don't ICE when emitting linker errors during -Z link-only

note that this still ICEs when passed `-Z link-only --error-format json` because i can't be bothered to fix it right now

compiler/rustc_errors/src/json.rs

@@ -45,7 +45,7 @@ pub struct JsonEmitter {
     #[setters(skip)]
     dst: IntoDynSyncSend<Box<dyn Write + Send>>,
     #[setters(skip)]
-    sm: Lrc<SourceMap>,
+    sm: Option<Lrc<SourceMap>>,
     fluent_bundle: Option<Lrc<FluentBundle>>,
     #[setters(skip)]
     fallback_bundle: LazyFallbackBundle,
@@ -65,7 +65,7 @@ pub struct JsonEmitter {
 impl JsonEmitter {
     pub fn new(
         dst: Box<dyn Write + Send>,
-        sm: Lrc<SourceMap>,
+        sm: Option<Lrc<SourceMap>>,
         fallback_bundle: LazyFallbackBundle,
         pretty: bool,
         json_rendered: HumanReadableErrorType,
@@ -171,7 +171,7 @@ impl Emitter for JsonEmitter {
     }
 
     fn source_map(&self) -> Option<&SourceMap> {
-        Some(&self.sm)
+        self.sm.as_deref()
     }
 
     fn should_show_explain(&self) -> bool {
@@ -371,7 +371,7 @@ impl Diagnostic {
         }
         HumanEmitter::new(dst, Lrc::clone(&je.fallback_bundle))
             .short_message(short)
-            .sm(Some(Lrc::clone(&je.sm)))
+            .sm(je.sm.clone())
             .fluent_bundle(je.fluent_bundle.clone())
             .diagnostic_width(je.diagnostic_width)
             .macro_backtrace(je.macro_backtrace)
@@ -458,22 +458,23 @@ impl DiagnosticSpan {
         mut backtrace: impl Iterator<Item = ExpnData>,
         je: &JsonEmitter,
     ) -> DiagnosticSpan {
-        let start = je.sm.lookup_char_pos(span.lo());
+        let sm = je.sm.as_ref().expect("JSON spanned output not supported with -Zlink-only");
+        let start = sm.lookup_char_pos(span.lo());
         // If this goes from the start of a line to the end and the replacement
         // is an empty string, increase the length to include the newline so we don't
         // leave an empty line
         if start.col.0 == 0
             && suggestion.map_or(false, |(s, _)| s.is_empty())
-            && let Ok(after) = je.sm.span_to_next_source(span)
+            && let Ok(after) = sm.span_to_next_source(span)
             && after.starts_with('\n')
         {
             span = span.with_hi(span.hi() + rustc_span::BytePos(1));
         }
-        let end = je.sm.lookup_char_pos(span.hi());
+        let end = sm.lookup_char_pos(span.hi());
         let backtrace_step = backtrace.next().map(|bt| {
             let call_site = Self::from_span_full(bt.call_site, false, None, None, backtrace, je);
             let def_site_span = Self::from_span_full(
-                je.sm.guess_head_span(bt.def_site),
+                sm.guess_head_span(bt.def_site),
                 false,
                 None,
                 None,
@@ -488,7 +489,7 @@ impl DiagnosticSpan {
         });
 
         DiagnosticSpan {
-            file_name: je.sm.filename_for_diagnostics(&start.file.name).to_string(),
+            file_name: sm.filename_for_diagnostics(&start.file.name).to_string(),
             byte_start: start.file.original_relative_byte_pos(span.lo()).0,
             byte_end: start.file.original_relative_byte_pos(span.hi()).0,
             line_start: start.line,
@@ -558,19 +559,20 @@ impl DiagnosticSpanLine {
     /// `span` within the line.
     fn from_span(span: Span, je: &JsonEmitter) -> Vec<DiagnosticSpanLine> {
         je.sm
-            .span_to_lines(span)
-            .map(|lines| {
+            .as_ref()
+            .and_then(|sm| {
+                let lines = sm.span_to_lines(span).ok()?;
                 // We can't get any lines if the source is unavailable.
                 if !should_show_source_code(
                     &je.ignored_directories_in_source_blocks,
-                    &je.sm,
+                    &sm,
                     &lines.file,
                 ) {
-                    return vec![];
+                    return None;
                 }
 
