Fix ICE caused by suggestion with no code substitutions

compiler/rustc_errors/src/diagnostic.rs

@@ -295,6 +295,7 @@ impl Diagnostic {
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
+        assert!(!suggestion.is_empty());
         self.suggestions.push(CodeSuggestion {
             substitutions: vec![Substitution {
                 parts: suggestion
@@ -318,6 +319,10 @@ impl Diagnostic {
         suggestions: Vec<Vec<(Span, String)>>,
         applicability: Applicability,
     ) -> &mut Self {
+        assert!(!suggestions.is_empty());
+        for s in &suggestions {
+            assert!(!s.is_empty());
+        }
         self.suggestions.push(CodeSuggestion {
             substitutions: suggestions
                 .into_iter()
@@ -348,6 +353,7 @@ impl Diagnostic {
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
+        assert!(!suggestion.is_empty());
         self.suggestions.push(CodeSuggestion {
             substitutions: vec![Substitution {
                 parts: suggestion

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -319,9 +319,13 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             .collect::<Vec<_>>();
         let crate_def_id = DefId::local(CRATE_DEF_INDEX);
         if candidates.is_empty() && is_expected(Res::Def(DefKind::Enum, crate_def_id)) {
-            let enum_candidates =
-                self.r.lookup_import_candidates(ident, ns, &self.parent_scope, is_enum_variant);
-
+            let mut enum_candidates: Vec<_> = self
+                .r
+                .lookup_import_candidates(ident, ns, &self.parent_scope, is_enum_variant)
+                .into_iter()
+                .map(|suggestion| import_candidate_to_enum_paths(&suggestion))
+                .filter(|(_, enum_ty_path)| enum_ty_path != "std::prelude::v1")
+                .collect();
             if !enum_candidates.is_empty() {
                 if let (PathSource::Type, Some(span)) =
                     (source, self.diagnostic_metadata.current_type_ascription.last())
@@ -340,10 +344,6 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     }
                 }
 
-                let mut enum_candidates = enum_candidates
-                    .iter()
-                    .map(|suggestion| import_candidate_to_enum_paths(&suggestion))
-                    .collect::<Vec<_>>();
                 enum_candidates.sort();
 
                 // Contextualize for E0412 "cannot find type", but don't belabor the point
@@ -363,19 +363,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                 err.span_suggestions(
                     span,
                     &msg,
-                    enum_candidates
-                        .into_iter()
-                        .map(|(_variant_path, enum_ty_path)| enum_ty_path)
-                        // Variants re-exported in prelude doesn't mean `prelude::v1` is the
-                        // type name!
-                        // FIXME: is there a more principled way to do this that
-                        // would work for other re-exports?
-                        .filter(|enum_ty_path| enum_ty_path != "std::prelude::v1")
-                        // Also write `Option` rather than `std::prelude::v1::Option`.
-                        .map(|enum_ty_path| {
-                            // FIXME #56861: DRY-er prelude filtering.
-                            enum_ty_path.trim_start_matches("std::prelude::v1::").to_owned()
-                        }),
+                    enum_candidates.into_iter().map(|(_variant_path, enum_ty_path)| enum_ty_path),
                     Applicability::MachineApplicable,
                 );
             }

src/test/ui/suggestions/do-not-attempt-to-add-suggestions-with-no-changes.rs

@@ -0,0 +1,5 @@
+use std::result;
+impl result { //~ ERROR expected type, found module `result`
+    fn into_future() -> Err {} //~ ERROR expected type, found variant `Err`
+}
+fn main() {}

src/test/ui/suggestions/do-not-attempt-to-add-suggestions-with-no-changes.stderr

@@ -0,0 +1,20 @@
+error[E0573]: expected type, found module `result`
+  --> $DIR/do-not-attempt-to-add-suggestions-with-no-changes.rs:2:6
+   |
+LL | impl result {
+   |      ^^^^^^ help: an enum with a similar name exists: `Result`
+   | 
+  ::: $SRC_DIR/core/src/result.rs:LL:COL
+   |
+LL | pub enum Result<T, E> {
+   | --------------------- similarly named enum `Result` defined here
+
+error[E0573]: expected type, found variant `Err`
+  --> $DIR/do-not-attempt-to-add-suggestions-with-no-changes.rs:3:25
+   |
+LL |     fn into_future() -> Err {}
+   |                         ^^^ not a type
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0573`.