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[NLL] Mutability errors #52405

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merged 4 commits into from
Jul 21, 2018
Merged

[NLL] Mutability errors #52405

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c3dbdfe
MIR changes to improve NLL cannot mutate errors
matthewjasper Jul 15, 2018
db568e4
Move mutability error reporting to its own file
matthewjasper Jul 15, 2018
13b5f69
Improve NLL mutability errors
matthewjasper Jul 15, 2018
a06b243
Update tests for new NLL mutability errors
matthewjasper Jul 17, 2018
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3 changes: 3 additions & 0 deletions src/librustc/mir/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -531,6 +531,8 @@ pub enum BindingForm<'tcx> {
Var(VarBindingForm<'tcx>),
/// Binding for a `self`/`&self`/`&mut self` binding where the type is implicit.
ImplicitSelf,
/// Reference used in a guard expression to ensure immutability.
RefForGuard,
}

CloneTypeFoldableAndLiftImpls! { BindingForm<'tcx>, }
Expand All @@ -555,6 +557,7 @@ mod binding_form_impl {
match self {
Var(binding) => binding.hash_stable(hcx, hasher),
ImplicitSelf => (),
RefForGuard => (),
}
}
}
Expand Down
20 changes: 19 additions & 1 deletion src/librustc_mir/borrow_check/error_reporting.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -202,7 +202,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
/// the local assigned at `location`.
/// This is done by searching in statements succeeding `location`
/// and originating from `maybe_closure_span`.
fn find_closure_span(
pub(super) fn find_closure_span(
&self,
maybe_closure_span: Span,
location: Location,
Expand Down Expand Up @@ -742,6 +742,24 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
autoderef,
&including_downcast,
)?;
} else if let Place::Local(local) = proj.base {
if let Some(ClearCrossCrate::Set(BindingForm::RefForGuard))
= self.mir.local_decls[local].is_user_variable {
self.append_place_to_string(
&proj.base,
buf,
autoderef,
&including_downcast,
)?;
} else {
buf.push_str(&"*");
self.append_place_to_string(
&proj.base,
buf,
autoderef,
&including_downcast,
)?;
}
} else {
buf.push_str(&"*");
self.append_place_to_string(
Expand Down
229 changes: 20 additions & 209 deletions src/librustc_mir/borrow_check/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,7 @@ use rustc::hir::def_id::DefId;
use rustc::hir::map::definitions::DefPathData;
use rustc::infer::InferCtxt;
use rustc::lint::builtin::UNUSED_MUT;
use rustc::mir::{self, AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
use rustc::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
use rustc::mir::{ClearCrossCrate, Local, Location, Mir, Mutability, Operand, Place};
use rustc::mir::{Field, Projection, ProjectionElem, Rvalue, Statement, StatementKind};
use rustc::mir::{Terminator, TerminatorKind};
Expand All @@ -43,27 +43,27 @@ use dataflow::{do_dataflow, DebugFormatted};
use dataflow::{EverInitializedPlaces, MovingOutStatements};
use dataflow::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
use util::borrowck_errors::{BorrowckErrors, Origin};
use util::collect_writes::FindAssignments;
use util::suggest_ref_mut;

use self::borrow_set::{BorrowData, BorrowSet};
use self::flows::Flows;
use self::location::LocationTable;
use self::prefixes::PrefixSet;
use self::MutateMode::{JustWrite, WriteAndRead};
use self::mutability_errors::AccessKind;

use self::path_utils::*;

crate mod borrow_set;
mod error_reporting;
mod flows;
mod location;
mod move_errors;
mod mutability_errors;
mod path_utils;
crate mod place_ext;
mod places_conflict;
mod prefixes;
mod used_muts;
mod move_errors;

pub(crate) mod nll;

Expand Down Expand Up @@ -922,7 +922,13 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
}

let mutability_error =
self.check_access_permissions(place_span, rw, is_local_mutation_allowed, flow_state);
self.check_access_permissions(
place_span,
rw,
is_local_mutation_allowed,
flow_state,
context.loc,
);
let conflict_error =
self.check_access_for_conflict(context, place_span, sd, rw, flow_state);

Expand Down Expand Up @@ -1668,6 +1674,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
}
}


/// Check the permissions for the given place and read or write kind
///
/// Returns true if an error is reported, false otherwise.
Expand All @@ -1677,17 +1684,13 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
kind: ReadOrWrite,
is_local_mutation_allowed: LocalMutationIsAllowed,
flow_state: &Flows<'cx, 'gcx, 'tcx>,
location: Location,
) -> bool {
debug!(
"check_access_permissions({:?}, {:?}, {:?})",
place, kind, is_local_mutation_allowed
);

#[derive(Copy, Clone, Debug)]
enum AccessKind {
MutableBorrow,
Mutate,
}
let error_access;
let the_place_err;

Expand Down Expand Up @@ -1756,206 +1759,14 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
}

// at this point, we have set up the error reporting state.

let mut err;
let item_msg = match self.describe_place(place) {
Some(name) => format!("immutable item `{}`", name),
None => "immutable item".to_owned(),
};

