librustc: Disallow mutation and assignment in pattern guards, and modify

the CFG for match statements.

There were two bugs in issue #14684. One was simply that the borrow
check didn't know about the correct CFG for match statements: the
pattern must be a predecessor of the guard. This disallows the bad
behavior if there are bindings in the pattern. But it isn't enough to
prevent the memory safety problem, because of wildcards; thus, this
patch introduces a more restrictive rule, which disallows assignments
and mutable borrows inside guards outright.

I discussed this with Niko and we decided this was the best plan of
action.

This breaks code that performs mutable borrows in pattern guards. Most
commonly, the code looks like this:

    impl Foo {
        fn f(&mut self, ...) {}
        fn g(&mut self, ...) {
            match bar {
                Baz if self.f(...) => { ... }
                _ => { ... }
            }
        }
    }

Change this code to not use a guard. For example:

    impl Foo {
        fn f(&mut self, ...) {}
        fn g(&mut self, ...) {
            match bar {
                Baz => {
                    if self.f(...) {
                        ...
                    } else {
                        ...
                    }
                }
                _ => { ... }
            }
        }
    }

Sometimes this can result in code duplication, but often it illustrates
a hidden memory safety problem.

Closes #14684.

[breaking-change]

src/libfmt_macros/lib.rs

@@ -352,8 +352,13 @@ impl<'a> Parser<'a> {
             None => {
                 let tmp = self.cur.clone();
                 match self.word() {
-                    word if word.len() > 0 && self.consume('$') => {
-                        CountIsName(word)
+                    word if word.len() > 0 => {
+                        if self.consume('$') {
+                            CountIsName(word)
+                        } else {
+                            self.cur = tmp;
+                            CountImplied
+                        }
                     }
                     _ => {
                         self.cur = tmp;

src/librustc/middle/cfg/construct.rs

@@ -333,32 +333,37 @@ impl<'a> CFGBuilder<'a> {
                 //    [discr]
                 //       |
                 //       v 2
-                //    [guard1]
+                //    [cond1]
                 //      /  \
                 //     |    \
-                //     v 3  |
-                //  [pat1]  |
-                //     |
-                //     v 4  |
-                // [body1]  v
-                //     |  [guard2]
-                //     |    /   \
-                //     | [body2] \
-                //     |    |   ...
-                //     |    |    |
-                //     v 5  v    v
-                //   [....expr....]
+                //     v 3   \
+                //  [pat1]    \
+                //     |       |
+                //     v 4     |
+                //  [guard1]   |
+                //     |       |
+                //     |       |
+                //     v 5     v
+                //  [body1]  [cond2]
+                //     |      /  \
+                //     |    ...  ...
+                //     |     |    |
+                //     v 6   v    v
+                //  [.....expr.....]
                 //
                 let discr_exit = self.expr(discr.clone(), pred);         // 1
 
                 let expr_exit = self.add_node(expr.id, []);
-                let mut guard_exit = discr_exit;
+                let mut cond_exit = discr_exit;
                 for arm in arms.iter() {
-                    guard_exit = self.opt_expr(arm.guard, guard_exit);   // 2
+                    cond_exit = self.add_dummy_node([cond_exit]);        // 2
                     let pats_exit = self.pats_any(arm.pats.as_slice(),
-                                                  guard_exit);           // 3
-                    let body_exit = self.expr(arm.body.clone(), pats_exit); // 4
-                    self.add_contained_edge(body_exit, expr_exit);       // 5
+                                                  cond_exit);            // 3
+                    let guard_exit = self.opt_expr(arm.guard,
+                                                   pats_exit);           // 4
+                    let body_exit = self.expr(arm.body.clone(),
+                                              guard_exit);               // 5
+                    self.add_contained_edge(body_exit, expr_exit);       // 6
                 }
                 expr_exit
             }

src/librustc/middle/check_match.rs

@@ -11,6 +11,10 @@
 use middle::const_eval::{compare_const_vals, const_bool, const_float, const_nil, const_val};
 use middle::const_eval::{const_expr_to_pat, eval_const_expr, lookup_const_by_id};
 use middle::def::*;
+use middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, Init};
+use middle::expr_use_visitor::{JustWrite, LoanCause, MutateMode};
+use middle::expr_use_visitor::{WriteAndRead};
+use middle::mem_categorization::cmt;
 use middle::pat_util::*;
 use middle::ty::*;
 use middle::ty;
@@ -143,7 +147,16 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
                                                 arm.pats.as_slice());
             }
 
