Reinstate single_match/single_match_else lints with comments (rust-lang#14420)

Commit efe3fe9 removed the ability for
`single_match` and `single_match_else` to trigger if comments were
present outside of the arms, as those comments would be lost while
rewriting the `match` expression.

This reinstates the lint, but prevents the suggestion from being applied
automatically in the presence of comments by using the `MaybeIncorrect`
applicability. Also, a note is added to the lint message to warn the
user about the need to preserve the comments if acting upon the
suggestion.

changelog: [`single_match`, `single_match_else`]: reinstate lint when
comments are inside the `match` but do not autofix the code

Fix rust-lang#14418

Author: Alexendoo

clippy_lints/src/matches/mod.rs

@@ -1110,11 +1110,9 @@ impl<'tcx> LateLintPass<'tcx> for Matches {
                             }
                         }
                     }
-                    // If there are still comments, it means they are outside of the arms, therefore
-                    // we should not lint.
-                    if match_comments.is_empty() {
-                        single_match::check(cx, ex, arms, expr);
-                    }
+                    // If there are still comments, it means they are outside of the arms. Tell the lint
+                    // code about it.
+                    single_match::check(cx, ex, arms, expr, !match_comments.is_empty());
                     match_bool::check(cx, ex, arms, expr);
                     overlapping_arms::check(cx, ex, arms);
                     match_wild_enum::check(cx, ex, arms);

clippy_lints/src/matches/single_match.rs

@@ -1,12 +1,12 @@
-use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::{SpanRangeExt, expr_block, snippet, snippet_block_with_context};
 use clippy_utils::ty::implements_trait;
 use clippy_utils::{
     is_lint_allowed, is_unit_expr, peel_blocks, peel_hir_pat_refs, peel_middle_ty_refs, peel_n_hir_expr_refs,
 };
 use core::ops::ControlFlow;
 use rustc_arena::DroplessArena;
-use rustc_errors::Applicability;
+use rustc_errors::{Applicability, Diag};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{Visitor, walk_pat};
 use rustc_hir::{Arm, Expr, ExprKind, HirId, Node, Pat, PatExpr, PatExprKind, PatKind, QPath, StmtKind};
@@ -32,7 +32,7 @@ fn empty_arm_has_comment(cx: &LateContext<'_>, span: Span) -> bool {
 }
 
 #[rustfmt::skip]
-pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, ex: &'tcx Expr<'_>, arms: &'tcx [Arm<'_>], expr: &'tcx Expr<'_>) {
+pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, ex: &'tcx Expr<'_>, arms: &'tcx [Arm<'_>], expr: &'tcx Expr<'_>, contains_comments: bool) {
     if let [arm1, arm2] = arms
         && arm1.guard.is_none()
         && arm2.guard.is_none()
@@ -77,15 +77,31 @@ pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, ex: &'tcx Expr<'_>, arms: &'tc
                 }
             }
 
-            report_single_pattern(cx, ex, arm1, expr, els);
+            report_single_pattern(cx, ex, arm1, expr, els, contains_comments);
         }
     }
 }
 
-fn report_single_pattern(cx: &LateContext<'_>, ex: &Expr<'_>, arm: &Arm<'_>, expr: &Expr<'_>, els: Option<&Expr<'_>>) {
+fn report_single_pattern(
+    cx: &LateContext<'_>,
+    ex: &Expr<'_>,
+    arm: &Arm<'_>,
+    expr: &Expr<'_>,
+    els: Option<&Expr<'_>>,
+    contains_comments: bool,
+) {
     let lint = if els.is_some() { SINGLE_MATCH_ELSE } else { SINGLE_MATCH };
     let ctxt = expr.span.ctxt();
-    let mut app = Applicability::MachineApplicable;
+    let note = |diag: &mut Diag<'_, ()>| {
+        if contains_comments {
+            diag.note("you might want to preserve the comments from inside the `match`");
+        }
+    };
+    let mut app = if contains_comments {
+        Applicability::MaybeIncorrect
+    } else {
+        Applicability::MachineApplicable
+    };
     let els_str = els.map_or(String::new(), |els| {
         format!(" else {}", expr_block(cx, els, ctxt, "..", Some(expr.span), &mut app))
     });
@@ -109,7 +125,10 @@ fn report_single_pattern(cx: &LateContext<'_>, ex: &Expr<'_>, arm: &Arm<'_>, exp
             }
             (sugg, "try")
         };
-        span_lint_and_sugg(cx, lint, expr.span, msg, help, sugg.to_string(), app);
+        span_lint_and_then(cx, lint, expr.span, msg, |diag| {
+            diag.span_suggestion(expr.span, help, sugg.to_string(), app);
+            note(diag);
+        });
         return;
     }
 
@@ -162,7 +181,10 @@ fn report_single_pattern(cx: &LateContext<'_>, ex: &Expr<'_>, arm: &Arm<'_>, exp
         (msg, sugg)
     };
 
-    span_lint_and_sugg(cx, lint, expr.span, msg, "try", sugg, app);
+    span_lint_and_then(cx, lint, expr.span, msg, |diag| {
+        diag.span_suggestion(expr.span, "try", sugg.to_string(), app);
+        note(diag);
+    });
 }
 
 struct PatVisitor<'tcx> {

tests/ui/single_match.fixed

@@ -1,3 +1,4 @@
+//@require-annotations-for-level: WARN
 #![warn(clippy::single_match)]
 #![allow(
     unused,
@@ -18,13 +19,9 @@ fn single_match() {
     //~^^^^^^ single_match
 
     let x = Some(1u8);
-    match x {
-        // Note the missing block braces.
-        // We suggest `if let Some(y) = x { .. }` because the macro
-        // is expanded before we can do anything.
-        Some(y) => println!("{:?}", y),
-        _ => (),
-    }
+    if let Some(y) = x { println!("{:?}", y) }
+    //~^^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
 
     let z = (1u8, 1u8);
     if let (2..=3, 7..=9) = z { dummy() };
@@ -358,21 +355,14 @@ fn irrefutable_match() {
 
     let mut x = vec![1i8];
 
-    // Should not lint.
-    match x.pop() {
-        // bla
-        Some(u) => println!("{u}"),
-        // more comments!
-        None => {},
-    }
-    // Should not lint.
-    match x.pop() {
-        // bla
-        Some(u) => {
-            // bla
-            println!("{u}");
-        },
+    if let Some(u) = x.pop() { println!("{u}") }
+    //~^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
+
+    if let Some(u) = x.pop() {
         // bla
-        None => {},
+        println!("{u}");
     }
+    //~^^^^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
 }

tests/ui/single_match.rs

@@ -1,3 +1,4 @@
+//@require-annotations-for-level: WARN
 #![warn(clippy::single_match)]
 #![allow(
     unused,
@@ -28,6 +29,8 @@ fn single_match() {
         Some(y) => println!("{:?}", y),
         _ => (),
     }
+    //~^^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
 
     let z = (1u8, 1u8);
     match z {
@@ -437,14 +440,15 @@ fn irrefutable_match() {
 
     let mut x = vec![1i8];
 
-    // Should not lint.
     match x.pop() {
         // bla
         Some(u) => println!("{u}"),
         // more comments!
         None => {},
     }
-    // Should not lint.
+    //~^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
+
     match x.pop() {
         // bla
         Some(u) => {
@@ -454,4 +458,6 @@ fn irrefutable_match() {
         // bla
         None => {},
     }
+    //~^^^^^^^^^ single_match
+    //~| NOTE: you might want to preserve the comments from inside the `match`
 }