Auto merge of rust-lang#12884 - y21:issue12881, r=Alexendoo

Only run `suboptimal_flops` on inherent method calls

Fixes rust-lang#12881

`suboptimal_flops` was making the wrong assumption that a `.log()` method call on a float literal must choose the inherent log method that will always have an argument present (in which case `args[0]` indexing would be fine), but that wasn't the case in the linked issue because at the point of the method call, the exact float type hadn't been inferred yet (and method selection can't select inherent methods when the exact float type has not yet been inferred, in which case it falls back to looking for trait impls and chooses the one that didn't have any parameters).

This fixes it by actually making sure it's a call to an inherent method (could also fix the linked ICE by simply using fallibly indexing via `.get()`, but this felt like it'd fix the root cause: even if there were one argument, it would still be wrong to emit a warning there because it's not the `log` method the lint was expecting). I'm not sure if we need that extra function be in `clippy_utils` but it feels like it could be useful.

changelog: Fixes an ICE in [`suboptimal_flops`]

clippy_lints/src/floating_point_arithmetic.rs

@@ -2,7 +2,8 @@ use clippy_utils::consts::Constant::{Int, F32, F64};
 use clippy_utils::consts::{constant, constant_simple, Constant};
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::{
-    eq_expr_value, get_parent_expr, higher, in_constant, is_no_std_crate, numeric_literal, peel_blocks, sugg,
+    eq_expr_value, get_parent_expr, higher, in_constant, is_inherent_method_call, is_no_std_crate, numeric_literal,
+    peel_blocks, sugg,
 };
 use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Expr, ExprKind, PathSegment, UnOp};
@@ -759,7 +760,7 @@ impl<'tcx> LateLintPass<'tcx> for FloatingPointArithmetic {
         if let ExprKind::MethodCall(path, receiver, args, _) = &expr.kind {
             let recv_ty = cx.typeck_results().expr_ty(receiver);
 
-            if recv_ty.is_floating_point() && !is_no_std_crate(cx) {
+            if recv_ty.is_floating_point() && !is_no_std_crate(cx) && is_inherent_method_call(cx, expr) {
                 match path.ident.name.as_str() {
                     "ln" => check_ln1p(cx, expr, receiver),
                     "log" => check_log_base(cx, expr, receiver, args),

clippy_utils/src/lib.rs

@@ -321,6 +321,15 @@ pub fn match_trait_method(cx: &LateContext<'_>, expr: &Expr<'_>, path: &[&str])
         .map_or(false, |trt_id| match_def_path(cx, trt_id, path))
 }
 
+/// Checks if the given method call expression calls an inherent method.
+pub fn is_inherent_method_call(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    if let Some(method_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
+        cx.tcx.trait_of_item(method_id).is_none()
+    } else {
+        false
+    }
+}
+
 /// Checks if a method is defined in an impl of a diagnostic item
 pub fn is_diag_item_method(cx: &LateContext<'_>, def_id: DefId, diag_item: Symbol) -> bool {
     if let Some(impl_did) = cx.tcx.impl_of_method(def_id) {

tests/ui/floating_point_log.fixed

@@ -55,4 +55,19 @@ fn check_ln1p() {
     let _ = (1.0 + x - 2.0).ln();
 }
 
+fn issue12881() {
+    pub trait MyLog {
+        fn log(&self) -> Self;
+    }
+
+    impl MyLog for f32 {
+        fn log(&self) -> Self {
+            4.
+        }
+    }
+
+    let x = 2.0;
+    x.log();
+}
+
 fn main() {}

tests/ui/floating_point_log.rs

@@ -55,4 +55,19 @@ fn check_ln1p() {
     let _ = (1.0 + x - 2.0).ln();
 }
 
+fn issue12881() {
+    pub trait MyLog {
+        fn log(&self) -> Self;
+    }
+
+    impl MyLog for f32 {
+        fn log(&self) -> Self {
+            4.
+        }
+    }
+
+    let x = 2.0;
+    x.log();
+}
+
 fn main() {}