Fix single_char_pattern crash (#3204)

This commit fixes the crash by removing constant checking from the lint.

Closes #3204.

Author: Michael Wright

clippy_lints/src/methods.rs

@@ -16,7 +16,6 @@ use crate::utils::{get_arg_name, get_trait_def_id, implements_trait, in_macro, i
             span_lint, span_lint_and_sugg, span_lint_and_then, span_note_and_lint, walk_ptrs_ty, walk_ptrs_ty_depth, SpanlessEq};
 use crate::utils::paths;
 use crate::utils::sugg;
-use crate::consts::{constant, Constant};
 use crate::rustc_errors::Applicability;
 
 #[derive(Clone)]
@@ -1914,8 +1913,11 @@ fn lint_chars_last_cmp_with_unwrap<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, info: &
 
 /// lint for length-1 `str`s for methods in `PATTERN_METHODS`
 fn lint_single_char_pattern<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, _expr: &'tcx hir::Expr, arg: &'tcx hir::Expr) {
-    if let Some((Constant::Str(r), _)) = constant(cx, cx.tables, arg) {
-        if r.len() == 1 {
+    if_chain! {
+        if let hir::ExprKind::Lit(lit) = &arg.node;
+        if let ast::LitKind::Str(r, _) = lit.node;
+        if r.as_str().len() == 1;
+        then {
             let snip = snippet(cx, arg.span, "..");
             let hint = format!("'{}'", &snip[1..snip.len() - 1]);
             span_lint_and_sugg(

tests/ui/single_char_pattern.rs

@@ -45,4 +45,8 @@ fn main() {
 
     x.replace(";", ",").split(","); // issue #2978
     x.starts_with("\x03"); // issue #2996
+
+    // Issue #3204
+    const S: &str = "#";
+    x.find(S);
 }