special-case ranges

Author: ada4a

clippy_lints/src/no_effect.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
+use clippy_utils::higher::Range;
 use clippy_utils::source::SpanRangeExt;
 use clippy_utils::ty::{expr_type_is_certain, has_drop};
 use clippy_utils::{
@@ -347,6 +348,23 @@ fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec
         ExprKind::Cast(inner, _) if expr_type_is_certain(cx, inner) => {
             reduce_expression(cx, inner).or_else(|| Some(vec![inner]))
         },
+        // In the normal `Struct` case, we bail out if any of the fields has an uncertain type.
+        // But for two-sided ranges, we know that if the type of one of the sides is certain, then so is the other
+        // one's. So we only check that, more relaxed pre-condition.
+        //
+        // Note that that condition true in general for any struct with a generic present in two fields, but
+        // generalizing the check to those would be cumbersome.
+        ExprKind::Struct(..)
+            if let Some(range) = Range::hir(expr)
+                && let Some(start) = range.start
+                && let Some(end) = range.end =>
+        {
+            if [start, end].into_iter().any(|e| expr_type_is_certain(cx, e)) {
+                Some([start, end].into_iter().collect())
+            } else {
+                None
+            }
+        },
         ExprKind::Struct(_, fields, ref base) => {
             if fields.iter().any(|f| !expr_type_is_certain(cx, &f.expr))
                 || has_drop(cx, cx.typeck_results().expr_ty(expr))

tests/ui/unnecessary_operation.stderr

@@ -67,6 +67,12 @@ error: unnecessary operation
 LL |     ..get_number();
    |     ^^^^^^^^^^^^^^^ help: statement can be reduced to: `get_number();`
 
+error: unnecessary operation
+  --> tests/ui/unnecessary_operation.rs:93:5
+   |
+LL |     5..get_number();
+   |     ^^^^^^^^^^^^^^^^ help: statement can be reduced to: `5;get_number();`
+
 error: unnecessary operation
   --> tests/ui/unnecessary_operation.rs:95:5
    |
@@ -112,5 +118,5 @@ error: unnecessary operation
 LL |     [42, 55][get_usize()];
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42, 55].len() > get_usize());`
 
-error: aborting due to 18 previous errors
+error: aborting due to 19 previous errors