Auto merge of #10931 - y21:issue10000, r=Jarcho

[`unnecessary_fold`]: suggest turbofish if necessary

Fixes #10000

This adds turbofish `::<T>` to the suggestion in `unnecessary_fold`. This is necessary because the `Sum` trait is generic, which breaks inference when changing `fold()` to `sum()`.

changelog: [`unnecessary_fold`]: suggest turbofish if necessary

clippy_lints/src/methods/unnecessary_fold.rs

@@ -7,10 +7,113 @@ use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::PatKind;
 use rustc_lint::LateContext;
+use rustc_middle::ty;
 use rustc_span::{source_map::Span, sym};
 
 use super::UNNECESSARY_FOLD;
 
+/// Do we need to suggest turbofish when suggesting a replacement method?
+/// Changing `fold` to `sum` needs it sometimes when the return type can't be
+/// inferred. This checks for some common cases where it can be safely omitted
+fn needs_turbofish(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
+    let parent = cx.tcx.hir().get_parent(expr.hir_id);
+
+    // some common cases where turbofish isn't needed:
+    // - assigned to a local variable with a type annotation
+    if let hir::Node::Local(local) = parent
+        && local.ty.is_some()
+    {
+        return false;
+    }
+
+    // - part of a function call argument, can be inferred from the function signature (provided that
+    //   the parameter is not a generic type parameter)
+    if let hir::Node::Expr(parent_expr) = parent
+        && let hir::ExprKind::Call(recv, args) = parent_expr.kind
+        && let hir::ExprKind::Path(ref qpath) = recv.kind
+        && let Some(fn_def_id) = cx.qpath_res(qpath, recv.hir_id).opt_def_id()
+        && let fn_sig = cx.tcx.fn_sig(fn_def_id).skip_binder().skip_binder()
+        && let Some(arg_pos) = args.iter().position(|arg| arg.hir_id == expr.hir_id)
+        && let Some(ty) = fn_sig.inputs().get(arg_pos)
+        && !matches!(ty.kind(), ty::Param(_))
+    {
+        return false;
+    }
+
+    // if it's neither of those, stay on the safe side and suggest turbofish,
+    // even if it could work!
+    true
+}
+
+#[derive(Copy, Clone)]
+struct Replacement {
+    method_name: &'static str,
+    has_args: bool,
+    has_generic_return: bool,
+}
+
+fn check_fold_with_op(
+    cx: &LateContext<'_>,
+    expr: &hir::Expr<'_>,
+    acc: &hir::Expr<'_>,
+    fold_span: Span,
+    op: hir::BinOpKind,
+    replacement: Replacement,
+) {
+    if_chain! {
+        // Extract the body of the closure passed to fold
+        if let hir::ExprKind::Closure(&hir::Closure { body, .. }) = acc.kind;
+        let closure_body = cx.tcx.hir().body(body);
+        let closure_expr = peel_blocks(closure_body.value);
+
+        // Check if the closure body is of the form `acc <op> some_expr(x)`
+        if let hir::ExprKind::Binary(ref bin_op, left_expr, right_expr) = closure_expr.kind;
+        if bin_op.node == op;
+
+        // Extract the names of the two arguments to the closure
+        if let [param_a, param_b] = closure_body.params;
+        if let PatKind::Binding(_, first_arg_id, ..) = strip_pat_refs(param_a.pat).kind;
+        if let PatKind::Binding(_, second_arg_id, second_arg_ident, _) = strip_pat_refs(param_b.pat).kind;
+
+        if path_to_local_id(left_expr, first_arg_id);
+        if replacement.has_args || path_to_local_id(right_expr, second_arg_id);
+
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+
+            let turbofish = if replacement.has_generic_return {
+                format!("::<{}>", cx.typeck_results().expr_ty_adjusted(right_expr).peel_refs())
+            } else {
+                String::new()
+            };
+
+            let sugg = if replacement.has_args {
+                format!(
+                    "{method}{turbofish}(|{second_arg_ident}| {r})",
+                    method = replacement.method_name,
+                    r = snippet_with_applicability(cx, right_expr.span, "EXPR", &mut applicability),
+                )
+            } else {
+                format!(
+                    "{method}{turbofish}()",
+                    method = replacement.method_name,
+                )
+            };
+
+            span_lint_and_sugg(
+                cx,
+                UNNECESSARY_FOLD,
+                fold_span.with_hi(expr.span.hi()),
+                // TODO #2371 don't suggest e.g., .any(|x| f(x)) if we can suggest .any(f)
+                "this `.fold` can be written more succinctly using another method",
+                "try",
+                sugg,
+                applicability,
+            );
+        }
+    }
+}
+
 pub(super) fn check(
     cx: &LateContext<'_>,
     expr: &hir::Expr<'_>,
@@ -18,60 +121,6 @@ pub(super) fn check(
     acc: &hir::Expr<'_>,
     fold_span: Span,
 ) {
-    fn check_fold_with_op(
-        cx: &LateContext<'_>,
-        expr: &hir::Expr<'_>,
-        acc: &hir::Expr<'_>,
