Auto merge of rust-lang#9920 - naosense:suppress_lint_in_const, r=xFrednet

add `suppress_restriction_lint_in_const` config

According to rust-lang#9808 , add a new lint `suppress_lint_in_const` to report even in const context. BTW, i am not good at naming either, if anyone have a better idea, i am happy to change it.

This PR is still in progress, so i keep it draft.

- \[x] Followed [lint naming conventions][lint_naming]
- \[x] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[x] Executed `cargo dev update_lints`
- \[x] Added lint documentation
- \[x] Run `cargo dev fmt`

changelog: Enhancement: [`indexing_slicing`]: add new config `suppress-restriction-lint-in-const` to enable restriction lints, even if the suggestion might not be applicable

r? `@xFrendet`

Author: bors

clippy_lints/src/indexing_slicing.rs

@@ -1,13 +1,13 @@
 //! lint on indexing and slicing operations
 
 use clippy_utils::consts::{constant, Constant};
-use clippy_utils::diagnostics::{span_lint, span_lint_and_help};
+use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
 use clippy_utils::higher;
 use rustc_ast::ast::RangeLimits;
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
-use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_session::{declare_tool_lint, impl_lint_pass};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -82,15 +82,29 @@ declare_clippy_lint! {
     "indexing/slicing usage"
 }
 
-declare_lint_pass!(IndexingSlicing => [INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING]);
+impl_lint_pass!(IndexingSlicing => [INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING]);
+
+#[derive(Copy, Clone)]
+pub struct IndexingSlicing {
+    suppress_restriction_lint_in_const: bool,
+}
+
+impl IndexingSlicing {
+    pub fn new(suppress_restriction_lint_in_const: bool) -> Self {
+        Self {
+            suppress_restriction_lint_in_const,
+        }
+    }
+}
 
 impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if cx.tcx.hir().is_inside_const_context(expr.hir_id) {
+        if self.suppress_restriction_lint_in_const && cx.tcx.hir().is_inside_const_context(expr.hir_id) {
             return;
         }
 
         if let ExprKind::Index(array, index) = &expr.kind {
+            let note = "the suggestion might not be applicable in constant blocks";
             let ty = cx.typeck_results().expr_ty(array).peel_refs();
             if let Some(range) = higher::Range::hir(index) {
                 // Ranged indexes, i.e., &x[n..m], &x[n..], &x[..n] and &x[..]
@@ -141,7 +155,13 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
                     (None, None) => return, // [..] is ok.
                 };
 
-                span_lint_and_help(cx, INDEXING_SLICING, expr.span, "slicing may panic", None, help_msg);
+                span_lint_and_then(cx, INDEXING_SLICING, expr.span, "slicing may panic", |diag| {
+                    diag.help(help_msg);
+
+                    if cx.tcx.hir().is_inside_const_context(expr.hir_id) {
+                        diag.note(note);
+                    }
+                });
             } else {
                 // Catchall non-range index, i.e., [n] or [n << m]
                 if let ty::Array(..) = ty.kind() {
@@ -156,14 +176,13 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
                     }
                 }
 
-                span_lint_and_help(
-                    cx,
-                    INDEXING_SLICING,
-                    expr.span,
-                    "indexing may panic",
-                    None,
-                    "consider using `.get(n)` or `.get_mut(n)` instead",
-                );
+                span_lint_and_then(cx, INDEXING_SLICING, expr.span, "indexing may panic", |diag| {
+                    diag.help("consider using `.get(n)` or `.get_mut(n)` instead");
+
+                    if cx.tcx.hir().is_inside_const_context(expr.hir_id) {
+                        diag.note(note);
+                    }
+                });
             }
         }
     }