-            // Second, check for unreachable arms.
+            // Second, if there is a guard on each arm, make sure it isn't
+            // assigning or borrowing anything mutably.
+            for arm in arms.iter() {
+                match arm.guard {
+                    Some(guard) => check_for_mutation_in_guard(cx, &*guard),
+                    None => {}
+                }
+            }
+
+            // Third, check for unreachable arms.
             check_arms(cx, arms.as_slice());
 
             // Finally, check if the whole match expression is exhaustive.
@@ -903,3 +916,53 @@ fn check_legality_of_move_bindings(cx: &MatchCheckCtxt,
         });
     }
 }
+
+/// Ensures that a pattern guard doesn't borrow by mutable reference or
+/// assign.
+fn check_for_mutation_in_guard<'a>(cx: &'a MatchCheckCtxt<'a>, guard: &Expr) {
+    let mut checker = MutationChecker {
+        cx: cx,
+    };
+    let mut visitor = ExprUseVisitor::new(&mut checker, checker.cx.tcx);
+    visitor.walk_expr(guard);
+}
+
+struct MutationChecker<'a> {
+    cx: &'a MatchCheckCtxt<'a>,
+}
+
+impl<'a> Delegate for MutationChecker<'a> {
+    fn consume(&mut self, _: NodeId, _: Span, _: cmt, _: ConsumeMode) {}
+    fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
+    fn borrow(&mut self,
+              _: NodeId,
+              span: Span,
+              _: cmt,
+              _: Region,
+              kind: BorrowKind,
+              _: LoanCause) {
+        match kind {
+            MutBorrow => {
+                self.cx
+                    .tcx
+                    .sess
+                    .span_err(span,
+                              "cannot mutably borrow in a pattern guard")
+            }
+            ImmBorrow | UniqueImmBorrow => {}
+        }
+    }
+    fn decl_without_init(&mut self, _: NodeId, _: Span) {}
+    fn mutate(&mut self, _: NodeId, span: Span, _: cmt, mode: MutateMode) {
+        match mode {
+            JustWrite | WriteAndRead => {
+                self.cx
+                    .tcx
+                    .sess
+                    .span_err(span, "cannot assign in a pattern guard")
+            }
+            Init => {}
+        }
+    }
+}
+

src/librustc/middle/expr_use_visitor.rs

@@ -292,7 +292,7 @@ impl<'d,'t,TYPER:mc::Typer> ExprUseVisitor<'d,'t,TYPER> {
         self.walk_expr(expr)
     }
 
-    fn walk_expr(&mut self, expr: &ast::Expr) {
+    pub fn walk_expr(&mut self, expr: &ast::Expr) {
         debug!("walk_expr(expr={})", expr.repr(self.tcx()));
 
         self.walk_adjustment(expr);

src/librustdoc/html/render.rs

@@ -1730,17 +1730,19 @@ fn item_struct(w: &mut fmt::Formatter, it: &clean::Item,
         }
     }).peekable();
     match s.struct_type {
-        doctree::Plain if fields.peek().is_some() => {
-            try!(write!(w, "<h2 class='fields'>Fields</h2>\n<table>"));
-            for field in fields {
-                try!(write!(w, "<tr><td id='structfield.{name}'>\
-                                  {stab}<code>{name}</code></td><td>",
-                              stab = ConciseStability(&field.stability),
-                              name = field.name.get_ref().as_slice()));
-                try!(document(w, field));
-                try!(write!(w, "</td></tr>"));
+        doctree::Plain => {
+            if fields.peek().is_some() {
+                try!(write!(w, "<h2 class='fields'>Fields</h2>\n<table>"));
+                for field in fields {
+                    try!(write!(w, "<tr><td id='structfield.{name}'>\
+                                      {stab}<code>{name}</code></td><td>",
+                                  stab = ConciseStability(&field.stability),
+                                  name = field.name.get_ref().as_slice()));
+                    try!(document(w, field));
+                    try!(write!(w, "</td></tr>"));
+                }
+                try!(write!(w, "</table>"));
             }
-            try!(write!(w, "</table>"));
         }
         _ => {}
     }