From 5461ed670eecedad74c82dd91ba532249d2073fb Mon Sep 17 00:00:00 2001
From: Jason Newcomb <jsnewcomb@pm.me>
Date: Sun, 16 Jan 2022 09:46:02 -0500
Subject: [PATCH] Don't lint `if_same_then_else` with `if let` conditions

---
 clippy_lints/src/copies.rs     | 29 +++++++++++++++++------------
 tests/ui/if_same_then_else2.rs | 17 +++++++++++++++++
 2 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/clippy_lints/src/copies.rs b/clippy_lints/src/copies.rs
index 73ce656ad151..f44d370a8fd0 100644
--- a/clippy_lints/src/copies.rs
+++ b/clippy_lints/src/copies.rs
@@ -183,7 +183,7 @@ impl<'tcx> LateLintPass<'tcx> for CopyAndPaste {
                 lint_same_cond(cx, &conds);
                 lint_same_fns_in_if_cond(cx, &conds);
                 // Block duplication
-                lint_same_then_else(cx, &blocks, conds.len() == blocks.len(), expr);
+                lint_same_then_else(cx, &conds, &blocks, conds.len() == blocks.len(), expr);
             }
         }
     }
@@ -192,6 +192,7 @@ impl<'tcx> LateLintPass<'tcx> for CopyAndPaste {
 /// Implementation of `BRANCHES_SHARING_CODE` and `IF_SAME_THEN_ELSE` if the blocks are equal.
 fn lint_same_then_else<'tcx>(
     cx: &LateContext<'tcx>,
+    conds: &[&'tcx Expr<'_>],
     blocks: &[&Block<'tcx>],
     has_conditional_else: bool,
     expr: &'tcx Expr<'_>,
@@ -204,7 +205,7 @@ fn lint_same_then_else<'tcx>(
     // Check if each block has shared code
     let has_expr = blocks[0].expr.is_some();
 
-    let (start_eq, mut end_eq, expr_eq) = if let Some(block_eq) = scan_block_for_eq(cx, blocks) {
+    let (start_eq, mut end_eq, expr_eq) = if let Some(block_eq) = scan_block_for_eq(cx, conds, blocks) {
         (block_eq.start_eq, block_eq.end_eq, block_eq.expr_eq)
     } else {
         return;
@@ -316,14 +317,14 @@ struct BlockEqual {
 
 /// This function can also trigger the `IF_SAME_THEN_ELSE` in which case it'll return `None` to
 /// abort any further processing and avoid duplicate lint triggers.
-fn scan_block_for_eq(cx: &LateContext<'_>, blocks: &[&Block<'_>]) -> Option<BlockEqual> {
+fn scan_block_for_eq(cx: &LateContext<'_>, conds: &[&Expr<'_>], blocks: &[&Block<'_>]) -> Option<BlockEqual> {
     let mut start_eq = usize::MAX;
     let mut end_eq = usize::MAX;
     let mut expr_eq = true;
-    let mut iter = blocks.windows(2);
-    while let Some(&[win0, win1]) = iter.next() {
-        let l_stmts = win0.stmts;
-        let r_stmts = win1.stmts;
+    let mut iter = blocks.windows(2).enumerate();
+    while let Some((i, &[block0, block1])) = iter.next() {
+        let l_stmts = block0.stmts;
+        let r_stmts = block1.stmts;
 
         // `SpanlessEq` now keeps track of the locals and is therefore context sensitive clippy#6752.
         // The comparison therefore needs to be done in a way that builds the correct context.
@@ -340,22 +341,26 @@ fn scan_block_for_eq(cx: &LateContext<'_>, blocks: &[&Block<'_>]) -> Option<Bloc
             it1.zip(it2)
                 .fold(0, |acc, (l, r)| if evaluator.eq_stmt(l, r) { acc + 1 } else { 0 })
         };
-        let block_expr_eq = both(&win0.expr, &win1.expr, |l, r| evaluator.eq_expr(l, r));
+        let block_expr_eq = both(&block0.expr, &block1.expr, |l, r| evaluator.eq_expr(l, r));
 
         // IF_SAME_THEN_ELSE
         if_chain! {
             if block_expr_eq;
             if l_stmts.len() == r_stmts.len();
             if l_stmts.len() == current_start_eq;
-            if !is_lint_allowed(cx, IF_SAME_THEN_ELSE, win0.hir_id);
-            if !is_lint_allowed(cx, IF_SAME_THEN_ELSE, win1.hir_id);
+            // `conds` may have one last item than `blocks`.
+            // Any `i` from `blocks.windows(2)` will exist in `conds`, but `i+1` may not exist on the last iteration.
+            if !matches!(conds[i].kind, ExprKind::Let(..));
+            if !matches!(conds.get(i + 1).map(|e| &e.kind), Some(ExprKind::Let(..)));
+            if !is_lint_allowed(cx, IF_SAME_THEN_ELSE, block0.hir_id);
+            if !is_lint_allowed(cx, IF_SAME_THEN_ELSE, block1.hir_id);
             then {
                 span_lint_and_note(
                     cx,
                     IF_SAME_THEN_ELSE,
-                    win0.span,
+                    block0.span,
                     "this `if` has identical blocks",
-                    Some(win1.span),
+                    Some(block1.span),
                     "same as this",
                 );
 
diff --git a/tests/ui/if_same_then_else2.rs b/tests/ui/if_same_then_else2.rs
index 69189d9e0c00..0016009a02f5 100644
--- a/tests/ui/if_same_then_else2.rs
+++ b/tests/ui/if_same_then_else2.rs
@@ -138,6 +138,23 @@ fn if_same_then_else2() -> Result<&'static str, ()> {
         let (y, x) = (1, 2);
         return Ok(&foo[x..y]);
     }
+
+    // Issue #7579
+    let _ = if let Some(0) = None { 0 } else { 0 };
+
+    if true {
+        return Err(());
+    } else if let Some(0) = None {
+        return Err(());
+    }
+
+    let _ = if let Some(0) = None {
+        0
+    } else if let Some(1) = None {
+        0
+    } else {
+        0
+    };
 }
 
 fn main() {}