fix(cargo-fix): dont apply same suggestion twice

This assumes that if any of the machine applicable fixes (`solution`)
in a diagnostic is a duplicate, then `cargo fix` should only apply
the entire suggestion once.

src/cargo/ops/fix.rs

@@ -738,7 +738,17 @@ fn rustfix_and_fix(
         // As mentioned above in `rustfix_crate`, we don't immediately warn
         // about suggestions that fail to apply here, and instead we save them
         // off for later processing.
+        let mut already_applied = HashSet::new();
         for suggestion in suggestions.iter().rev() {
+            // Skip seemly duplicate suggestions.
+            if suggestion
+                .solutions
+                .iter()
+                .any(|sol| !already_applied.insert(sol))
+            {
+                // already applied
+                break;
+            }
             match fixed.apply(suggestion) {
                 Ok(()) => fixed_file.fixes_applied += 1,
                 Err(e) => fixed_file.errors_applying_fixes.push(e.to_string()),

tests/testsuite/fix.rs

@@ -1924,18 +1924,18 @@ fn fix_only_once_for_duplicates() {
         .build();
 
     p.cargo("fix --allow-no-vcs")
-        .with_stderr_contains(
+        .with_stderr(
             "\
 [CHECKING] foo v0.0.1 ([CWD])
-[FIXED] src/lib.rs (2 fixes)
+[FIXED] src/lib.rs (1 fix)
+[FINISHED] `dev` profile [..]
 ",
         )
-        .with_stderr_contains("[WARNING] unnecessary `unsafe` block[..]")
         .run();
 
     assert_eq!(
         p.read_file("src/lib.rs").matches("unsafe").count(),
-        5,
-        "2 in lint name, 1 from original unsafe fn, 2 from newly-applied unsafe blocks"
+        4,
+        "2 in lint name, 1 from original unsafe fn, 1 from newly-applied unsafe blocks"
     );
 }