Fixed FP in unused_io_amount for Ok(lit), unrachable!

We introduce the following rules for match exprs.
- `panic!` and `unreachable!` are treated as consumption.
- guard expressions in any arm imply consumption.

For match exprs:
- Lint only if exacrtly 2 non-consuming arms exist
- Lint only if one arm is an `Ok(_)` and the other is `Err(_)`

Added additional requirement that for a block return expression
that is a match, the source must be `Normal`.

changelog: FP [`unused_io_amount`] when matching Ok(literal)

clippy_lints/src/unused_io_amount.rs

@@ -1,5 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::{is_res_lang_ctor, is_trait_method, match_trait_method, paths};
+use clippy_utils::macros::{is_panic, root_macro_call_first_node};
+use clippy_utils::{is_res_lang_ctor, is_trait_method, match_trait_method, paths, peel_blocks};
 use hir::{ExprKind, PatKind};
 use rustc_hir as hir;
 use rustc_lint::{LateContext, LateLintPass};
@@ -82,37 +83,72 @@ fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'tcx>)
         }
 
         if let Some(exp) = block.expr
-            && matches!(exp.kind, hir::ExprKind::If(_, _, _) | hir::ExprKind::Match(_, _, _))
+            && matches!(
+                exp.kind,
+                hir::ExprKind::If(_, _, _) | hir::ExprKind::Match(_, _, hir::MatchSource::Normal)
+            )
         {
             check_expr(cx, exp);
         }
     }
 }
 
+fn non_consuming_err_arm<'a>(cx: &LateContext<'a>, arm: &hir::Arm<'a>) -> bool {
+    // if there is a guard, we consider the result to be consumed
+    if arm.guard.is_some() {
+        return false;
+    }
+    if is_unreachable_or_panic(cx, arm.body) {
+        // if the body is unreachable or there is a panic,
+        // we consider the result to be consumed
+        return false;
+    }
+
+    if let PatKind::TupleStruct(ref path, [inner_pat], _) = arm.pat.kind {
+        return is_res_lang_ctor(cx, cx.qpath_res(path, inner_pat.hir_id), hir::LangItem::ResultErr);
+    }
+
+    false
+}
+
+fn non_consuming_ok_arm<'a>(cx: &LateContext<'a>, arm: &hir::Arm<'a>) -> bool {
+    // if there is a guard, we consider the result to be consumed
+    if arm.guard.is_some() {
+        return false;
+    }
+    if is_unreachable_or_panic(cx, arm.body) {
+        // if the body is unreachable or there is a panic,
+        // we consider the result to be consumed
+        return false;
+    }
+
+    if is_ok_wild_or_dotdot_pattern(cx, arm.pat) {
+        return true;
+    }
+    false
+}
+
 fn check_expr<'a>(cx: &LateContext<'a>, expr: &'a hir::Expr<'a>) {
     match expr.kind {
         hir::ExprKind::If(cond, _, _)
             if let ExprKind::Let(hir::Let { pat, init, .. }) = cond.kind
-                && pattern_is_ignored_ok(cx, pat)
+                && is_ok_wild_or_dotdot_pattern(cx, pat)
                 && let Some(op) = should_lint(cx, init) =>
         {
             emit_lint(cx, cond.span, op, &[pat.span]);
         },
-        hir::ExprKind::Match(expr, arms, hir::MatchSource::Normal) if let Some(op) = should_lint(cx, expr) => {
+        // we will capture only the case where the match is Ok( ) or Err( )
-            let found_arms: Vec<_> = arms
+        // prefer to match the minimum possible, and expand later if needed
-                .iter()
+        // to avoid false positives on something as used as this
-                .filter_map(|arm| {
+        hir::ExprKind::Match(expr, [arm1, arm2], hir::MatchSource::Normal) if let Some(op) = should_lint(cx, expr) => {
-                    if pattern_is_ignored_ok(cx, arm.pat) {
+            if non_consuming_ok_arm(cx, arm1) && non_consuming_err_arm(cx, arm2) {
-                        Some(arm.span)
+                emit_lint(cx, expr.span, op, &[arm1.pat.span]);
-                    } else {
+            }
-                        None
+            if non_consuming_ok_arm(cx, arm2) && non_consuming_err_arm(cx, arm1) {
-                    }
+                emit_lint(cx, expr.span, op, &[arm2.pat.span]);
-                })
-                .collect();
-            if !found_arms.is_empty() {
-                emit_lint(cx, expr.span, op, found_arms.as_slice());
             }
         },
+        hir::ExprKind::Match(_, _, hir::MatchSource::Normal) => {},
         _ if let Some(op) = should_lint(cx, expr) => {
             emit_lint(cx, expr.span, op, &[]);
         },
@@ -130,25 +166,40 @@ fn should_lint<'a>(cx: &LateContext<'a>, mut inner: &'a hir::Expr<'a>) -> Option
     check_io_mode(cx, inner)
 }
 
