented_debug_info,
             ALIGN,
-            comment(tcx, var_debug_info.source_info),
+            comment(tcx, var_debug_info.source_info, body.span),
         )?;
     }
 
@@ -585,7 +591,7 @@ fn write_scope_tree(
             indented_decl,
             ALIGN,
             local_name,
-            comment(tcx, local_decl.source_info),
+            comment(tcx, local_decl.source_info, body.span),
         )?;
     }
 
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 501997679f4..ef314115043 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1379,6 +1379,8 @@ options! {
         "use like `-Zmir-enable-passes=+DestProp,-InstCombine`. Forces the specified passes to be \
         enabled, overriding all other checks. Passes that are not specified are enabled or \
         disabled by other flags as usual."),
+    mir_pretty_relative_line_numbers: bool = (false, parse_bool, [UNTRACKED],
+        "use line numbers relative to the function in mir pretty printing"),
     #[cfg_attr(not(bootstrap), rustc_lint_opt_deny_field_access("use `Session::mir_opt_level` instead of this field"))]
     mir_opt_level: Option<usize> = (None, parse_opt_number, [TRACKED],
         "MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)"),
diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 47159584afe..28381157d50 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -463,6 +463,33 @@ impl SourceMap {
         self.span_to_string(sp, FileNameDisplayPreference::Remapped)
     }
 
+    /// Format the span location suitable for pretty printing anotations with relative line numbers
+    pub fn span_to_relative_line_string(&self, sp: Span, relative_to: Span) -> String {
+        if self.files.borrow().source_files.is_empty() || sp.is_dummy() || relative_to.is_dummy() {
+            return "no-location".to_string();
+        }
+
+        let lo = self.lookup_char_pos(sp.lo());
+        let hi = self.lookup_char_pos(sp.hi());
+        let offset = self.lookup_char_pos(relative_to.lo());
+
+        if lo.file.name != offset.file.name {
+            return self.span_to_embeddable_string(sp);
+        }
+
+        let lo_line = lo.line.saturating_sub(offset.line);
+        let hi_line = hi.line.saturating_sub(offset.line);
+
+        format!(
+            "{}:+{}:{}: +{}:{}",
+            lo.file.name.display(FileNameDisplayPreference::Remapped),
+            lo_line,
+            lo.col.to_usize() + 1,
+            hi_line,
+            hi.col.to_usize() + 1,
+        )
+    }
+
     /// Format the span location to be printed in diagnostics. Must not be emitted
     /// to build artifacts as this may leak local file paths. Use span_to_embeddable_string
     /// for string suitable for embedding.