single_match: suggest if over if let when possible

clippy_lints/src/matches.rs

@@ -2,10 +2,10 @@ use crate::consts::{constant, miri_to_const, Constant};
 use crate::utils::sugg::Sugg;
 use crate::utils::usage::is_unused;
 use crate::utils::{
-    expr_block, get_arg_name, get_parent_expr, in_macro, indent_of, is_allowed, is_expn_of, is_refutable,
-    is_type_diagnostic_item, is_wild, match_qpath, match_type, match_var, meets_msrv, multispan_sugg, remove_blocks,
-    snippet, snippet_block, snippet_opt, snippet_with_applicability, span_lint_and_help, span_lint_and_note,
-    span_lint_and_sugg, span_lint_and_then,
+    expr_block, get_arg_name, get_parent_expr, implements_trait, in_macro, indent_of, is_allowed, is_expn_of,
+    is_refutable, is_type_diagnostic_item, is_wild, match_qpath, match_type, match_var, meets_msrv, multispan_sugg,
+    peel_hir_pat_refs, peel_mid_ty_refs, peel_n_hir_expr_refs, remove_blocks, snippet, snippet_block, snippet_opt,
+    snippet_with_applicability, span_lint_and_help, span_lint_and_note, span_lint_and_sugg, span_lint_and_then,
 };
 use crate::utils::{paths, search_same, SpanlessEq, SpanlessHash};
 use if_chain::if_chain;
@@ -728,20 +728,60 @@ fn report_single_match_single_pattern(
     let els_str = els.map_or(String::new(), |els| {
         format!(" else {}", expr_block(cx, els, None, "..", Some(expr.span)))
     });
+
+    let (msg, sugg) = if_chain! {
+        let (pat, pat_ref_count) = peel_hir_pat_refs(arms[0].pat);
+        if let PatKind::Path(_) | PatKind::Lit(_) = pat.kind;
+        let (ty, ty_ref_count) = peel_mid_ty_refs(cx.typeck_results().expr_ty(ex));
+        if let Some(trait_id) = cx.tcx.lang_items().structural_peq_trait();
+        if ty.is_integral() || ty.is_char() || ty.is_str() || implements_trait(cx, ty, trait_id, &[]);
+        then {
+            // scrutinee derives PartialEq and the pattern is a constant.
+            let pat_ref_count = match pat.kind {
+                // string literals are already a reference.
+                PatKind::Lit(Expr { kind: ExprKind::Lit(lit), .. }) if lit.node.is_str() => pat_ref_count + 1,
+                _ => pat_ref_count,
+            };
+            // References are only implicitly added to the pattern, so no overflow here.
+            // e.g. will work: match &Some(_) { Some(_) => () }
+            // will not: match Some(_) { &Some(_) => () }
+            let ref_count_diff = ty_ref_count - pat_ref_count;
+
+            // Try to remove address of expressions first.
+            let (ex, removed) = peel_n_hir_expr_refs(ex, ref_count_diff);
+            let ref_count_diff = ref_count_diff - removed;
+
+            let msg = "you seem to be trying to use `match` for an equality check. Consider using `if`";
+            let sugg = format!(
+                "if {} == {}{} {}{}",
+                snippet(cx, ex.span, ".."),
+                // PartialEq for different reference counts may not exist.
+                "&".repeat(ref_count_diff),
+                snippet(cx, arms[0].pat.span, ".."),
+                expr_block(cx, &arms[0].body, None, "..", Some(expr.span)),
+                els_str,
+            );
+            (msg, sugg)
+        } else {
+            let msg = "you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`";
+            let sugg = format!(
+                "if let {} = {} {}{}",
+                snippet(cx, arms[0].pat.span, ".."),
+                snippet(cx, ex.span, ".."),
+                expr_block(cx, &arms[0].body, None, "..", Some(expr.span)),
+                els_str,
+            );
+            (msg, sugg)
+        }
+    };
+
     span_lint_and_sugg(
         cx,
         lint,
         expr.span,
-        "you seem to be trying to use match for destructuring a single pattern. Consider using `if \
-         let`",
+        msg,
         "try this",
-        format!(
-            "if let {} = {} {}{}",
-            snippet(cx, arms[0].pat.span, ".."),
-            snippet(cx, ex.span, ".."),
-            expr_block(cx, &arms[0].body, None, "..", Some(expr.span)),
-            els_str,
-        ),
+        sugg,
         Applicability::HasPlaceholders,
     );
 }

clippy_lints/src/utils/mod.rs

@@ -1668,6 +1668,44 @@ where
     match_expr_list
 }
 
+/// Peels off all references on the pattern. Returns the underlying pattern and the number of
+/// references removed.
+pub fn peel_hir_pat_refs(pat: &'a Pat<'a>) -> (&'a Pat<'a>, usize) {
+    fn peel(pat: &'a Pat<'a>, count: usize) -> (&'a Pat<'a>, usize) {
+        if let PatKind::Ref(pat, _) = pat.kind {
+            peel(pat, count + 1)
+        } else {
+            (pat, count)
+        }
+    }
+    peel(pat, 0)
+}
+
+/// Peels off up to the given number of references on the expression. Returns the underlying
+/// expression and the number of references removed.
+pub fn peel_n_hir_expr_refs(expr: &'a Expr<'a>, count: usize) -> (&'a Expr<'a>, usize) {
+    fn f(expr: &'a Expr<'a>, count: usize, target: usize) -> (&'a Expr<'a>, usize) {
+        match expr.kind {
+            ExprKind::AddrOf(_, _, expr) if count != target => f(expr, count + 1, target),
+            _ => (expr, count),
+        }
+    }
+    f(expr, 0, count)
+}
+
+/// Peels off all references on the type. Returns the underlying type and the number of references
+/// removed.
+pub fn peel_mid_ty_refs(ty: Ty<'_>) -> (Ty<'_>, usize) {
+    fn peel(ty: Ty<'_>, count: usize) -> (Ty<'_>, usize) {
+        if let ty::Ref(_, ty, _) = ty.kind() {
+            peel(ty, count + 1)
+        } else {
+            (ty, count)
+        }
+    }
+    peel(ty, 0)
+}
+
 #[macro_export]
 macro_rules! unwrap_cargo_metadata {
     ($cx: ident, $lint: ident, $deps: expr) => {{