clippy_lints/src/lib.rs

@@ -561,6 +561,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let avoid_breaking_exported_api = conf.avoid_breaking_exported_api;
     let allow_expect_in_tests = conf.allow_expect_in_tests;
     let allow_unwrap_in_tests = conf.allow_unwrap_in_tests;
+    let suppress_restriction_lint_in_const = conf.suppress_restriction_lint_in_const;
     store.register_late_pass(move |_| Box::new(approx_const::ApproxConstant::new(msrv())));
     store.register_late_pass(move |_| {
         Box::new(methods::Methods::new(
@@ -682,7 +683,11 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(inherent_impl::MultipleInherentImpl));
     store.register_late_pass(|_| Box::new(neg_cmp_op_on_partial_ord::NoNegCompOpForPartialOrd));
     store.register_late_pass(|_| Box::new(unwrap::Unwrap));
-    store.register_late_pass(|_| Box::new(indexing_slicing::IndexingSlicing));
+    store.register_late_pass(move |_| {
+        Box::new(indexing_slicing::IndexingSlicing::new(
+            suppress_restriction_lint_in_const,
+        ))
+    });
     store.register_late_pass(|_| Box::new(non_copy_const::NonCopyConst));
     store.register_late_pass(|_| Box::new(ptr_offset_with_cast::PtrOffsetWithCast));
     store.register_late_pass(|_| Box::new(redundant_clone::RedundantClone));

clippy_lints/src/utils/conf.rs

@@ -406,6 +406,14 @@ define_Conf! {
     ///
     /// Whether to allow mixed uninlined format args, e.g. `format!("{} {}", a, foo.bar)`
     (allow_mixed_uninlined_format_args: bool = true),
+    /// Lint: INDEXING_SLICING
+    ///
+    /// Whether to suppress a restriction lint in constant code. In same
+    /// cases the restructured operation might not be unavoidable, as the
+    /// suggested counterparts are unavailable in constant code. This
+    /// configuration will cause restriction lints to trigger even
+    /// if no suggestion can be made.
+    (suppress_restriction_lint_in_const: bool = false),
 }
 
 /// Search for the configuration file.

tests/ui-toml/suppress_lint_in_const/clippy.toml

@@ -0,0 +1 @@
+suppress-restriction-lint-in-const = true

tests/ui-toml/suppress_lint_in_const/test.rs

@@ -0,0 +1,60 @@
+#![feature(inline_const)]
+#![warn(clippy::indexing_slicing)]
+// We also check the out_of_bounds_indexing lint here, because it lints similar things and
+// we want to avoid false positives.
+#![warn(clippy::out_of_bounds_indexing)]
+#![allow(unconditional_panic, clippy::no_effect, clippy::unnecessary_operation)]
+
+const ARR: [i32; 2] = [1, 2];
+const REF: &i32 = &ARR[idx()]; // Ok, should not produce stderr, since `suppress-restriction-lint-in-const` is set true.
+const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
+
+const fn idx() -> usize {
+    1
+}
+const fn idx4() -> usize {
+    4
+}
+
+fn main() {
+    let x = [1, 2, 3, 4];
+    let index: usize = 1;
+    x[index];
+    x[4]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+    x[1 << 3]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+
+    x[0]; // Ok, should not produce stderr.
+    x[3]; // Ok, should not produce stderr.
+    x[const { idx() }]; // Ok, should not produce stderr.
+    x[const { idx4() }]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+    const { &ARR[idx()] }; // Ok, should not produce stderr, since `suppress-restriction-lint-in-const` is set true.
+    const { &ARR[idx4()] }; // Ok, should not produce stderr, since `suppress-restriction-lint-in-const` is set true.
+
+    let y = &x;
+    y[0]; // Ok, referencing shouldn't affect this lint. See the issue 6021
+    y[4]; // Ok, rustc will handle references too.
+
+    let v = vec![0; 5];
+    v[0];
+    v[10];
+    v[1 << 3];
+
+    const N: usize = 15; // Out of bounds
+    const M: usize = 3; // In bounds
+    x[N]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+    x[M]; // Ok, should not produce stderr.
+    v[N];
+    v[M];
+}
+
+/// An opaque integer representation
+pub struct Integer<'a> {
+    /// The underlying data
+    value: &'a [u8],
+}
+impl<'a> Integer<'a> {
+    // Check whether `self` holds a negative number or not
+    pub const fn is_negative(&self) -> bool {
+        self.value[0] & 0b1000_0000 != 0
+    }
+}

