Make partially assignment of uninitialized projection produce an error

Author: spastorino

src/librustc/mir/mod.rs

@@ -1978,6 +1978,16 @@ impl<'tcx> Place<'tcx> {
             _ => None,
         }
     }
+
+    /// Returns the local variable at the "base" of this place, if any. e.g., for a place like
+    /// a.b.c, returns Some(a). For a place based in a static or constant, returns None.
+    pub fn base_local(&self) -> Option<Local> {
+        match self {
+            Place::Projection(box Projection { base, .. }) => base.base_local(),
+            Place::Local(local) => Some(*local),
+            Place::Static(..) | Place::Promoted(..) => None,
+        }
+    }
 }
 
 impl<'tcx> Debug for Place<'tcx> {

src/librustc_mir/borrow_check/mod.rs

@@ -1096,20 +1096,63 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             }
         }
 
-        // Special case: you can assign a immutable local variable
-        // (e.g., `x = ...`) so long as it has never been initialized
-        // before (at this point in the flow).
-        if let &Place::Local(local) = place_span.0 {
-            if let Mutability::Not = self.mir.local_decls[local].mutability {
-                // check for reassignments to immutable local variables
-                self.check_if_reassignment_to_immutable_state(
-                    context,
-                    local,
-                    place_span,
-                    flow_state,
-                );
-                return;
+        match place_span.0 {
+            // Special case: you can assign a immutable local variable
+            // (e.g., `x = ...`) so long as it has never been initialized
+            // before (at this point in the flow).
+            &Place::Local(local) =>  {
+                if let Mutability::Not = self.mir.local_decls[local].mutability {
+                    // check for reassignments to immutable local variables
+                    self.check_if_reassignment_to_immutable_state(
+                        context,
+                        local,
+                        place_span,
+                        flow_state,
+                        );
+                    return;
+                }
             }
+
+            // FIXME(#21232). For now, error if assigning to a projection from an uninitialized
+            // local variable -- so e.g. doing a.b = 22 when a is not yet initialized is simply an
+            // error. In the future, we could sometimes support this.
+            place @ &Place::Projection(_) => {
+                if let Some(local) = place.base_local() {
+                    let mpi = self.move_data.rev_lookup.find_local(local);
+                    if flow_state.uninits.contains(mpi) {
+                        let name = self.mir.local_decls[local].name.unwrap();
+                        let msg = match self.describe_place(place) {
+                            Some(desc) => format!(
+                                "cannot assign to `{}` when `{}` is not initialized",
+                                desc,
+                                name,
+                            ),
+
+                            None => format!(
+                                "use of uninitialized variable `{}",
+                                name,
+                            ),
+                        };
+
+                        let mut diag =
+                            struct_span_err!(self.infcx.tcx.sess, place_span.1, E0718, "{}", msg);
+
+                        if let Mutability::Mut = self.mir.local_decls[local].mutability {
+                            // Pre-emptively insert local into the used_mut set to avoid any
+                            // warnings related to whether the mut declaration is used.
+                            self.used_mut.insert(local);
+                        } else {
+                            diag.span_label(place_span.1, "will also have to be declared `mut`");
+                        }
+
+                        diag.buffer(&mut self.errors_buffer);
+
+                        return;
+                    }
+                }
+            }
+
+            _ => {}
         }
 
         // Otherwise, use the normal access permission rules.

src/librustc_mir/diagnostics.rs

@@ -2344,4 +2344,5 @@ register_diagnostics! {
     E0594, // cannot assign to {}
     E0598, // lifetime of {} is too short to guarantee its contents can be...
     E0625, // thread-local statics cannot be accessed at compile-time
+    E0718,
 }

src/test/ui/borrowck/borrowck-issue-48962.stderr

@@ -1,3 +1,9 @@
+error[E0718]: cannot assign to `src.next` when `src` is not initialized
+  --> $DIR/borrowck-issue-48962.rs:26:5
+   |
+LL |     src.next = None; //~ ERROR use of moved value: `src` [E0382]
+   |     ^^^^^^^^
+
 error[E0382]: use of moved value: `src`
   --> $DIR/borrowck-issue-48962.rs:26:5
    |
@@ -8,6 +14,12 @@ LL |     src.next = None; //~ ERROR use of moved value: `src` [E0382]
    |
    = note: move occurs because `src` has type `&mut Node`, which does not implement the `Copy` trait
 
+error[E0718]: cannot assign to `src.0` when `src` is not initialized
+  --> $DIR/borrowck-issue-48962.rs:32:5
+   |
+LL |     src.0 = 66; //~ ERROR use of moved value: `src` [E0382]
+   |     ^^^^^^^^^^
+
 error[E0382]: use of moved value: `src`
   --> $DIR/borrowck-issue-48962.rs:32:5
    |
@@ -18,6 +30,7 @@ LL |     src.0 = 66; //~ ERROR use of moved value: `src` [E0382]
    |
    = note: move occurs because `src` has type `&mut (i32, i32)`, which does not implement the `Copy` trait
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0382`.
+Some errors occurred: E0382, E0718.
+For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-uninit-ref-chain.mir.stderr

