use structured fields in some existing warnings

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Author: hawkw

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -844,18 +844,17 @@ fn link_natively<'a, B: ArchiveBuilder<'a>>(
         if out.contains(msg_segv) || out.contains(msg_bus) {
             warn!(
                 "looks like the linker segfaulted when we tried to call it, \
-                 automatically retrying again. cmd = {:?}, out = {}.",
-                cmd, out,
+                 automatically retrying again",
+                ?cmd, %out,
             );
             continue;
         }
 
         if is_illegal_instruction(&output.status) {
             warn!(
                 "looks like the linker hit an illegal instruction when we \
-                 tried to call it, automatically retrying again. cmd = {:?}, ]\
-                 out = {}, status = {}.",
-                cmd, out, output.status,
+                 tried to call it, automatically retrying again."
+                ?cmd, %out, status = %output.status,
             );
             continue;
         }

compiler/rustc_errors/src/emitter.rs

@@ -2308,7 +2308,7 @@ pub fn is_case_difference(sm: &SourceMap, suggested: &str, sp: Span) -> bool {
     let found = match sm.span_to_snippet(sp) {
         Ok(snippet) => snippet,
         Err(e) => {
-            warn!("Invalid span {:?}. Err={:?}", sp, e);
+            warn!(error = ?e, "Invalid span {:?}", sp);
             return false;
         }
     };