-        fold_span: Span,
-        op: hir::BinOpKind,
-        replacement_method_name: &str,
-        replacement_has_args: bool,
-    ) {
-        if_chain! {
-            // Extract the body of the closure passed to fold
-            if let hir::ExprKind::Closure(&hir::Closure { body, .. }) = acc.kind;
-            let closure_body = cx.tcx.hir().body(body);
-            let closure_expr = peel_blocks(closure_body.value);
-
-            // Check if the closure body is of the form `acc <op> some_expr(x)`
-            if let hir::ExprKind::Binary(ref bin_op, left_expr, right_expr) = closure_expr.kind;
-            if bin_op.node == op;
-
-            // Extract the names of the two arguments to the closure
-            if let [param_a, param_b] = closure_body.params;
-            if let PatKind::Binding(_, first_arg_id, ..) = strip_pat_refs(param_a.pat).kind;
-            if let PatKind::Binding(_, second_arg_id, second_arg_ident, _) = strip_pat_refs(param_b.pat).kind;
-
-            if path_to_local_id(left_expr, first_arg_id);
-            if replacement_has_args || path_to_local_id(right_expr, second_arg_id);
-
-            then {
-                let mut applicability = Applicability::MachineApplicable;
-                let sugg = if replacement_has_args {
-                    format!(
-                        "{replacement_method_name}(|{second_arg_ident}| {r})",
-                        r = snippet_with_applicability(cx, right_expr.span, "EXPR", &mut applicability),
-                    )
-                } else {
-                    format!(
-                        "{replacement_method_name}()",
-                    )
-                };
-
-                span_lint_and_sugg(
-                    cx,
-                    UNNECESSARY_FOLD,
-                    fold_span.with_hi(expr.span.hi()),
-                    // TODO #2371 don't suggest e.g., .any(|x| f(x)) if we can suggest .any(f)
-                    "this `.fold` can be written more succinctly using another method",
-                    "try",
-                    sugg,
-                    applicability,
-                );
-            }
-        }
-    }
-
     // Check that this is a call to Iterator::fold rather than just some function called fold
     if !is_trait_method(cx, expr, sym::Iterator) {
         return;
@@ -80,11 +129,59 @@ pub(super) fn check(
     // Check if the first argument to .fold is a suitable literal
     if let hir::ExprKind::Lit(lit) = init.kind {
         match lit.node {
-            ast::LitKind::Bool(false) => check_fold_with_op(cx, expr, acc, fold_span, hir::BinOpKind::Or, "any", true),
-            ast::LitKind::Bool(true) => check_fold_with_op(cx, expr, acc, fold_span, hir::BinOpKind::And, "all", true),
-            ast::LitKind::Int(0, _) => check_fold_with_op(cx, expr, acc, fold_span, hir::BinOpKind::Add, "sum", false),
+            ast::LitKind::Bool(false) => {
+                check_fold_with_op(
+                    cx,
+                    expr,
+                    acc,
+                    fold_span,
+                    hir::BinOpKind::Or,
+                    Replacement {
+                        has_args: true,
+                        has_generic_return: false,
+                        method_name: "any",
+                    },
+                );
+            },
+            ast::LitKind::Bool(true) => {
+                check_fold_with_op(
+                    cx,
+                    expr,
+                    acc,
+                    fold_span,
+                    hir::BinOpKind::And,
+                    Replacement {
+                        has_args: true,
+                        has_generic_return: false,
+                        method_name: "all",
+                    },
+                );
+            },
+            ast::LitKind::Int(0, _) => check_fold_with_op(
+                cx,
+                expr,
+                acc,
+                fold_span,
+                hir::BinOpKind::Add,
+                Replacement {
+                    has_args: false,
+                    has_generic_return: needs_turbofish(cx, expr),
+                    method_name: "sum",
+                },
+            ),
             ast::LitKind::Int(1, _) => {
-                check_fold_with_op(cx, expr, acc, fold_span, hir::BinOpKind::Mul, "product", false);
+                check_fold_with_op(
+                    cx,
+                    expr,
+                    acc,
+                    fold_span,
+                    hir::BinOpKind::Mul,
+                    Replacement {
+                        has_args: false,
+                        has_generic_return: needs_turbofish(cx, expr),
+                        method_name: "product",
+                    },
+                );
             },
             _ => (),
         }