tests/ui-toml/suppress_lint_in_const/test.stderr

@@ -0,0 +1,70 @@
+error[E0080]: evaluation of `main::{constant#3}` failed
+  --> $DIR/test.rs:31:14
+   |
+LL |     const { &ARR[idx4()] }; // Ok, should not produce stderr, since `suppress-restriction-lint-in-const` is set true.
+   |              ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
+
+note: erroneous constant used
+  --> $DIR/test.rs:31:5
+   |
+LL |     const { &ARR[idx4()] }; // Ok, should not produce stderr, since `suppress-restriction-lint-in-const` is set true.
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: indexing may panic
+  --> $DIR/test.rs:22:5
+   |
+LL |     x[index];
+   |     ^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+   = note: `-D clippy::indexing-slicing` implied by `-D warnings`
+
+error: indexing may panic
+  --> $DIR/test.rs:38:5
+   |
+LL |     v[0];
+   |     ^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+
+error: indexing may panic
+  --> $DIR/test.rs:39:5
+   |
+LL |     v[10];
+   |     ^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+
+error: indexing may panic
+  --> $DIR/test.rs:40:5
+   |
+LL |     v[1 << 3];
+   |     ^^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+
+error: indexing may panic
+  --> $DIR/test.rs:46:5
+   |
+LL |     v[N];
+   |     ^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+
+error: indexing may panic
+  --> $DIR/test.rs:47:5
+   |
+LL |     v[M];
+   |     ^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/test.rs:10:24
+   |
+LL | const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
+   |                        ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui-toml/toml_unknown_key/conf_unknown_key.stderr

@@ -35,6 +35,7 @@ error: error reading Clippy's configuration file `$DIR/clippy.toml`: unknown fie
            pass-by-value-size-limit
            single-char-binding-names-threshold
            standard-macro-braces
+           suppress-restriction-lint-in-const
            third-party
            too-large-for-stack
            too-many-arguments-threshold

tests/ui/indexing_slicing_index.rs

@@ -6,7 +6,7 @@
 #![allow(unconditional_panic, clippy::no_effect, clippy::unnecessary_operation)]
 
 const ARR: [i32; 2] = [1, 2];
-const REF: &i32 = &ARR[idx()]; // Ok, should not produce stderr.
+const REF: &i32 = &ARR[idx()]; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
 const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
 
 const fn idx() -> usize {
@@ -27,8 +27,8 @@ fn main() {
     x[3]; // Ok, should not produce stderr.
     x[const { idx() }]; // Ok, should not produce stderr.
     x[const { idx4() }]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
-    const { &ARR[idx()] }; // Ok, should not produce stderr.
-    const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
+    const { &ARR[idx()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
+    const { &ARR[idx4()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
 
     let y = &x;
     y[0]; // Ok, referencing shouldn't affect this lint. See the issue 6021

tests/ui/indexing_slicing_index.stderr

@@ -1,13 +1,32 @@
+error: indexing may panic
+  --> $DIR/indexing_slicing_index.rs:9:20
+   |
+LL | const REF: &i32 = &ARR[idx()]; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
+   |                    ^^^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+   = note: the suggestion might not be applicable in constant blocks
+   = note: `-D clippy::indexing-slicing` implied by `-D warnings`
+
+error: indexing may panic
+  --> $DIR/indexing_slicing_index.rs:10:24
+   |
+LL | const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
+   |                        ^^^^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+   = note: the suggestion might not be applicable in constant blocks
+
 error[E0080]: evaluation of `main::{constant#3}` failed
   --> $DIR/indexing_slicing_index.rs:31:14
    |
-LL |     const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
+LL |     const { &ARR[idx4()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
    |              ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
 
 note: erroneous constant used
   --> $DIR/indexing_slicing_index.rs:31:5
    |
-LL |     const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
+LL |     const { &ARR[idx4()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: indexing may panic
@@ -17,7 +36,24 @@ LL |     x[index];
    |     ^^^^^^^^
    |
    = help: consider using `.get(n)` or `.get_mut(n)` instead
-   = note: `-D clippy::indexing-slicing` implied by `-D warnings`
+
+error: indexing may panic
+  --> $DIR/indexing_slicing_index.rs:30:14
+   |
+LL |     const { &ARR[idx()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
+   |              ^^^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+   = note: the suggestion might not be applicable in constant blocks
+
+error: indexing may panic
+  --> $DIR/indexing_slicing_index.rs:31:14
+   |
+LL |     const { &ARR[idx4()] }; // This should be linted, since `suppress-restriction-lint-in-const` default is false.
+   |              ^^^^^^^^^^^
+   |
+   = help: consider using `.get(n)` or `.get_mut(n)` instead
+   = note: the suggestion might not be applicable in constant blocks
 
 error: indexing may panic
   --> $DIR/indexing_slicing_index.rs:38:5
@@ -65,6 +101,6 @@ error[E0080]: evaluation of constant value failed
 LL | const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
    |                        ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
 
-error: aborting due to 8 previous errors
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0080`.