WIP: Suggest similar type or module on resolve failure

src/librustc_resolve/diagnostics.rs

@@ -797,6 +797,14 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    fn typo_suggestion_text(&self, suggestion: &TypoSuggestion) -> String {
+        format!(
+            "{} {} with a similar name exists",
+            suggestion.res.article(),
+            suggestion.res.descr()
+        )
+    }
+
     crate fn add_typo_suggestion(
         &self,
         err: &mut DiagnosticBuilder<'_>,
@@ -809,14 +817,9 @@ impl<'a> Resolver<'a> {
                 return false;
             }
 
-            let msg = format!(
-                "{} {} with a similar name exists",
-                suggestion.res.article(),
-                suggestion.res.descr()
-            );
             err.span_suggestion(
                 span,
-                &msg,
+                &self.typo_suggestion_text(&suggestion),
                 suggestion.candidate.to_string(),
                 Applicability::MaybeIncorrect,
             );
@@ -999,6 +1002,32 @@ impl<'a> Resolver<'a> {
 
         err.emit();
     }
+
+    crate fn make_undeclared_type_suggestion(
+        &mut self,
+        ident: Ident,
+        parent_scope: &ParentScope<'a>,
+        ns: Namespace,
+    ) -> (String, Option<Suggestion>) {
+        let typo_suggestion = self.early_lookup_typo_candidate(
+            ScopeSet::All(ns, false),
+            parent_scope,
+            ident,
+            &|_| true,
+        );
+        if let Some(typo_suggestion) = typo_suggestion {
+            (
+                format!("use of undeclared type or module `{}`", ident),
+                Some((
+                    vec![(ident.span, format!("{}", typo_suggestion.candidate.as_str()))],
+                    self.typo_suggestion_text(&typo_suggestion),
+                    Applicability::MaybeIncorrect,
+                )),
+            )
+        } else {
+            (format!("use of undeclared type or module `{}`", ident), None)
+        }
+    }
 }
 
 impl<'a, 'b> ImportResolver<'a, 'b> {

src/librustc_resolve/lib.rs

@@ -2195,7 +2195,7 @@ impl<'a> Resolver<'a> {
                             return PathResult::NonModule(PartialRes::new(Res::Err));
                         }
                     } else if i == 0 {
-                        (format!("use of undeclared type or module `{}`", ident), None)
+                        self.make_undeclared_type_suggestion(ident, &parent_scope, ns)
                     } else {
                         (format!("could not find `{}` in `{}`", ident, path[i - 1].ident), None)
                     };

src/test/ui/macros/macro-inner-attributes.stderr

@@ -2,7 +2,10 @@ error[E0433]: failed to resolve: use of undeclared type or module `a`
   --> $DIR/macro-inner-attributes.rs:17:5
    |
 LL |     a::bar();
-   |     ^ use of undeclared type or module `a`
+   |     ^
+   |     |
+   |     use of undeclared type or module `a`
+   |     help: a module with a similar name exists: `b`
 
 error: aborting due to previous error
 

src/test/ui/pattern/pattern-error-continue.stderr

@@ -2,7 +2,10 @@ error[E0433]: failed to resolve: use of undeclared type or module `E`
   --> $DIR/pattern-error-continue.rs:33:9
    |
 LL |         E::V => {}
-   |         ^ use of undeclared type or module `E`
+   |         ^
+   |         |
+   |         use of undeclared type or module `E`
+   |         help: an enum with a similar name exists: `A`
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `A::D`
   --> $DIR/pattern-error-continue.rs:18:9

src/test/ui/resolve/suggest-type.rs

@@ -0,0 +1,24 @@
+use std::ffi::CString;
+
+mod foo {
+    use std::collections::HashMap;
+
+    fn bar() {
+        let _ = HashNap::new();
+        //~^ ERROR failed to resolve: use of undeclared type or module `HashNap`
+        //~| HELP a struct with a similar name exists
+        //~| SUGGESTION HashMap
+    }
+}
+
+fn main() {
+    let _ = Cstring::new("hello").unwrap();
+    //~^ ERROR failed to resolve: use of undeclared type or module `Cstring`
+    //~| HELP a struct with a similar name exists
+    //~| SUGGESTION CString
+
+    let _ = foO::bar();
+    //~^ ERROR failed to resolve: use of undeclared type or module `foO`
+    //~| HELP a module with a similar name exists
+    //~| SUGGESTION foo
+}

src/test/ui/resolve/suggest-type.stderr

@@ -0,0 +1,30 @@
+error[E0433]: failed to resolve: use of undeclared type or module `HashNap`
+  --> $DIR/suggest-type.rs:7:17
+   |
+LL |         let _ = HashNap::new();
+   |                 ^^^^^^^
+   |                 |
+   |                 use of undeclared type or module `HashNap`
+   |                 help: a struct with a similar name exists: `HashMap`
+
+error[E0433]: failed to resolve: use of undeclared type or module `Cstring`
+  --> $DIR/suggest-type.rs:15:13
+   |
+LL |     let _ = Cstring::new("hello").unwrap();
+   |             ^^^^^^^
+   |             |
+   |             use of undeclared type or module `Cstring`
+   |             help: a struct with a similar name exists (notice the capitalization): `CString`
+
+error[E0433]: failed to resolve: use of undeclared type or module `foO`
+  --> $DIR/suggest-type.rs:20:13
+   |
+LL |     let _ = foO::bar();
+   |             ^^^
+   |             |
+   |             use of undeclared type or module `foO`
+   |             help: a module with a similar name exists (notice the capitalization): `foo`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0433`.