nll: strange suggestion to "consider removing the *

@nikomatsakis

For this code:

rust/src/test/ui/borrowck/borrowck-move-from-unsafe-ptr.rs

Lines 12 to 15 in fe8ace8

    
           unsafe fn foo(x: *const Box<isize>) -> Box<isize> { 
        
               let y = *x; //~ ERROR cannot move out of dereference of raw pointer 
        
               return y; 
        
           }

which produces this diagnostic output:

rust/src/test/ui/borrowck/borrowck-move-from-unsafe-ptr.nll.stderr

Lines 4 to 8 in fe8ace8

    
           LL |     let y = *x; //~ ERROR cannot move out of dereference of raw pointer 
        
              |             ^^ 
        
              |             | 
        
              |             cannot move out of dereference of raw pointer 
        
              |             help: consider removing the `*`: `x`

@nikomatsakis made this comment about the HELP on line 8:

Pre-existing but: it is...strange that we make this suggestion. It seems almost certainly wrong. It doesn't preserve the type or "intent" of the code in any particular way...? It's also distinct from what the AST checker says, which is to suggest &*x (though I would argue that is also wrong, for similar reasons). It feels like the best suggestion might be ptr::read but I'm sort of inclined to just not suggest anything at all.

rust/src/librustc_mir/borrow_check/move_errors.rs

Lines 353 to 406 in b8b4150

    
           fn add_move_hints( 
        
               &self, 
        
               error: GroupedMoveError<'tcx>, 
        
               err: &mut DiagnosticBuilder<'a>, 
        
               span: Span, 
        
           ) { 
        
               let snippet = self.infcx.tcx.sess.source_map().span_to_snippet(span).unwrap(); 
        
               match error { 
        
                   GroupedMoveError::MovesFromPlace { 
        
                       mut binds_to, 
        
                       move_from, 
        
                       .. 
        
                   } => { 
        
                       let try_remove_deref = match move_from { 
        
                           Place::Projection(box PlaceProjection { 
        
                               elem: ProjectionElem::Deref, 
        
                               .. 
        
                           }) => true, 
        
                           _ => false, 
        
                       }; 
        
                       if try_remove_deref && snippet.starts_with('*') { 
        
                           // The snippet doesn't start with `*` in (e.g.) index 
        
                           // expressions `a[b]`, which roughly desugar to 
        
                           // `*Index::index(&a, b)` or 
        
                           // `*IndexMut::index_mut(&mut a, b)`. 
        
                           err.span_suggestion_with_applicability( 
        
                               span, 
        
                               "consider removing the `*`", 
        
                               snippet[1..].to_owned(), 
        
                               Applicability::Unspecified, 
        
                           ); 
        
                       } else { 
        
                           err.span_suggestion_with_applicability( 
        
                               span, 
        
                               "consider borrowing here", 
        
                               format!("&{}", snippet), 
        
                               Applicability::Unspecified, 
        
                           ); 
        
                       } 
        
                       binds_to.sort(); 
        
                       binds_to.dedup(); 
        
                       self.add_move_error_details(err, &binds_to); 
        
                   } 
        
                   GroupedMoveError::MovesFromValue { mut binds_to, .. } => { 
        
                       binds_to.sort(); 
        
                       binds_to.dedup(); 
        
                       self.add_move_error_suggestions(err, &binds_to); 
        
                       self.add_move_error_details(err, &binds_to); 
        
                   } 
        
                   // No binding. Nothing to suggest. 
        
                   GroupedMoveError::OtherIllegalMove { .. } => (), 
        
               } 
        
           }

How bad might it be to just remove this suggestion entirely (vs attempting to fine tune when we emit it)? Can we easily weigh the good it brings vs the harm it causes?

I'd prefer to just remove it entirely. I think it adds pretty little value.

I could imagine using an extended error code to offer more detailed hints

or else a reference to something online?

marking as NLL-deferred as this should not block RC2 (or even the Release, IMO).

NLL triage. Tagging as P-medium priority.

Diagnostics triage: marking as E-easy since it's just removing the suggestion

Rollup merge of rust-lang#61332 - kennethbgoodin:borrowck-remove-aste…

Shouldn't this be closed because #61332 is now merged?

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nll: strange suggestion to "consider removing the `*` #54985

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	unsafe fn foo(x: *const Box<isize>) -> Box<isize> {
	let y = *x; //~ ERROR cannot move out of dereference of raw pointer
	return y;
	}

	LL \| let y = *x; //~ ERROR cannot move out of dereference of raw pointer
	\| ^^
	\| \|
	\| cannot move out of dereference of raw pointer
	\| help: consider removing the `*`: `x`

	fn add_move_hints(
	&self,
	error: GroupedMoveError<'tcx>,
	err: &mut DiagnosticBuilder<'a>,
	span: Span,
	) {
	let snippet = self.infcx.tcx.sess.source_map().span_to_snippet(span).unwrap();
	match error {
	GroupedMoveError::MovesFromPlace {
	mut binds_to,
	move_from,
	..
	} => {
	let try_remove_deref = match move_from {
	Place::Projection(box PlaceProjection {
	elem: ProjectionElem::Deref,
	..
	}) => true,
	_ => false,
	};
	if try_remove_deref && snippet.starts_with('*') {
	// The snippet doesn't start with `*` in (e.g.) index
	// expressions `a[b]`, which roughly desugar to
	// `*Index::index(&a, b)` or
	// `*IndexMut::index_mut(&mut a, b)`.
	err.span_suggestion_with_applicability(
	span,
	"consider removing the `*`",
	snippet[1..].to_owned(),
	Applicability::Unspecified,
	);
	} else {
	err.span_suggestion_with_applicability(
	span,
	"consider borrowing here",
	format!("&{}", snippet),
	Applicability::Unspecified,
	);
	}

	binds_to.sort();
	binds_to.dedup();
	self.add_move_error_details(err, &binds_to);
	}
	GroupedMoveError::MovesFromValue { mut binds_to, .. } => {
	binds_to.sort();
	binds_to.dedup();
	self.add_move_error_suggestions(err, &binds_to);
	self.add_move_error_details(err, &binds_to);
	}
	// No binding. Nothing to suggest.
	GroupedMoveError::OtherIllegalMove { .. } => (),
	}
	}

nll: strange suggestion to "consider removing the * #54985

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estebank commented on Oct 11, 2018

pnkfelix commented on Oct 16, 2018

nikomatsakis commented on Oct 16, 2018

nikomatsakis commented on Oct 16, 2018

nikomatsakis commented on Oct 16, 2018

pnkfelix commented on Oct 16, 2018

pnkfelix commented on Mar 20, 2019

oli-obk commented on May 24, 2019

awaitlink commented on Jun 17, 2019

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nll: strange suggestion to "consider removing the `*` #54985