fix: reconcile the ERROR markers with what the checker reports

Checking the markers for the first time found five disagreements in areas that
are implemented. Four were the marker's fault:

- own/badarg pinned "self", but the rule being broken is that a returned
  reference must derive from a parameter -- `self` has nothing to do with it.
- own/badbrmov pinned line 9, which is the closing brace; the second destroy is
  on line 8.
- own/badloop pinned line 8, the destroy after the loop. The diagnostic is on
  line 6, inside it, and line 6 is right: the second iteration moves the same
  value again, so the loop body is where it is caught. Whoever wrote the marker
  expected the error after the loop.
- optional/badcatch pinned line 14, the body of the catch block. The catch
  expression on line 13 is what cannot fall through.

The fifth was the compiler's. own/badweak assigns a `&mut i32` to a `&i32` and
got "initializer type mismatch", which says nothing about why. Weakening an
exclusive borrow to a shared one is a specific rule and now says so, for
references and slices alike.

own/ is fully green: 50/50. Overall 133 -> 138 of 188. The six remaining marker
disagreements are all under units/ and generic/, where nothing is implemented
yet, so there is no diagnostic to compare against and no way to tell whether
the marker is right.

fec/src/check.c

@@ -2489,8 +2489,22 @@ static void m7_check_decl_stmt(FeCheckerState *s, FeNode *n, int mutable)
     if (expected && expected->kind==FE_TYPE_VOID)
         err(s->c,n->loc,"variable cannot have void type");
     if (n->b && !fe_type_equal(expected,stored) &&
-        !m7_actual_compatible(expected,stored,n->b))
-        err(s->c,n->loc,"initializer type mismatch");
+        !m7_actual_compatible(expected,stored,n->b)) {
+        /* Say which rule was hit. Weakening &mut to & is a distinct thing from
+           two unrelated types not matching, and "type mismatch" told the reader
+           nothing about why the exclusive borrow could not be shared. */
+        if (expected && stored && expected->kind==FE_TYPE_REF &&
+            stored->kind==FE_TYPE_REF && !expected->ref_mut && stored->ref_mut)
+            err(s->c,n->loc,
+                "cannot rebind a mut borrow as a shared reference");
+        else if (expected && stored && expected->kind==FE_TYPE_SLICE &&
+                 stored->kind==FE_TYPE_SLICE && !expected->ref_mut &&
+                 stored->ref_mut)
+            err(s->c,n->loc,
+                "cannot rebind a mut slice as a shared slice");
+        else
+            err(s->c,n->loc,"initializer type mismatch");
+    }
     /* Rules the M6 declaration case carried that this one has to repeat now
        that it is the only declaration case. */
     if (n->b && stored && stored->kind==FE_TYPE_VOID)

fec/tests/optional/badcatch.fe

@@ -1,4 +1,4 @@
-// ERROR:14:catch
+// ERROR:13:catch
 unit badcatch;
 
 error E {

fec/tests/own/badarg.fe

@@ -1,4 +1,4 @@
-// ERROR:8:self
+// ERROR:8:derived from a parameter
 unit badarg;
 
 struct Box {

fec/tests/own/badbrmov.fe

@@ -1,4 +1,4 @@
-// ERROR:9:move
+// ERROR:8:move
 unit badbrmov;
 
 fn bad(p: ^i32, consume: bool) -> void {

fec/tests/own/badloop.fe

@@ -1,4 +1,4 @@
-// ERROR:8:move
+// ERROR:6:move
 unit badloop;
 
 fn bad(p: ^i32, again: bool) -> void {