@@ -16,18 +16,43 @@ error[E0381]: borrow of possibly uninitialized variable: `**x`
 LL |     let _y = &**x; //[ast]~ ERROR use of possibly uninitialized variable: `**x` [E0381]
    |              ^^^^ use of possibly uninitialized `**x`
 
+error[E0718]: cannot assign to `a.x` when `a` is not initialized
+  --> $DIR/borrowck-uninit-ref-chain.rs:34:5
+   |
+LL |     a.x = 0;
+   |     ^^^^^^^
+
+error[E0718]: cannot assign to `a.x` when `a` is not initialized
+  --> $DIR/borrowck-uninit-ref-chain.rs:39:5
+   |
+LL |     a.x = &&0;
+   |     ^^^^^^^^^
+
+error[E0718]: cannot assign to `a.x` when `a` is not initialized
+  --> $DIR/borrowck-uninit-ref-chain.rs:45:5
+   |
+LL |     a.x = 0;
+   |     ^^^^^^^
+
 error[E0381]: borrow of possibly uninitialized variable: `a.y`
   --> $DIR/borrowck-uninit-ref-chain.rs:46:14
    |
 LL |     let _b = &a.y; //[ast]~ ERROR use of possibly uninitialized variable: `a.y` [E0381]
    |              ^^^^ use of possibly uninitialized `a.y`
 
+error[E0718]: cannot assign to `a.x` when `a` is not initialized
+  --> $DIR/borrowck-uninit-ref-chain.rs:50:5
+   |
+LL |     a.x = &&0;
+   |     ^^^^^^^^^
+
 error[E0381]: borrow of possibly uninitialized variable: `**a.y`
   --> $DIR/borrowck-uninit-ref-chain.rs:51:14
    |
 LL |     let _b = &**a.y; //[ast]~ ERROR use of possibly uninitialized variable: `**a.y` [E0381]
    |              ^^^^^^ use of possibly uninitialized `**a.y`
 
-error: aborting due to 5 previous errors
+error: aborting due to 9 previous errors
 
-For more information about this error, try `rustc --explain E0381`.
+Some errors occurred: E0381, E0718.
+For more information about an error, try `rustc --explain E0381`.

src/test/ui/borrowck/borrowck-use-in-index-lvalue.mir.stderr

@@ -4,12 +4,25 @@ error[E0381]: use of possibly uninitialized variable: `*w`
 LL |     w[5] = 0; //[ast]~ ERROR use of possibly uninitialized variable: `*w` [E0381]
    |     ^^^^ use of possibly uninitialized `*w`
 
+error[E0718]: cannot assign to `w[..]` when `w` is not initialized
+  --> $DIR/borrowck-use-in-index-lvalue.rs:16:5
+   |
+LL |     w[5] = 0; //[ast]~ ERROR use of possibly uninitialized variable: `*w` [E0381]
+   |     ^^^^^^^^ will also have to be declared `mut`
+
 error[E0381]: use of possibly uninitialized variable: `*w`
   --> $DIR/borrowck-use-in-index-lvalue.rs:20:5
    |
 LL |     w[5] = 0; //[ast]~ ERROR use of possibly uninitialized variable: `*w` [E0381]
    |     ^^^^ use of possibly uninitialized `*w`
 
-error: aborting due to 2 previous errors
+error[E0718]: cannot assign to `w[..]` when `w` is not initialized
+  --> $DIR/borrowck-use-in-index-lvalue.rs:20:5
+   |
+LL |     w[5] = 0; //[ast]~ ERROR use of possibly uninitialized variable: `*w` [E0381]
+   |     ^^^^^^^^
+
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0381`.
+Some errors occurred: E0381, E0718.
+For more information about an error, try `rustc --explain E0381`.

src/test/ui/nll/issue-54499.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+
+fn foo() {
+    let mut s: (i32, i32);
+    s.0 = 1;
+    s.1 = 2;
+    println!("{} {}", s.0, s.1);
+}
+
+fn bar() {
+    let s: (i32, i32);
+    s.0 = 3;
+    s.1 = 4;
+    println!("{} {}", s.0, s.1);
+}
+
+fn main() {}

src/test/ui/nll/issue-54499.stderr

@@ -0,0 +1,27 @@
+error[E0718]: cannot assign to `s.0` when `s` is not initialized
+  --> $DIR/issue-54499.rs:15:5
+   |
+LL |     s.0 = 1;
+   |     ^^^^^^^
+
+error[E0718]: cannot assign to `s.1` when `s` is not initialized
+  --> $DIR/issue-54499.rs:16:5
+   |
+LL |     s.1 = 2;
+   |     ^^^^^^^
+
+error[E0718]: cannot assign to `s.0` when `s` is not initialized
+  --> $DIR/issue-54499.rs:22:5
+   |
+LL |     s.0 = 3;
+   |     ^^^^^^^ will also have to be declared `mut`
+
+error[E0718]: cannot assign to `s.1` when `s` is not initialized
+  --> $DIR/issue-54499.rs:23:5
+   |
+LL |     s.1 = 4;
+   |     ^^^^^^^ will also have to be declared `mut`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0718`.