Edit's refusals name the pattern, not the line, and say when dot is empty

A ctl Edit that missed answered `Edit: no match for regexp "Edit
/nomatch/d"`, quoting the whole program; it now names the address that
missed (`no match for regexp /nomatch/`) and quotes no line after. An s
on an empty dot says `no substitution: dot is empty`. A \\1 is blamed
only once its pattern has matched, as before. tags.md says & is special
in s and not in c, a or i, as in sam.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

3 files changed, 20 insertions, 5 deletions

diff --git a/docs/tags.md b/docs/tags.md
index c1034cb4..bfca4f0e 100644
--- a/docs/tags.md
+++ b/docs/tags.md
@@ -62,7 +62,11 @@ if every command ran; an error says why in acme's words and changes
 nothing. `p` and `=` print to the directory's `+Errors`. Left out: the file
 commands `b B D e r w f X Y`, the pipes `< | >`, and `\1`-`\9` in `s`,
 since mvzr keeps no submatches; acme applies changes that come out of
-sequence with a warning, pardes refuses the Edit.
+sequence with a warning, pardes refuses the Edit. In `s`, `&` in the
+replacement is the matched text (`\&` a plain `&`); in `c`, `a` and `i` an
+`&` is only an `&`, as in sam. A pattern that finds nothing says so with the
+pattern (`no match for regexp /nomatch/`), and an `s` on an empty dot says
+`no substitution: dot is empty`.
 
 `Undo` and `Redo` are acme's: typed or clicked in a pane's tag, or written
 to its `ctl`, they step the body back and forward through its edits, as the
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index e1dd3fc5..2fdc47cc 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -332,7 +332,10 @@ pub fn runBuiltin(p: *Pardes, req: Req, id: usize, line: []const u8) ?Reply {
     if (!created) noteResult(p, context);
     if (p.fs.refused) return refuse(p, req, e_prompt, line);
     if (p.fs.failure_len == 0) return null;
-    const refusal = refuse(p, req, p.fs.failure[0..p.fs.failure_len], line);
+    // An Edit's failure names what failed (the pattern that missed): its
+    // line, a program, is not quoted after it.
+    const edit = std.mem.startsWith(u8, line, "Edit ") or std.mem.eql(u8, line, "Edit");
+    const refusal = if (edit) tree.failText(req.tag, E.IO, p.fs.failure[0..p.fs.failure_len]) else refuse(p, req, p.fs.failure[0..p.fs.failure_len], line);
     return .{ .tag = req.tag, .status = .err, .errno = E.IO, .ename = refusal.ename };
 }
 
diff --git a/src/sam_edit.zig b/src/sam_edit.zig
index 679823da..29dec726 100644
--- a/src/sam_edit.zig
+++ b/src/sam_edit.zig
@@ -390,7 +390,11 @@ const Exec = struct {
     fn address(ex: *Exec, expr: ?[]const u8, dot: Range) Failure!Range {
         const e = expr orelse return dot;
         var a: addr_lang.Addr = .{ .text = ex.text, .lim = null, .expr = e };
-        const r = a.address(dot) orelse return fail(ex.why, "{s}", .{a.err});
+        // A miss names the pattern that missed, not the whole Edit line.
+        const r = a.address(dot) orelse return if (std.mem.eql(u8, a.err, addr_lang.e_no_match))
+            fail(ex.why, "no match for regexp {s}", .{e})
+        else
+            fail(ex.why, "{s}", .{a.err});
         if (a.i < e.len) return fail(ex.why, "bad address syntax", .{});
         if (r.q0 > r.q1) return fail(ex.why, "addresses out of order", .{});
         return r;
@@ -537,7 +541,10 @@ const Exec = struct {
             did = true;
             if (!c.global) break;
         }
-        if (!did and ex.nest == 0) return fail(ex.why, "no substitution", .{});
+        if (!did and ex.nest == 0) return if (r.q0 == r.q1)
+            fail(ex.why, "no substitution: dot is empty", .{})
+        else
+            fail(ex.why, "no substitution", .{});
         return r;
     }
 
@@ -679,8 +686,9 @@ test "an Edit that fails halfway changes nothing, and says why in acme's words"
     var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
     defer arena_state.deinit();
     for ([_][2][]const u8{
-        .{ ",x/foo/c/bar/\n/nomatch/d", "no match for regexp" },
+        .{ ",x/foo/c/bar/\n/nomatch/d", "no match for regexp /nomatch/" },
         .{ ",s/zzz/y/", "no substitution" },
+        .{ "s/foo/y/", "no substitution: dot is empty" },
         .{ "k", "unknown command k" },
         .{ ",x/a/{\na/1/\ni/2/\n}", "changes out of sequence" },
         .{ "}", "right brace with no left brace" },