-fn pattern_is_ignored_ok(cx: &LateContext<'_>, pat: &hir::Pat<'_>) -> bool {
+fn is_ok_wild_or_dotdot_pattern<'a>(cx: &LateContext<'a>, pat: &hir::Pat<'a>) -> bool {
     // the if checks whether we are in a result Ok( ) pattern
     // and the return checks whether it is unhandled
 
-    if let PatKind::TupleStruct(ref path, inner_pat, ddp) = pat.kind
+    if let PatKind::TupleStruct(ref path, inner_pat, _) = pat.kind
         // we check against Result::Ok to avoid linting on Err(_) or something else.
         && is_res_lang_ctor(cx, cx.qpath_res(path, pat.hir_id), hir::LangItem::ResultOk)
     {
-        return match (inner_pat, ddp.as_opt_usize()) {
+        if matches!(inner_pat, []) {
-            // Ok(_) pattern
+            return true;
-            ([inner_pat], None) if matches!(inner_pat.kind, PatKind::Wild) => true,
+        }
-            // Ok(..) pattern
+
-            ([], Some(0)) => true,
+        if let [cons_pat] = inner_pat
-            _ => false,
+            && matches!(cons_pat.kind, PatKind::Wild)
-        };
+        {
+            return true;
+        }
+        return false;
     }
     false
 }
 
+// this is partially taken from panic_unimplemented
+fn is_unreachable_or_panic(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
+    let expr = peel_blocks(expr);
+    let Some(macro_call) = root_macro_call_first_node(cx, expr) else {
+        return false;
+    };
+    if is_panic(cx, macro_call.def_id) {
+        return !cx.tcx.hir().is_inside_const_context(expr.hir_id);
+    }
+    matches!(cx.tcx.item_name(macro_call.def_id).as_str(), "unreachable")
+}
+
 fn unpack_call_chain<'a>(mut expr: &'a hir::Expr<'a>) -> &'a hir::Expr<'a> {
     while let hir::ExprKind::MethodCall(path, receiver, ..) = expr.kind {
         if matches!(

tests/ui/unused_io_amount.rs

@@ -229,4 +229,47 @@ fn on_return_should_not_raise<T: io::Read + io::Write>(s: &mut T) -> io::Result<
     s.read(&mut buf)
 }
 
+pub fn unwrap_in_block(rdr: &mut dyn std::io::Read) -> std::io::Result<usize> {
+    let read = { rdr.read(&mut [0])? };
+    Ok(read)
+}
+
+pub fn consumed_example(rdr: &mut dyn std::io::Read) {
+    match rdr.read(&mut [0]) {
+        Ok(0) => println!("EOF"),
+        Ok(_) => println!("fully read"),
+        Err(_) => println!("fail"),
+    };
+    match rdr.read(&mut [0]) {
+        Ok(0) => println!("EOF"),
+        Ok(_) => println!("fully read"),
+        Err(_) => println!("fail"),
+    }
+}
+
+pub fn unreachable_or_panic(rdr: &mut dyn std::io::Read) {
+    {
+        match rdr.read(&mut [0]) {
+            Ok(_) => unreachable!(),
+            Err(_) => println!("expected"),
+        }
+    }
+
+    {
+        match rdr.read(&mut [0]) {
+            Ok(_) => panic!(),
+            Err(_) => println!("expected"),
+        }
+    }
+}
+
+pub fn wildcards(rdr: &mut dyn std::io::Read) {
+    {
+        match rdr.read(&mut [0]) {
+            Ok(1) => todo!(),
+            _ => todo!(),
+        }
+    }
+}
+
 fn main() {}

tests/ui/unused_io_amount.stderr

@@ -180,7 +180,7 @@ note: the result is consumed here, but the amount of I/O bytes remains unhandled
   --> $DIR/unused_io_amount.rs:149:9
    |
 LL |         Ok(_) => todo!(),
-   |         ^^^^^^^^^^^^^^^^
+   |         ^^^^^
 
 error: read amount is not handled
   --> $DIR/unused_io_amount.rs:155:11
@@ -193,7 +193,7 @@ note: the result is consumed here, but the amount of I/O bytes remains unhandled
   --> $DIR/unused_io_amount.rs:157:9
    |
 LL |         Ok(_) => todo!(),
-   |         ^^^^^^^^^^^^^^^^
+   |         ^^^^^
 
 error: read amount is not handled
   --> $DIR/unused_io_amount.rs:164:11
@@ -206,7 +206,7 @@ note: the result is consumed here, but the amount of I/O bytes remains unhandled
   --> $DIR/unused_io_amount.rs:166:9
    |
 LL |         Ok(_) => todo!(),
-   |         ^^^^^^^^^^^^^^^^
+   |         ^^^^^
 
 error: written amount is not handled
   --> $DIR/unused_io_amount.rs:173:11
@@ -219,7 +219,7 @@ note: the result is consumed here, but the amount of I/O bytes remains unhandled
   --> $DIR/unused_io_amount.rs:175:9
    |
 LL |         Ok(_) => todo!(),
-   |         ^^^^^^^^^^^^^^^^
+   |         ^^^^^
 
 error: read amount is not handled
   --> $DIR/unused_io_amount.rs:186:8