Implement if_same_cond_fn lint

Run ./util/dev

Revert changelog entry

Rename lint to same_functions_in_if_condition and add a doc example

Add testcases with different arg in fn invocation

clippy_lints/src/copies.rs

@@ -40,6 +40,53 @@ declare_clippy_lint! {
     "consecutive `ifs` with the same condition"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for consecutive `if`s with the same function call.
+    ///
+    /// **Why is this bad?** This is probably a copy & paste error.
+    /// Despite the fact that function can have side effects and `if` works as
+    /// intended, such an approach is implicit and can be considered a "code smell".
+    ///
+    /// **Known problems:** Hopefully none.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// if foo() == bar {
+    ///     …
+    /// } else if foo() == bar {
+    ///     …
+    /// }
+    /// ```
+    ///
+    /// This probably should be:
+    /// ```ignore
+    /// if foo() == bar {
+    ///     …
+    /// } else if foo() == baz {
+    ///     …
+    /// }
+    /// ```
+    ///
+    /// or if the original code was not a typo and called function mutates a state,
+    /// consider move the mutation out of the `if` condition to avoid similarity to
+    /// a copy & paste error:
+    ///
+    /// ```ignore
+    /// let first = foo();
+    /// if first == bar {
+    ///     …
+    /// } else {
+    ///     let second = foo();
+    ///     if second == bar {
+    ///     …
+    ///     }
+    /// }
+    /// ```
+    pub SAME_FUNCTIONS_IN_IF_CONDITION,
+    pedantic,
+    "consecutive `ifs` with the same function call"
+}
+
 declare_clippy_lint! {
     /// **What it does:** Checks for `if/else` with the same body as the *then* part
     /// and the *else* part.
@@ -102,7 +149,7 @@ declare_clippy_lint! {
     "`match` with identical arm bodies"
 }
 
-declare_lint_pass!(CopyAndPaste => [IFS_SAME_COND, IF_SAME_THEN_ELSE, MATCH_SAME_ARMS]);
+declare_lint_pass!(CopyAndPaste => [IFS_SAME_COND, SAME_FUNCTIONS_IN_IF_CONDITION, IF_SAME_THEN_ELSE, MATCH_SAME_ARMS]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CopyAndPaste {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
@@ -119,6 +166,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CopyAndPaste {
             let (conds, blocks) = if_sequence(expr);
             lint_same_then_else(cx, &blocks);
             lint_same_cond(cx, &conds);
+            lint_same_fns_in_if_cond(cx, &conds);
             lint_match_arms(cx, expr);
         }
     }
@@ -163,6 +211,34 @@ fn lint_same_cond(cx: &LateContext<'_, '_>, conds: &[&Expr]) {
     }
 }
 
+/// Implementation of `SAME_FUNCTIONS_IN_IF_CONDITION`.
+fn lint_same_fns_in_if_cond(cx: &LateContext<'_, '_>, conds: &[&Expr]) {
+    let hash: &dyn Fn(&&Expr) -> u64 = &|expr| -> u64 {
+        let mut h = SpanlessHash::new(cx, cx.tables);
+        h.hash_expr(expr);
+        h.finish()
+    };
+
+    let eq: &dyn Fn(&&Expr, &&Expr) -> bool = &|&lhs, &rhs| -> bool {
+        // Do not spawn warning if `IFS_SAME_COND` already produced it.
+        if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs, rhs) {
+            return false;
+        }
+        SpanlessEq::new(cx).eq_expr(lhs, rhs)
+    };
+
+    for (i, j) in search_same(conds, hash, eq) {
+        span_note_and_lint(
+            cx,
+            SAME_FUNCTIONS_IN_IF_CONDITION,
+            j.span,
+            "this `if` has the same function call as a previous if",
+            i.span,
+            "same as this",
+        );
+    }
+}
+
 /// Implementation of `MATCH_SAME_ARMS`.
 fn lint_match_arms<'tcx>(cx: &LateContext<'_, 'tcx>, expr: &Expr) {
     fn same_bindings<'tcx>(

clippy_lints/src/lib.rs

@@ -471,6 +471,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &collapsible_if::COLLAPSIBLE_IF,
         &comparison_chain::COMPARISON_CHAIN,
         &copies::IFS_SAME_COND,
+        &copies::SAME_FUNCTIONS_IN_IF_CONDITION,
         &copies::IF_SAME_THEN_ELSE,
         &copies::MATCH_SAME_ARMS,
         &copy_iterator::COPY_ITERATOR,
@@ -989,6 +990,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
     store.register_group(true, "clippy::pedantic", Some("clippy_pedantic"), vec![
         LintId::of(&attrs::INLINE_ALWAYS),
         LintId::of(&checked_conversions::CHECKED_CONVERSIONS),
+        LintId::of(&copies::SAME_FUNCTIONS_IN_IF_CONDITION),
         LintId::of(&copies::MATCH_SAME_ARMS),
         LintId::of(&copy_iterator::COPY_ITERATOR),
         LintId::of(&default_trait_access::DEFAULT_TRAIT_ACCESS),