Add help for trying to do C-like pointer arithmetics #112261

jieyouxu · 2023-06-04T02:25:50Z

This PR adds help messages for these cases:

fn main() {
    let ptr1: *const u32 = std::ptr::null();
    let ptr2: *const u32 = std::ptr::null();
    let a = ptr1 + 5;
    let b = ptr1 - 5;
    let c = ptr2 - ptr1;
    let d = ptr1[5];
}

Current Output

error[E0369]: cannot add `{integer}` to `*const u32`
 --> tests/ui/typeck/issue-112252-ptr-arithmetics-help.rs:4:18
  |
4 |     let a = ptr1 + 5; //~ ERROR cannot add
  |             ---- ^ - {integer}
  |             |
  |             *const u32

error[E0369]: cannot subtract `{integer}` from `*const u32`
 --> tests/ui/typeck/issue-112252-ptr-arithmetics-help.rs:5:18
  |
5 |     let b = ptr1 - 5; //~ ERROR cannot subtract
  |             ---- ^ - {integer}
  |             |
  |             *const u32

error[E0369]: cannot subtract `*const u32` from `*const u32`
 --> tests/ui/typeck/issue-112252-ptr-arithmetics-help.rs:6:18
  |
6 |     let c = ptr2 - ptr1; //~ ERROR cannot subtract
  |             ---- ^ ---- *const u32
  |             |
  |             *const u32

error[E0608]: cannot index into a value of type `*const u32`
 --> tests/ui/typeck/issue-112252-ptr-arithmetics-help.rs:7:13
  |
7 |     let d = ptr1[5]; //~ ERROR cannot index
  |             ^^^^^^^

error: aborting due to 4 previous errors

Output After This PR

error[E0369]: cannot add `{integer}` to `*const u32`
  --> $DIR/issue-112252-ptr-arithmetics-help.rs:6:20
   |
LL |     let _a = _ptr1 + 5;
   |              ------^--
   |              |       |
   |              |       {integer}
   |              *const u32
   |              help: consider using `wrapping_add` or `add` for pointer + {integer}: `_ptr1.wrapping_add(5)`

error[E0369]: cannot subtract `{integer}` from `*const u32`
  --> $DIR/issue-112252-ptr-arithmetics-help.rs:7:20
   |
LL |     let _b = _ptr1 - 5;
   |              ------^--
   |              |       |
   |              |       {integer}
   |              *const u32
   |              help: consider using `offset` for pointer - {integer}: `unsafe { _ptr1.offset(-5) }`

error[E0369]: cannot subtract `*const u32` from `*const u32`
  --> $DIR/issue-112252-ptr-arithmetics-help.rs:8:20
   |
LL |     let _c = _ptr2 - _ptr1;
   |              ------^------
   |              |       |
   |              |       *const u32
   |              *const u32
   |              help: consider using `offset_from` for pointer - pointer if the pointers point to the same allocation: `_ptr2.offset_from(_ptr1)`

error[E0608]: cannot index into a value of type `*const u32`
  --> $DIR/issue-112252-ptr-arithmetics-help.rs:9:14
   |
LL |     let _d = _ptr1[5];
   |              ^^^^^^^^
   |
help: consider using `wrapping_add` or `add` for indexing into raw pointer
   |
LL |     let _d = _ptr1.wrapping_add(5);
   |              ~~~~~~~~~~~~~~~~~~~~~

error: aborting due to 4 previous errors

Closes #112252.

rustbot · 2023-06-04T02:25:58Z

r? @eholk

(rustbot has picked a reviewer for you, use r? to override)

compiler-errors · 2023-06-04T04:58:05Z

compiler/rustc_hir_typeck/src/expr.rs

@@ -2870,6 +2870,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                            );
                        }
                    }
+
+                    if let ty::RawPtr(..) = base_t.kind() && idx_t.is_integral() {
+                        err.span_help(


Can this be a span_suggestion

I was hesistant to make them suggestions because ptr::add and friends are unsafe functions.

If I were to make it into a suggestion, it would become something like

error[E0608]: cannot index into a value of type `*const u32` --> tests/ui/typeck/issue-112252-ptr-arithmetics-help.rs:9:13 | 9 | let d = ptr1[5]; //~ ERROR cannot index | ^^^^^^^ help: consider using `add` for indexing into raw pointer: `unsafe { ptr1.add(5) }`

wrapping_add isn't unsafe. Although C pointer arithmetic does have the same preconditions as add I think. But I don't think suggestions should include unsafe code. add should still be mentioned in the help though.

wrapping_add isn't unsafe.

Good catch, I'll refer to that instead of add.

But I don't think suggestions should include unsafe code.

I think I agree with this. I can undo the change to make them suggestions and revert them to being help notes if this is deemed more suitable.

