From 130d02b62e65c5f2a434eaec63c4249e9d508487 Mon Sep 17 00:00:00 2001
From: Nick Lamb <git@tree.tlrmx.org>
Date: Sat, 14 Jan 2023 14:51:17 +0000
Subject: [PATCH] Improve E0308: suggest user meant to use byte literal, w/
tests and fix suggested by Nilstrieb
Co-authored-by: nils <48135649+Nilstrieb@users.noreply.github.com>
---
.../src/infer/error_reporting/mod.rs | 16 +++++++
.../suggestions/type-mismatch-byte-literal.rs | 18 ++++++++
.../type-mismatch-byte-literal.stderr | 42 +++++++++++++++++++
3 files changed, 76 insertions(+)
create mode 100644 tests/ui/suggestions/type-mismatch-byte-literal.rs
create mode 100644 tests/ui/suggestions/type-mismatch-byte-literal.stderr
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 533a3c768eb16..abd99fc74dacc 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -1923,6 +1923,22 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
(ty::Tuple(fields), _) => {
self.emit_tuple_wrap_err(&mut err, span, found, fields)
}
+ // If a byte was expected and the found expression is a char literal
+ // containing a single ASCII character, perhaps the user meant to write `b'c'` to
+ // specify a byte literal
+ (ty::Uint(ty::UintTy::U8), ty::Char) => {
+ if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
+ && let Some(code) = code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
+ && code.chars().next().map_or(false, |c| c.is_ascii())
+ {
+ err.span_suggestion(
+ span,
+ "if you meant to write a byte literal, prefix with `b`",
+ format!("b'{}'", escape_literal(code)),
+ Applicability::MachineApplicable,
+ );
+ }
+ }
// 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)
diff --git a/tests/ui/suggestions/type-mismatch-byte-literal.rs b/tests/ui/suggestions/type-mismatch-byte-literal.rs
new file mode 100644
index 0000000000000..34199f8c37c10
--- /dev/null
+++ b/tests/ui/suggestions/type-mismatch-byte-literal.rs
@@ -0,0 +1,18 @@
+// Tests that a suggestion is issued for type mismatch errors when a
+// u8 is expected and a char literal which is ASCII is supplied.
+
+fn foo(_t: u8) {}
+
+fn main() {
+ let _x: u8 = 'X';
+ //~^ ERROR: mismatched types [E0308]
+ //~| HELP: if you meant to write a byte literal, prefix with `b`
+
+ foo('#');
+ //~^ ERROR: mismatched types [E0308]
+ //~| HELP: if you meant to write a byte literal, prefix with `b`
+
+ // Do not issue the suggestion if the char literal isn't ASCII
+ let _t: u8 = '€';
+ //~^ ERROR: mismatched types [E0308]
+}
diff --git a/tests/ui/suggestions/type-mismatch-byte-literal.stderr b/tests/ui/suggestions/type-mismatch-byte-literal.stderr
new file mode 100644
index 0000000000000..c9c2e7498d058
--- /dev/null
+++ b/tests/ui/suggestions/type-mismatch-byte-literal.stderr
@@ -0,0 +1,42 @@
+error[E0308]: mismatched types
+ --> $DIR/type-mismatch-byte-literal.rs:7:18
+ |
+LL | let _x: u8 = 'X';
+ | -- ^^^ expected `u8`, found `char`
+ | |
+ | expected due to this
+ |
+help: if you meant to write a byte literal, prefix with `b`
+ |
+LL | let _x: u8 = b'X';
+ | ~~~~
+
+error[E0308]: mismatched types
+ --> $DIR/type-mismatch-byte-literal.rs:11:9
+ |
+LL | foo('#');
+ | --- ^^^ expected `u8`, found `char`
+ | |
+ | arguments to this function are incorrect
+ |
+note: function defined here
+ --> $DIR/type-mismatch-byte-literal.rs:4:4
+ |
+LL | fn foo(_t: u8) {}
+ | ^^^ ------
+help: if you meant to write a byte literal, prefix with `b`
+ |
+LL | foo(b'#');
+ | ~~~~
+
+error[E0308]: mismatched types
+ --> $DIR/type-mismatch-byte-literal.rs:16:18
+ |
+LL | let _t: u8 = '€';
+ | -- ^^^ expected `u8`, found `char`
+ | |
+ | expected due to this
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.