tests/ui/unnecessary_fold.fixed

@@ -49,4 +49,28 @@ fn unnecessary_fold_over_multiple_lines() {
         .any(|x| x > 2);
 }
 
+fn issue10000() {
+    use std::collections::HashMap;
+    use std::hash::BuildHasher;
+
+    fn anything<T>(_: T) {}
+    fn num(_: i32) {}
+    fn smoketest_map<S: BuildHasher>(mut map: HashMap<i32, i32, S>) {
+        map.insert(0, 0);
+        assert_eq!(map.values().sum::<i32>(), 0);
+
+        // more cases:
+        let _ = map.values().sum::<i32>();
+        let _ = map.values().product::<i32>();
+        let _: i32 = map.values().sum();
+        let _: i32 = map.values().product();
+        anything(map.values().sum::<i32>());
+        anything(map.values().product::<i32>());
+        num(map.values().sum());
+        num(map.values().product());
+    }
+
+    smoketest_map(HashMap::new());
+}
+
 fn main() {}

tests/ui/unnecessary_fold.rs

@@ -49,4 +49,28 @@ fn unnecessary_fold_over_multiple_lines() {
         .fold(false, |acc, x| acc || x > 2);
 }
 
+fn issue10000() {
+    use std::collections::HashMap;
+    use std::hash::BuildHasher;
+
+    fn anything<T>(_: T) {}
+    fn num(_: i32) {}
+    fn smoketest_map<S: BuildHasher>(mut map: HashMap<i32, i32, S>) {
+        map.insert(0, 0);
+        assert_eq!(map.values().fold(0, |x, y| x + y), 0);
+
+        // more cases:
+        let _ = map.values().fold(0, |x, y| x + y);
+        let _ = map.values().fold(1, |x, y| x * y);
+        let _: i32 = map.values().fold(0, |x, y| x + y);
+        let _: i32 = map.values().fold(1, |x, y| x * y);
+        anything(map.values().fold(0, |x, y| x + y));
+        anything(map.values().fold(1, |x, y| x * y));
+        num(map.values().fold(0, |x, y| x + y));
+        num(map.values().fold(1, |x, y| x * y));
+    }
+
+    smoketest_map(HashMap::new());
+}
+
 fn main() {}

tests/ui/unnecessary_fold.stderr

@@ -36,5 +36,59 @@ error: this `.fold` can be written more succinctly using another method
 LL |         .fold(false, |acc, x| acc || x > 2);
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `any(|x| x > 2)`
 
-error: aborting due to 6 previous errors
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:60:33
+   |
+LL |         assert_eq!(map.values().fold(0, |x, y| x + y), 0);
+   |                                 ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:63:30
+   |
+LL |         let _ = map.values().fold(0, |x, y| x + y);
+   |                              ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:64:30
+   |
+LL |         let _ = map.values().fold(1, |x, y| x * y);
+   |                              ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:65:35
+   |
+LL |         let _: i32 = map.values().fold(0, |x, y| x + y);
+   |                                   ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:66:35
+   |
+LL |         let _: i32 = map.values().fold(1, |x, y| x * y);
+   |                                   ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:67:31
+   |
+LL |         anything(map.values().fold(0, |x, y| x + y));
+   |                               ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:68:31
+   |
+LL |         anything(map.values().fold(1, |x, y| x * y));
+   |                               ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:69:26
+   |
+LL |         num(map.values().fold(0, |x, y| x + y));
+   |                          ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:70:26
+   |
+LL |         num(map.values().fold(1, |x, y| x * y));
+   |                          ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`
+
+error: aborting due to 15 previous errors