Suggest changing quotes when str/char type mismatch

10 files changed, 143 insertions, 3 deletions

diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 9a76c05e4f6..f0c73d0c2f3 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -2041,11 +2041,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 if let ValuePairs::Types(ty::error::ExpectedFound { expected, found }) =
                     trace.values
                 {
-                    // If a tuple of length one was expected and the found expression has
-                    // parentheses around it, perhaps the user meant to write `(expr,)` to
-                    // build a tuple (issue #86100)
                     match (expected.kind(), found.kind()) {
                         (ty::Tuple(_), ty::Tuple(_)) => {}
+                        // If a tuple of length one was expected and the found expression has
+                        // parentheses around it, perhaps the user meant to write `(expr,)` to
+                        // build a tuple (issue #86100)
                         (ty::Tuple(_), _) if expected.tuple_fields().count() == 1 => {
                             if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
                                 if let Some(code) =
@@ -2060,6 +2060,41 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 }
                             }
                         }
+                        // If a character was expected and the found expression is a string literal
+                        // containing a single character, perhaps the user meant to write `'c'` to
+                        // specify a character literal (issue #92479)
+                        (ty::Char, ty::Ref(_, r, _)) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
+                                if let Some(code) =
+                                    code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
+                                {
+                                    if code.chars().nth(1).is_none() {
+                                        err.span_suggestion(
+                                            span,
+                                            "if you meant to write a `char` literal, use single quotes",
+                                            format!("'{}'", code),
+                                            Applicability::MachineApplicable,
+                                        );
+                                    }
+                                }
+                            }
+                        }
+                        // If a string was expected and the found expression is a character literal,
+                        // perhaps the user meant to write `"s"` to specify a string literal.
+                        (ty::Ref(_, r, _), ty::Char) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
+                                if let Some(code) =
+                                    code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
+                                {
+                                    err.span_suggestion(
+                                        span,
+                                        "if you meant to write a `str` literal, use double quotes",
+                                        format!("\"{}\"", code),
+                                        Applicability::MachineApplicable,
+                                    );
+                                }
+                            }
+                        }
                         _ => {}
                     }
                 }
diff --git a/src/test/ui/inference/char-as-str-multi.rs b/src/test/ui/inference/char-as-str-multi.rs
new file mode 100644
index 00000000000..21bbc6f20b2
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-multi.rs
@@ -0,0 +1,6 @@
+// When a MULTI-character string literal is used where a char should be,
+// DO NOT suggest changing to single quotes.
+
+fn main() {
+    let _: char = "foo"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-multi.stderr b/src/test/ui/inference/char-as-str-multi.stderr
new file mode 100644
index 00000000000..c3ba17a5579
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-multi.stderr
@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-multi.rs:5:19
+   |
+LL |     let _: char = "foo";
+   |            ----   ^^^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/inference/char-as-str-single.fixed b/src/test/ui/inference/char-as-str-single.fixed
new file mode 100644
index 00000000000..e401492a830
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.fixed
@@ -0,0 +1,11 @@
+// When a SINGLE-character string literal is used where a char should be,
+// suggest changing to single quotes.
+
+// Testing both single-byte and multi-byte characters, as we should handle both.
+
+// run-rustfix
+
+fn main() {
+    let _: char = 'a'; //~ ERROR mismatched types
+    let _: char = '人'; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-single.rs b/src/test/ui/inference/char-as-str-single.rs
new file mode 100644
index 00000000000..4f23cea5354
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.rs
@@ -0,0 +1,11 @@
+// When a SINGLE-character string literal is used where a char should be,
+// suggest changing to single quotes.
+
+// Testing both single-byte and multi-byte characters, as we should handle both.
+
+// run-rustfix
+
+fn main() {
+    let _: char = "a"; //~ ERROR mismatched types
+    let _: char = "人"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-single.stderr b/src/test/ui/inference/char-as-str-single.stderr
new file mode 100644
index 00000000000..29075c15414
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.stderr
@@ -0,0 +1,29 @@
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-single.rs:9:19
+   |
+LL |     let _: char = "a";
+   |            ----   ^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `char` literal, use single quotes
+   |
+LL |     let _: char = 'a';
+   |                   ~~~
+
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-single.rs:10:19
+   |
+LL |     let _: char = "人";
+   |            ----   ^^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `char` literal, use single quotes
+   |
+LL |     let _: char = '人';
+   |                   ~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/inference/str-as-char.fixed b/src/test/ui/inference/str-as-char.fixed
new file mode 100644
index 00000000000..09f3dec5a17
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.fixed
@@ -0,0 +1,8 @@
+// When a char literal is used where a str should be,
+// suggest changing to double quotes.
+
+// run-rustfix
+
+fn main() {
+    let _: &str = "a"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/str-as-char.rs b/src/test/ui/inference/str-as-char.rs
new file mode 100644
index 00000000000..7092a612442
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.rs
@@ -0,0 +1,8 @@
+// When a char literal is used where a str should be,
+// suggest changing to double quotes.
+
+// run-rustfix
+
+fn main() {
+    let _: &str = 'a'; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/str-as-char.stderr b/src/test/ui/inference/str-as-char.stderr
new file mode 100644
index 00000000000..ebbe7c80f77
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.stderr
@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/str-as-char.rs:7:19
+   |
+LL |     let _: &str = 'a';
+   |            ----   ^^^ expected `&str`, found `char`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     let _: &str = "a";
+   |                   ~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/issues/issue-23589.stderr b/src/test/ui/issues/issue-23589.stderr
index d126e1bf0b5..e065e17c280 100644
--- a/src/test/ui/issues/issue-23589.stderr
+++ b/src/test/ui/issues/issue-23589.stderr
@@ -12,6 +12,11 @@ error[E0308]: mismatched types
    |
 LL |     let v: Vec(&str) = vec!['1', '2'];
    |                             ^^^ expected `&str`, found `char`
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     let v: Vec(&str) = vec!["1", '2'];
+   |                             ~~~
 
 error: aborting due to 2 previous errors