Auto merge of rust-lang#12780 - future-highway:str-to-string-expansions, r=Manishearth

Ignore `_to_string` lints in code `from_expansion`

Includes the `string_to_string` and `str_to_string` lints.

changelog: [`str_to_string`]: Ignore code from expansion
changelog: [`string_to_string`]: Ignore code from expansion

Author: bors

Diff for: clippy_lints/src/strings.rs

@@ -389,6 +389,10 @@ declare_lint_pass!(StrToString => [STR_TO_STRING]);
 
 impl<'tcx> LateLintPass<'tcx> for StrToString {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
         if let ExprKind::MethodCall(path, self_arg, ..) = &expr.kind
             && path.ident.name == sym::to_string
             && let ty = cx.typeck_results().expr_ty(self_arg)
@@ -437,6 +441,10 @@ declare_lint_pass!(StringToString => [STRING_TO_STRING]);
 
 impl<'tcx> LateLintPass<'tcx> for StringToString {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
         if let ExprKind::MethodCall(path, self_arg, ..) = &expr.kind
             && path.ident.name == sym::to_string
             && let ty = cx.typeck_results().expr_ty(self_arg)