                 let sf = &*lines.file;
-                lines
+                let span_lines = lines
                     .lines
                     .iter()
                     .map(|line| {
@@ -581,8 +583,9 @@ impl DiagnosticSpanLine {
                             line.end_col.0 + 1,
                         )
                     })
-                    .collect()
+                    .collect();
+                Some(span_lines)
             })
-            .unwrap_or_else(|_| vec![])
+            .unwrap_or_default()
     }
 }

compiler/rustc_errors/src/json/tests.rs

@@ -47,7 +47,7 @@ fn test_positions(code: &str, span: (u32, u32), expected_output: SpanTestData) {
         let output = Arc::new(Mutex::new(Vec::new()));
         let je = JsonEmitter::new(
             Box::new(Shared { data: output.clone() }),
-            sm,
+            Some(sm),
             fallback_bundle,
             true, // pretty
             HumanReadableErrorType::Short,

compiler/rustc_session/src/session.rs

@@ -890,13 +890,16 @@ fn default_emitter(
         }
         t => t,
     };
+
+    let source_map = if sopts.unstable_opts.link_only { None } else { Some(source_map) };
+
     match sopts.error_format {
         config::ErrorOutputType::HumanReadable(kind, color_config) => {
             let short = kind.short();
 
             if let HumanReadableErrorType::AnnotateSnippet = kind {
                 let emitter = AnnotateSnippetEmitter::new(
-                    Some(source_map),
+                    source_map,
                     bundle,
                     fallback_bundle,
                     short,
@@ -906,7 +909,7 @@ fn default_emitter(
             } else {
                 let emitter = HumanEmitter::new(stderr_destination(color_config), fallback_bundle)
                     .fluent_bundle(bundle)
-                    .sm(Some(source_map))
+                    .sm(source_map)
                     .short_message(short)
                     .teach(sopts.unstable_opts.teach)
                     .diagnostic_width(sopts.diagnostic_width)
@@ -1437,7 +1440,7 @@ fn mk_emitter(output: ErrorOutputType) -> Box<DynEmitter> {
         config::ErrorOutputType::Json { pretty, json_rendered, color_config } => {
             Box::new(JsonEmitter::new(
                 Box::new(io::BufWriter::new(io::stderr())),
-                Lrc::new(SourceMap::new(FilePathMapping::empty())),
+                Some(Lrc::new(SourceMap::new(FilePathMapping::empty()))),
                 fallback_bundle,
                 pretty,
                 json_rendered,

src/librustdoc/core.rs

@@ -178,7 +178,7 @@ pub(crate) fn new_dcx(
             Box::new(
                 JsonEmitter::new(
                     Box::new(io::BufWriter::new(io::stderr())),
-                    source_map,
+                    Some(source_map),
                     fallback_bundle,
                     pretty,
                     json_rendered,

tests/run-make/linker-warning/rmake.rs

@@ -44,4 +44,33 @@ fn main() {
         .assert_stderr_contains("object files omitted")
         .assert_stderr_contains_regex(r"\{")
         .assert_stderr_not_contains_regex(r"lib(/|\\\\)libstd");
+
+    // Make sure we show linker warnings even across `-Z no-link`
+    rustc()
+        .arg("-Zno-link")
+        .input("-")
+        .stdin_buf("#![deny(linker_messages)] \n fn main() {}")
+        .run()
+        .assert_stderr_equals("");
+    rustc()
+        .arg("-Zlink-only")
+        .arg("rust_out.rlink")
+        .linker("./fake-linker")
+        .link_arg("run_make_warn")
+        .run_fail()
+        // NOTE: the error message here is quite bad (we don't have a source
+        // span, but still try to print the lint source). But `-Z link-only` is
+        // unstable and this still shows the linker warning itself so this is
+        // probably good enough.
+        .assert_stderr_contains("linker stderr: bar");
+
+    // Same thing, but with json output. This currently isn't supported and just ICEs.
+    rustc()
+        .arg("-Zlink-only")
+        .arg("--error-format=json")
+        .arg("rust_out.rlink")
+        .linker("./fake-linker")
+        .link_arg("run_make_warn")
+        .run_fail()
+        .assert_stderr_contains("JSON spanned output not supported with -Zlink-only");
 }