Suggest ..= when someone tries to create an overflowing range

compiler/rustc_lint/messages.ftl

@@ -197,7 +197,9 @@ lint_drop_glue =
     types that do not implement `Drop` can still have drop glue, consider instead using `{$needs_drop}` to detect whether a type is trivially dropped
 
 lint_range_endpoint_out_of_range = range endpoint is out of range for `{$ty}`
-    .suggestion = use an inclusive range instead
+
+lint_range_use_inclusive_range = use an inclusive range instead
+
 
 lint_overflowing_bin_hex = literal out of range for `{$ty}`
     .negative_note = the literal `{$lit}` (decimal `{$dec}`) does not fit into the type `{$ty}`

compiler/rustc_lint/src/lints.rs

@@ -1210,11 +1210,33 @@ impl<'a> DecorateLint<'a, ()> for DropGlue<'_> {
 #[diag(lint_range_endpoint_out_of_range)]
 pub struct RangeEndpointOutOfRange<'a> {
     pub ty: &'a str,
-    #[suggestion(code = "{start}..={literal}{suffix}", applicability = "machine-applicable")]
-    pub suggestion: Span,
-    pub start: String,
-    pub literal: u128,
-    pub suffix: &'a str,
+    #[subdiagnostic]
+    pub sub: UseInclusiveRange<'a>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum UseInclusiveRange<'a> {
+    #[suggestion(
+        lint_range_use_inclusive_range,
+        code = "{start}..={literal}{suffix}",
+        applicability = "machine-applicable"
+    )]
+    WithoutParen {
+        #[primary_span]
+        sugg: Span,
+        start: String,
+        literal: u128,
+        suffix: &'a str,
+    },
+    #[multipart_suggestion(lint_range_use_inclusive_range, applicability = "machine-applicable")]
+    WithParen {
+        #[suggestion_part(code = "=")]
+        eq_sugg: Span,
+        #[suggestion_part(code = "{literal}{suffix}")]
+        lit_sugg: Span,
+        literal: u128,
+        suffix: &'a str,
+    },
 }
 
 #[derive(LintDiagnostic)]

compiler/rustc_lint/src/types.rs

@@ -4,7 +4,8 @@ use crate::{
         AtomicOrderingFence, AtomicOrderingLoad, AtomicOrderingStore, ImproperCTypes,
         InvalidAtomicOrderingDiag, OnlyCastu8ToChar, OverflowingBinHex, OverflowingBinHexSign,
         OverflowingBinHexSub, OverflowingInt, OverflowingIntHelp, OverflowingLiteral,
-        OverflowingUInt, RangeEndpointOutOfRange, UnusedComparisons, VariantSizeDifferencesDiag,
+        OverflowingUInt, RangeEndpointOutOfRange, UnusedComparisons, UseInclusiveRange,
+        VariantSizeDifferencesDiag,
     },
 };
 use crate::{LateContext, LateLintPass, LintContext};
@@ -136,6 +137,14 @@ fn lint_overflowing_range_endpoint<'tcx>(
     expr: &'tcx hir::Expr<'tcx>,
     ty: &str,
 ) -> bool {
+    // Look past casts to support cases like `0..256 as u8`
+    let (expr, lit_span) = if let Node::Expr(par_expr) = cx.tcx.hir().get(cx.tcx.hir().parent_id(expr.hir_id))
+      && let ExprKind::Cast(_, _) = par_expr.kind {
+        (par_expr, expr.span)
+    } else {
+        (expr, expr.span)
+    };
+
     // We only want to handle exclusive (`..`) ranges,
     // which are represented as `ExprKind::Struct`.
     let par_id = cx.tcx.hir().parent_id(expr.hir_id);
@@ -155,7 +164,6 @@ fn lint_overflowing_range_endpoint<'tcx>(
     if !(eps[1].expr.hir_id == expr.hir_id && lit_val - 1 == max) {
         return false;
     };
-    let Ok(start) = cx.sess().source_map().span_to_snippet(eps[0].span) else { return false };
 
     use rustc_ast::{LitIntType, LitKind};
     let suffix = match lit.node {
@@ -164,16 +172,28 @@ fn lint_overflowing_range_endpoint<'tcx>(
         LitKind::Int(_, LitIntType::Unsuffixed) => "",
         _ => bug!(),
     };
-    cx.emit_spanned_lint(
-        OVERFLOWING_LITERALS,
-        struct_expr.span,
-        RangeEndpointOutOfRange {
-            ty,
-            suggestion: struct_expr.span,
+
+    let sub_sugg = if expr.span.lo() == lit_span.lo() {
+        let Ok(start) = cx.sess().source_map().span_to_snippet(eps[0].span) else { return false };
+        UseInclusiveRange::WithoutParen {
+            sugg: struct_expr.span.shrink_to_lo().to(lit_span.shrink_to_hi()),
             start,
             literal: lit_val - 1,
             suffix,
-        },
+        }
+    } else {
+        UseInclusiveRange::WithParen {
+            eq_sugg: expr.span.shrink_to_lo(),
+            lit_sugg: lit_span,
+            literal: lit_val - 1,
+            suffix,
+        }
+    };
+
+    cx.emit_spanned_lint(
+        OVERFLOWING_LITERALS,
+        struct_expr.span,
+        RangeEndpointOutOfRange { ty, sub: sub_sugg },
     );
 
     // We've just emitted a lint, special cased for `(...)..MAX+1` ranges,

tests/ui/lint/issue-109529.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    for _ in 0..=255 as u8 {} //~ ERROR range endpoint is out of range
+    for _ in 0..=(255 as u8) {} //~ ERROR range endpoint is out of range
+}

tests/ui/lint/issue-109529.rs

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    for _ in 0..256 as u8 {} //~ ERROR range endpoint is out of range
+    for _ in 0..(256 as u8) {} //~ ERROR range endpoint is out of range
+}

tests/ui/lint/issue-109529.stderr

@@ -0,0 +1,23 @@
+error: range endpoint is out of range for `u8`
+  --> $DIR/issue-109529.rs:4:14
+   |
+LL |     for _ in 0..256 as u8 {}
+   |              ------^^^^^^
+   |              |
+   |              help: use an inclusive range instead: `0..=255`
+   |
+   = note: `#[deny(overflowing_literals)]` on by default
+
+error: range endpoint is out of range for `u8`
+  --> $DIR/issue-109529.rs:5:14
+   |
+LL |     for _ in 0..(256 as u8) {}
+   |              ^^^^^^^^^^^^^^
+   |
+help: use an inclusive range instead
+   |
+LL |     for _ in 0..=(255 as u8) {}
+   |                 + ~~~
+
+error: aborting due to 2 previous errors
+