Change temporary variable name if it would conflict

Author: camelid

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1337,15 +1337,12 @@ impl<'a> Parser<'a> {
         kind: IncDecRecovery,
         (pre_span, post_span): (Span, Span),
     ) {
-        let mut msg = format!("use `{}= 1` instead", kind.op.chr());
-        if base_src.trim() == "tmp" {
-            msg.push_str(" (rename `tmp` so it doesn't conflict with your variable)");
-        }
+        let tmp_var = if base_src.trim() == "tmp" { "tmp_" } else { "tmp" };
         err.multipart_suggestion(
-            &msg,
+            &format!("use `{}= 1` instead", kind.op.chr()),
             vec![
-                (pre_span, "{ let tmp = ".to_string()),
-                (post_span, format!("; {} {}= 1; tmp }}", base_src, kind.op.chr())),
+                (pre_span, format!("{{ let {} = ", tmp_var)),
+                (post_span, format!("; {} {}= 1; {} }}", base_src, kind.op.chr(), tmp_var)),
             ],
             Applicability::HasPlaceholders,
         );

src/test/ui/parser/increment-autofix.stderr

@@ -36,10 +36,10 @@ error: Rust has no postfix increment operator
 LL |     tmp++;
    |        ^^ not a valid postfix operator
    |
-help: use `+= 1` instead (rename `tmp` so it doesn't conflict with your variable)
+help: use `+= 1` instead
    |
-LL |     { let tmp = tmp; tmp += 1; tmp };
-   |     +++++++++++    ~~~~~~~~~~~~~~~~~
+LL |     { let tmp_ = tmp; tmp += 1; tmp_ };
+   |     ++++++++++++    ~~~~~~~~~~~~~~~~~~
 help: or, if you don't need to use it as an expression, change it to this
    |
 LL -     tmp++;
@@ -52,10 +52,10 @@ error: Rust has no postfix increment operator
 LL |     while tmp++ < 5 {
    |              ^^ not a valid postfix operator
    |
-help: use `+= 1` instead (rename `tmp` so it doesn't conflict with your variable)
+help: use `+= 1` instead
    |
-LL |     while { let tmp = tmp; tmp += 1; tmp } < 5 {
-   |           +++++++++++    ~~~~~~~~~~~~~~~~~
+LL |     while { let tmp_ = tmp; tmp += 1; tmp_ } < 5 {
+   |           ++++++++++++    ~~~~~~~~~~~~~~~~~~
 help: or, if you don't need to use it as an expression, change it to this
    |
 LL -     while tmp++ < 5 {