~~You can have both a suggestion and a note~~ although having add in the suggestion text is also fine

compiler-errors · 2023-06-04T05:07:34Z

compiler/rustc_hir_typeck/src/op.rs

+                    if op.node == hir::BinOpKind::Add {
+                        match (lhs_ty.kind(), rhs_ty) {
+                            (RawPtr(..), rhs_ty) if rhs_ty.is_integral() => {
+                                err.span_help(


similarly, and all other usages of span_help

compiler/rustc_hir_typeck/src/op.rs

eholk · 2023-06-05T21:55:11Z

compiler/rustc_hir_typeck/src/op.rs

+                                    err.span_suggestion(
+                                        expr.span,
+                                        "consider using `offset_from` for pointer - pointer",
+                                        format!("unsafe {{ {lhs_snip}.offset_from({rhs_snip}) }}"),


Is there a safe suggestion we could make here? I didn't find one immediately after poking around the docs a little...

I couldn't find one either...

you should at least add "if the pointers point to the same allocation"

compiler/rustc_hir_typeck/src/expr.rs

compiler/rustc_hir_typeck/src/op.rs

WaffleLapkin · 2023-06-12T07:14:34Z

@bors r+ rollup

bors · 2023-06-12T07:14:37Z

📌 Commit 63d643d has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2023-06-12T07:15:23Z

⌛ Testing commit 63d643d with merge fd0a331...

bors · 2023-06-12T10:04:40Z

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing fd0a331 to master...

rust-timer · 2023-06-12T11:19:31Z

Finished benchmarking commit (fd0a331): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.3%, 0.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.1%, -1.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-1.5%, -1.5%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 647.92s -> 648.279s (0.06%)

rustbot assigned eholk Jun 4, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 4, 2023

compiler-errors reviewed Jun 4, 2023

View reviewed changes

jieyouxu force-pushed the c-like-ptr-arithmetics-diagnostics branch 2 times, most recently from 50e4123 to d09f33d Compare June 4, 2023 08:22

deltragon reviewed Jun 4, 2023

View reviewed changes

compiler/rustc_hir_typeck/src/op.rs Outdated Show resolved Hide resolved

jieyouxu force-pushed the c-like-ptr-arithmetics-diagnostics branch from d09f33d to 7ab91e5 Compare June 4, 2023 09:34

eholk reviewed Jun 5, 2023

View reviewed changes

jieyouxu force-pushed the c-like-ptr-arithmetics-diagnostics branch from 7ab91e5 to 4ae70a7 Compare June 6, 2023 07:19

WaffleLapkin reviewed Jun 8, 2023

View reviewed changes

compiler/rustc_hir_typeck/src/expr.rs Outdated Show resolved Hide resolved

compiler/rustc_hir_typeck/src/op.rs Outdated Show resolved Hide resolved

compiler/rustc_hir_typeck/src/op.rs Outdated Show resolved Hide resolved

Add help for trying to do C-like pointer arithmetics

63d643d

jieyouxu force-pushed the c-like-ptr-arithmetics-diagnostics branch from 4ae70a7 to 63d643d Compare June 9, 2023 02:01

WaffleLapkin approved these changes Jun 12, 2023

View reviewed changes

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 12, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 12, 2023

bors merged commit fd0a331 into rust-lang:master Jun 12, 2023

rustbot added this to the 1.72.0 milestone Jun 12, 2023

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Add help for trying to do C-like pointer arithmetics #112261

Add help for trying to do C-like pointer arithmetics #112261

jieyouxu commented Jun 4, 2023 •

edited

Loading

rustbot commented Jun 4, 2023

compiler-errors Jun 4, 2023

jieyouxu Jun 4, 2023 •

edited

Loading

Noratrieb Jun 4, 2023

jieyouxu Jun 4, 2023 •

edited

Loading

WaffleLapkin Jun 8, 2023 •

edited

Loading

compiler-errors Jun 4, 2023

eholk Jun 5, 2023

jieyouxu Jun 5, 2023

Noratrieb Jun 6, 2023

WaffleLapkin commented Jun 12, 2023

bors commented Jun 12, 2023

bors commented Jun 12, 2023

bors commented Jun 12, 2023

rust-timer commented Jun 12, 2023

Add help for trying to do C-like pointer arithmetics #112261

Add help for trying to do C-like pointer arithmetics #112261

Conversation

jieyouxu commented Jun 4, 2023 • edited Loading

Current Output

Output After This PR

rustbot commented Jun 4, 2023

Choose a reason for hiding this comment

jieyouxu Jun 4, 2023 • edited Loading

Choose a reason for hiding this comment

Choose a reason for hiding this comment

jieyouxu Jun 4, 2023 • edited Loading

Choose a reason for hiding this comment

WaffleLapkin Jun 8, 2023 • edited Loading

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

WaffleLapkin commented Jun 12, 2023

bors commented Jun 12, 2023

bors commented Jun 12, 2023

bors commented Jun 12, 2023

rust-timer commented Jun 12, 2023

Overall result: ❌✅ regressions and improvements - no action needed

jieyouxu commented Jun 4, 2023 •

edited

Loading

jieyouxu Jun 4, 2023 •

edited

Loading

jieyouxu Jun 4, 2023 •

edited

Loading

WaffleLapkin Jun 8, 2023 •

edited

Loading