// `act` and `acted_on` are strings that let us abstract over
// the verbs used in some diagnostic messages.
let act;
let acted_on;

match error_access {
AccessKind::Mutate => {
let item_msg = match the_place_err {
Place::Projection(box Projection {
base: _,
elem: ProjectionElem::Deref,
}) => match self.describe_place(place) {
Some(description) => {
format!("`{}` which is behind a `&` reference", description)
}
None => format!("data in a `&` reference"),
},
_ => item_msg,
};
err = self.tcx.cannot_assign(span, &item_msg, Origin::Mir);
act = "assign";
acted_on = "written";
}
AccessKind::MutableBorrow => {
err = self
.tcx
.cannot_borrow_path_as_mutable(span, &item_msg, Origin::Mir);
act = "borrow as mutable";
acted_on = "borrowed as mutable";
}
}

match the_place_err {
// We want to suggest users use `let mut` for local (user
// variable) mutations...
Place::Local(local) if self.mir.local_decls[*local].can_be_made_mutable() => {
// ... but it doesn't make sense to suggest it on
// variables that are `ref x`, `ref mut x`, `&self`,
// or `&mut self` (such variables are simply not
// mutable)..
let local_decl = &self.mir.local_decls[*local];
assert_eq!(local_decl.mutability, Mutability::Not);

err.span_label(span, format!("cannot {ACT}", ACT = act));
err.span_suggestion(
local_decl.source_info.span,
"consider changing this to be mutable",
format!("mut {}", local_decl.name.unwrap()),
);
}

// complete hack to approximate old AST-borrowck
// diagnostic: if the span starts with a mutable borrow of
// a local variable, then just suggest the user remove it.
Place::Local(_)
if {
if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(span) {
snippet.starts_with("&mut ")
} else {
false
}
} =>
{
err.span_label(span, format!("cannot {ACT}", ACT = act));
err.span_label(span, "try removing `&mut` here");
}

// We want to point out when a `&` can be readily replaced
// with an `&mut`.
//
// FIXME: can this case be generalized to work for an
// arbitrary base for the projection?
Place::Projection(box Projection {
base: Place::Local(local),
elem: ProjectionElem::Deref,
}) if self.mir.local_decls[*local].is_user_variable.is_some() => {
let local_decl = &self.mir.local_decls[*local];
let suggestion = match local_decl.is_user_variable.as_ref().unwrap() {
ClearCrossCrate::Set(mir::BindingForm::ImplicitSelf) => {
Some(suggest_ampmut_self(local_decl))
},

ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
binding_mode: ty::BindingMode::BindByValue(_),
opt_ty_info,
..
})) => Some(suggest_ampmut(
self.tcx,
self.mir,
*local,
local_decl,
*opt_ty_info,
)),

ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
binding_mode: ty::BindingMode::BindByReference(_),
..
})) => suggest_ref_mut(self.tcx, local_decl.source_info.span),

ClearCrossCrate::Clear => bug!("saw cleared local state"),
};

if let Some((err_help_span, suggested_code)) = suggestion {
err.span_suggestion(
err_help_span,
"consider changing this to be a mutable reference",
suggested_code,
);
}

if let Some(name) = local_decl.name {
err.span_label(
span,
format!(
"`{NAME}` is a `&` reference, \
so the data it refers to cannot be {ACTED_ON}",
NAME = name,
ACTED_ON = acted_on
),
);
} else {
err.span_label(
span,
format!("cannot {ACT} through `&`-reference", ACT = act),
);
}
}

_ => {
err.span_label(span, format!("cannot {ACT}", ACT = act));
}
}

err.emit();
self.report_mutability_error(
place,
span,
the_place_err,
error_access,
location,
);
return true;

fn suggest_ampmut_self<'cx, 'gcx, 'tcx>(
local_decl: &mir::LocalDecl<'tcx>,
) -> (Span, String) {
(local_decl.source_info.span, "&mut self".to_string())
}

// When we want to suggest a user change a local variable to be a `&mut`, there
// are three potential "obvious" things to highlight:
//
// let ident [: Type] [= RightHandSideExpression];
// ^^^^^ ^^^^ ^^^^^^^^^^^^^^^^^^^^^^^
// (1.) (2.) (3.)
//
// We can always fallback on highlighting the first. But chances are good that
// the user experience will be better if we highlight one of the others if possible;
// for example, if the RHS is present and the Type is not, then the type is going to
// be inferred *from* the RHS, which means we should highlight that (and suggest
// that they borrow the RHS mutably).
//
// This implementation attempts to emulate AST-borrowck prioritization
// by trying (3.), then (2.) and finally falling back on (1.).
fn suggest_ampmut<'cx, 'gcx, 'tcx>(
tcx: TyCtxt<'cx, 'gcx, 'tcx>,
mir: &Mir<'tcx>,
local: Local,
local_decl: &mir::LocalDecl<'tcx>,
opt_ty_info: Option<Span>,
) -> (Span, String) {
let locations = mir.find_assignments(local);
if locations.len() > 0 {
let assignment_rhs_span = mir.source_info(locations[0]).span;
let snippet = tcx.sess.codemap().span_to_snippet(assignment_rhs_span);
if let Ok(src) = snippet {
if src.starts_with('&') {
let borrowed_expr = src[1..].to_string();
return (
assignment_rhs_span,
format!("&mut {}", borrowed_expr),
);
}
}
}

let highlight_span = match opt_ty_info {
// if this is a variable binding with an explicit type,
// try to highlight that for the suggestion.
Some(ty_span) => ty_span,

// otherwise, just highlight the span associated with
// the (MIR) LocalDecl.
None => local_decl.source_info.span,
};

let ty_mut = local_decl.ty.builtin_deref(true).unwrap();
assert_eq!(ty_mut.mutbl, hir::MutImmutable);
(highlight_span, format!("&mut {}", ty_mut.ty))
}
}

/// Adds the place into the used mutable variables set
Expand Down
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