Auto merge of #11837 - y21:issue11835, r=dswij

[`missing_asserts_for_indexing`]: accept length equality checks

Fixes #11835

The lint now allows indexing with indices 0 and 1 when an `assert!(x.len() == 2);` is found.
(Also fixed a typo in the doc example)

changelog: [`missing_asserts_for_indexing`]: accept len equality checks as a valid assertion

3 files changed, 83 insertions, 1 deletions

diff --git a/tests/ui/missing_asserts_for_indexing.fixed b/tests/ui/missing_asserts_for_indexing.fixed
index a96827259f5..ac44a6f3fdb 100644
--- a/tests/ui/missing_asserts_for_indexing.fixed
+++ b/tests/ui/missing_asserts_for_indexing.fixed
@@ -118,4 +118,19 @@ fn index_different_slice_in_same_expr(v1: &[u8], v2: &[u8]) {
     let _ = v1[0] + v2[1];
 }
 
+fn issue11835(v1: &[u8], v2: &[u8], v3: &[u8], v4: &[u8]) {
+    assert!(v1.len() == 3);
+    assert!(v2.len() == 4);
+    assert!(v3.len() == 3);
+    assert!(4 == v4.len());
+
+    let _ = v1[0] + v1[1] + v1[2];
+    //~^ ERROR: indexing into a slice multiple times with an `assert` that does not cover the
+    let _ = v2[0] + v2[1] + v2[2];
+
+    let _ = v3[0] + v3[1] + v3[2];
+    //~^ ERROR: indexing into a slice multiple times with an `assert` that does not cover the
+    let _ = v4[0] + v4[1] + v4[2];
+}
+
 fn main() {}
diff --git a/tests/ui/missing_asserts_for_indexing.rs b/tests/ui/missing_asserts_for_indexing.rs
index 0b4b883acf8..f05d5fea57d 100644
--- a/tests/ui/missing_asserts_for_indexing.rs
+++ b/tests/ui/missing_asserts_for_indexing.rs
@@ -118,4 +118,19 @@ fn index_different_slice_in_same_expr(v1: &[u8], v2: &[u8]) {
     let _ = v1[0] + v2[1];
 }
 
+fn issue11835(v1: &[u8], v2: &[u8], v3: &[u8], v4: &[u8]) {
+    assert!(v1.len() == 2);
+    assert!(v2.len() == 4);
+    assert!(2 == v3.len());
+    assert!(4 == v4.len());
+
+    let _ = v1[0] + v1[1] + v1[2];
+    //~^ ERROR: indexing into a slice multiple times with an `assert` that does not cover the
+    let _ = v2[0] + v2[1] + v2[2];
+
+    let _ = v3[0] + v3[1] + v3[2];
+    //~^ ERROR: indexing into a slice multiple times with an `assert` that does not cover the
+    let _ = v4[0] + v4[1] + v4[2];
+}
+
 fn main() {}
diff --git a/tests/ui/missing_asserts_for_indexing.stderr b/tests/ui/missing_asserts_for_indexing.stderr
index a3e66d7958e..61dce6ccc6c 100644
--- a/tests/ui/missing_asserts_for_indexing.stderr
+++ b/tests/ui/missing_asserts_for_indexing.stderr
@@ -249,5 +249,57 @@ LL |     let _ = v1[0] + v1[12];
    |                     ^^^^^^
    = note: asserting the length before indexing will elide bounds checks
 
-error: aborting due to 9 previous errors
+error: indexing into a slice multiple times with an `assert` that does not cover the highest index
+  --> $DIR/missing_asserts_for_indexing.rs:127:13
+   |
+LL |     assert!(v1.len() == 2);
+   |     ---------------------- help: provide the highest index that is indexed with: `assert!(v1.len() == 3)`
+...
+LL |     let _ = v1[0] + v1[1] + v1[2];
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:127:13
+   |
+LL |     let _ = v1[0] + v1[1] + v1[2];
+   |             ^^^^^
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:127:21
+   |
+LL |     let _ = v1[0] + v1[1] + v1[2];
+   |                     ^^^^^
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:127:29
+   |
+LL |     let _ = v1[0] + v1[1] + v1[2];
+   |                             ^^^^^
+   = note: asserting the length before indexing will elide bounds checks
+
+error: indexing into a slice multiple times with an `assert` that does not cover the highest index
+  --> $DIR/missing_asserts_for_indexing.rs:131:13
+   |
+LL |     assert!(2 == v3.len());
+   |     ---------------------- help: provide the highest index that is indexed with: `assert!(v3.len() == 3)`
+...
+LL |     let _ = v3[0] + v3[1] + v3[2];
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:131:13
+   |
+LL |     let _ = v3[0] + v3[1] + v3[2];
+   |             ^^^^^
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:131:21
+   |
+LL |     let _ = v3[0] + v3[1] + v3[2];
+   |                     ^^^^^
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing.rs:131:29
+   |
+LL |     let _ = v3[0] + v3[1] + v3[2];
+   |                             ^^^^^
+   = note: asserting the length before indexing will elide bounds checks
+
+error: aborting due to 11 previous errors