use ptr_null where appropriate

src/machine.rs

@@ -130,7 +130,7 @@ impl MemoryExtra {
                 // This should be all-zero, pointer-sized.
                 let layout = this.layout_of(this.tcx.types.usize)?;
                 let place = this.allocate(layout, MiriMemoryKind::Machine.into());
-                this.write_scalar(Scalar::from_machine_usize(0, &*this.tcx), place.into())?;
+                this.write_scalar(Scalar::from_machine_usize(0, this), place.into())?;
                 Self::add_extern_static(this, "__cxa_thread_atexit_impl", place.ptr);
                 // "environ"
                 Self::add_extern_static(this, "environ", this.machine.env_vars.environ.unwrap().ptr);

src/shims/env.rs

@@ -337,14 +337,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             // This is memory backing an extern static, hence `Machine`, not `Env`.
             let layout = this.layout_of(this.tcx.types.usize)?;
             let place = this.allocate(layout, MiriMemoryKind::Machine.into());
-            this.write_scalar(Scalar::from_machine_usize(0, &*this.tcx), place.into())?;
             this.machine.env_vars.environ = Some(place);
         }
 
         // Collect all the pointers to each variable in a vector.
         let mut vars: Vec<Scalar<Tag>> = this.machine.env_vars.map.values().map(|&ptr| ptr.into()).collect();
         // Add the trailing null pointer.
-        vars.push(Scalar::from_machine_usize(0, this));
+        vars.push(Scalar::ptr_null(this));
         // Make an array with all these pointers inside Miri.
         let tcx = this.tcx;
         let vars_layout =

src/shims/fs.rs

@@ -860,7 +860,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             }
             Err(e) => {
                 this.set_last_error_from_io_error(e)?;
-                Ok(Scalar::from_machine_usize(0, this))
+                Ok(Scalar::ptr_null(this))
             }
         }
     }

src/shims/intrinsics.rs

@@ -2,7 +2,6 @@ use std::iter;
 use std::convert::TryFrom;
 
 use rustc::mir;
-use rustc::mir::interpret::{InterpResult, PointerArithmetic};
 use rustc::ty;
 use rustc::ty::layout::{Align, LayoutOf};
 use rustc_apfloat::Float;