tests/ui/single_match.rs

@@ -81,6 +81,62 @@ fn single_match_know_enum() {
     }
 }
 
+// issue #173
+fn if_suggestion() {
+    let x = "test";
+    match x {
+        "test" => println!(),
+        _ => (),
+    }
+
+    #[derive(PartialEq, Eq)]
+    enum Foo {
+        A,
+        B,
+        C(u32),
+    }
+
+    let x = Foo::A;
+    match x {
+        Foo::A => println!(),
+        _ => (),
+    }
+
+    const FOO_C: Foo = Foo::C(0);
+    match x {
+        FOO_C => println!(),
+        _ => (),
+    }
+
+    match &&x {
+        Foo::A => println!(),
+        _ => (),
+    }
+
+    let x = &x;
+    match &x {
+        Foo::A => println!(),
+        _ => (),
+    }
+
+    enum Bar {
+        A,
+        B,
+    }
+    impl PartialEq for Bar {
+        fn eq(&self, rhs: &Self) -> bool {
+            matches!((self, rhs), (Self::A, Self::A) | (Self::B, Self::B))
+        }
+    }
+    impl Eq for Bar {}
+
+    let x = Bar::A;
+    match x {
+        Bar::A => println!(),
+        _ => (),
+    }
+}
+
 macro_rules! single_match {
     ($num:literal) => {
         match $num {

tests/ui/single_match.stderr

@@ -1,4 +1,4 @@
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:8:5
    |
 LL | /     match x {
@@ -17,7 +17,7 @@ LL |         println!("{:?}", y);
 LL |     };
    |
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:16:5
    |
 LL | /     match x {
@@ -29,7 +29,7 @@ LL | |         _ => (),
 LL | |     }
    | |_____^ help: try this: `if let Some(y) = x { println!("{:?}", y) }`
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:25:5
    |
 LL | /     match z {
@@ -38,7 +38,7 @@ LL | |         _ => {},
 LL | |     };
    | |_____^ help: try this: `if let (2..=3, 7..=9) = z { dummy() }`
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:54:5
    |
 LL | /     match x {
@@ -47,7 +47,7 @@ LL | |         None => (),
 LL | |     };
    | |_____^ help: try this: `if let Some(y) = x { dummy() }`
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:59:5
    |
 LL | /     match y {
@@ -56,7 +56,7 @@ LL | |         Err(..) => (),
 LL | |     };
    | |_____^ help: try this: `if let Ok(y) = y { dummy() }`
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match.rs:66:5
    |
 LL | /     match c {
@@ -65,5 +65,59 @@ LL | |         Cow::Owned(..) => (),
 LL | |     };
    | |_____^ help: try this: `if let Cow::Borrowed(..) = c { dummy() }`
 
-error: aborting due to 6 previous errors
+error: you seem to be trying to use `match` for an equality check. Consider using `if`
+  --> $DIR/single_match.rs:87:5
+   |
+LL | /     match x {
+LL | |         "test" => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if x == "test" { println!() }`
+
+error: you seem to be trying to use `match` for an equality check. Consider using `if`
+  --> $DIR/single_match.rs:100:5
+   |
+LL | /     match x {
+LL | |         Foo::A => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if x == Foo::A { println!() }`
+
+error: you seem to be trying to use `match` for an equality check. Consider using `if`
+  --> $DIR/single_match.rs:106:5
+   |
+LL | /     match x {
+LL | |         FOO_C => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if x == FOO_C { println!() }`
+
+error: you seem to be trying to use `match` for an equality check. Consider using `if`
+  --> $DIR/single_match.rs:111:5
+   |
+LL | /     match &&x {
+LL | |         Foo::A => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if x == Foo::A { println!() }`
+
+error: you seem to be trying to use `match` for an equality check. Consider using `if`
+  --> $DIR/single_match.rs:117:5
+   |
+LL | /     match &x {
+LL | |         Foo::A => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if x == &Foo::A { println!() }`
+
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
+  --> $DIR/single_match.rs:134:5
+   |
+LL | /     match x {
+LL | |         Bar::A => println!(),
+LL | |         _ => (),
+LL | |     }
+   | |_____^ help: try this: `if let Bar::A = x { println!() }`
+
+error: aborting due to 12 previous errors
 

tests/ui/single_match_else.stderr

@@ -1,4 +1,4 @@
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match_else.rs:14:5
    |
 LL | /     match ExprNode::Butterflies {
@@ -19,7 +19,7 @@ LL |         None
 LL |     }
    |
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match_else.rs:70:5
    |
 LL | /     match Some(1) {
@@ -39,7 +39,7 @@ LL |         return
 LL |     }
    |
 
-error: you seem to be trying to use match for destructuring a single pattern. Consider using `if let`
+error: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> $DIR/single_match_else.rs:79:5
    |
 LL | /     match Some(1) {