auto-fix `if_not_else` (#13809)

fix #13411

The `if_not_else` lint can be fixed automatically, but the issue above
reports that there is no implementation to do so. Therefore, this PR
implements it.

----

changelog: [`if_not_else`]: make suggestions for modified code

1 files changed, 51 insertions, 3 deletions

diff --git a/clippy_lints/src/if_not_else.rs b/clippy_lints/src/if_not_else.rs
index 120c5396a1c..2806d4d0e5d 100644
--- a/clippy_lints/src/if_not_else.rs
+++ b/clippy_lints/src/if_not_else.rs
@@ -1,9 +1,13 @@
 use clippy_utils::consts::{ConstEvalCtxt, Constant};
-use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
 use clippy_utils::is_else_clause;
+use clippy_utils::source::{HasSession, indent_of, reindent_multiline, snippet};
+use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Expr, ExprKind, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
+use rustc_span::Span;
+use std::borrow::Cow;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -54,7 +58,7 @@ fn is_zero_const(expr: &Expr<'_>, cx: &LateContext<'_>) -> bool {
 
 impl LateLintPass<'_> for IfNotElse {
     fn check_expr(&mut self, cx: &LateContext<'_>, e: &Expr<'_>) {
-        if let ExprKind::If(cond, _, Some(els)) = e.kind
+        if let ExprKind::If(cond, cond_inner, Some(els)) = e.kind
             && let ExprKind::DropTemps(cond) = cond.kind
             && let ExprKind::Block(..) = els.kind
         {
@@ -79,8 +83,52 @@ impl LateLintPass<'_> for IfNotElse {
             // }
             // ```
             if !e.span.from_expansion() && !is_else_clause(cx.tcx, e) {
-                span_lint_and_help(cx, IF_NOT_ELSE, e.span, msg, None, help);
+                match cond.kind {
+                    ExprKind::Unary(UnOp::Not, _) | ExprKind::Binary(_, _, _) => span_lint_and_sugg(
+                        cx,
+                        IF_NOT_ELSE,
+                        e.span,
+                        msg,
+                        "try",
+                        make_sugg(cx, &cond.kind, cond_inner.span, els.span, "..", Some(e.span)).to_string(),
+                        Applicability::MachineApplicable,
+                    ),
+                    _ => span_lint_and_help(cx, IF_NOT_ELSE, e.span, msg, None, help),
+                }
             }
         }
     }
 }
+
+fn make_sugg<'a>(
+    sess: &impl HasSession,
+    cond_kind: &'a ExprKind<'a>,
+    cond_inner: Span,
+    els_span: Span,
+    default: &'a str,
+    indent_relative_to: Option<Span>,
+) -> Cow<'a, str> {
+    let cond_inner_snip = snippet(sess, cond_inner, default);
+    let els_snip = snippet(sess, els_span, default);
+    let indent = indent_relative_to.and_then(|s| indent_of(sess, s));
+
+    let suggestion = match cond_kind {
+        ExprKind::Unary(UnOp::Not, cond_rest) => {
+            format!(
+                "if {} {} else {}",
+                snippet(sess, cond_rest.span, default),
+                els_snip,
+                cond_inner_snip
+            )
+        },
+        ExprKind::Binary(_, lhs, rhs) => {
+            let lhs_snip = snippet(sess, lhs.span, default);
+            let rhs_snip = snippet(sess, rhs.span, default);
+
+            format!("if {lhs_snip} == {rhs_snip} {els_snip} else {cond_inner_snip}")
+        },
+        _ => String::new(),
+    };
+
+    reindent_multiline(suggestion.into(), true, indent)
+}