docs: fix crashes on searching documentation of various expressions (#59742)

Fix a crash when calling `@doc` on non-document-able expressions so that
docs (or lack thereof) can be successfully printed. For example, looking
up docs on sad faces, doesn't crash and cause sad faces after this
change:

```
help?> :()
  Expr(head::Symbol, args...)
```

Details: expands `astname` to handle valid expression types explicitly,
removes the unused, undocumented `@var` export from Base.Docs, and
updates the error message to be clearer.

🤖 Generated with some assistance from Claude Code.

Author: vtjnash

base/docs/Docs.jl

@@ -295,16 +295,29 @@ catdoc(xs...) = vcat(xs...)
 
 const keywords = Dict{Symbol, DocStr}()
 
-namify(@nospecialize x) = astname(x, isexpr(x, :macro))::Union{Symbol,Expr,GlobalRef}
+namify(@nospecialize x) = astname(x, isexpr(x, :macro))
 
 function astname(x::Expr, ismacro::Bool)
     head = x.head
     if head === :.
         ismacro ? macroname(x) : x
-    elseif head === :call && isexpr(x.args[1], :(::))
-        return astname((x.args[1]::Expr).args[end], ismacro)
+elseif head === :call && length(x.args) >= 1 && isexpr(x.args[1], :(::))
+        # for documenting (x::y)(args...), extract the name from y
+        # otherwise, for documenting `x::y`, it will be extracted from x
+        astname((x.args[1]::Expr).args[end], ismacro)
     else
-        n = isexpr(x, (:module, :struct)) ? 2 : 1
+        n = if isexpr(x, (:module, :struct))
+            2
+        elseif isexpr(x, (:call, :macrocall, :function, :(=), :macro, :where, :curly,
+                          :(::), :(<:), :(>:), :local, :global, :const, :atomic,
+                          :copyast, :quote, :inert, :primitive, :abstract,
+                          :escape, :var"hygienic-scope"))
+            # similar to is_function_def, but without -> and with various assignments, quoted statements, and miscellaneous that might be encountered in struct definitions also
+            1
+        else
+            return x # nothing to see here--bindingexpr will convert this to an error if defining a doc
+        end
+        length(x.args) < n && return x
         astname(x.args[n], ismacro)
     end
 end
@@ -356,7 +369,7 @@ function metadata(__source__, __module__, expr, ismodule)
             if isa(eachex, Symbol) || isexpr(eachex, :(::))
                 # a field declaration
                 if last_docstr !== nothing
-                    push!(fields, P(namify(eachex::Union{Symbol,Expr}), last_docstr))
+                    push!(fields, P(namify(eachex), last_docstr))
                     last_docstr = nothing
                 end
             elseif isexpr(eachex, :function) || isexpr(eachex, :(=))
@@ -610,7 +623,13 @@ function simple_lookup_doc(ex)
     elseif !isa(ex, Expr) && !isa(ex, Symbol)
         return :($(_doc)($(typeof)($(esc(ex)))))
     end
-    binding = esc(bindingexpr(namify(ex)))
+    name = namify(ex)
+    # If namify couldn't extract a meaningful name and returned an Expr
+    # that can't be converted to a binding, treat it like a value
+    if isa(name, Expr) && !isexpr(name, :(.))
+        return :($(_doc)($(typeof)($(esc(ex)))))
+    end
+    binding = esc(bindingexpr(name))
     if isexpr(ex, :call) || isexpr(ex, :macrocall) || isexpr(ex, :where)
         sig = esc(signature(ex))
         :($(_doc)($binding, $sig))

base/docs/bindings.jl

@@ -1,7 +1,5 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-export @var
-
 struct Binding
     mod::Module
     var::Symbol
@@ -20,17 +18,11 @@ defined(b::Binding) = invokelatest(isdefinedglobal, b.mod, b.var)
 resolve(b::Binding) = invokelatest(getglobal, b.mod, b.var)
 
 function splitexpr(x::Expr)
-    isexpr(x, :macrocall) ? splitexpr(x.args[1]) :
-    isexpr(x, :.)         ? (x.args[1], x.args[2]) :
-    error("Invalid @var syntax `$x`.")
+    isexpr(x, :.) ? (x.args[1], x.args[2]) : error("Could not find something to document in `$x`.")
 end
-splitexpr(s::Symbol) = Expr(:macrocall, getfield(Base, Symbol("@__MODULE__")), nothing), quot(s)
+splitexpr(s::Symbol) = :($Base.@__MODULE__), quot(s) # this somewhat complex form allows deferring resolving the Module for module docstring until after the module is created
 splitexpr(r::GlobalRef) = r.mod, quot(r.name)
-splitexpr(other)     = error("Invalid @var syntax `$other`.")
-
-macro var(x)
-    esc(bindingexpr(x))
-end
+splitexpr(other)     = error("Could not find something to document in `$other`.")
 
 function Base.show(io::IO, b::Binding)
     if b.mod === Base.active_module()

stdlib/REPL/src/docview.jl

@@ -256,7 +256,7 @@ doc(obj::UnionAll) = doc(Base.unwrap_unionall(obj))
 doc(object, sig::Type = Union{}) = doc(aliasof(object, typeof(object)), sig)
 doc(object, sig...)              = doc(object, Tuple{sig...})
 
-function lookup_doc(ex)
+function lookup_doc(@nospecialize(ex))
     if isa(ex, Expr) && ex.head !== :(.) && Base.isoperator(ex.head)
         # handle syntactic operators, e.g. +=, ::, .=
         ex = ex.head
@@ -284,7 +284,13 @@ function lookup_doc(ex)
             end
         end
     end
-    binding = esc(bindingexpr(namify(ex)))
+    name = namify(ex)
+    # If namify couldn't extract a meaningful name and returned an Expr
+    # that can't be converted to a binding, treat it like a value
+    if isa(name, Expr) && !isexpr(name, :(.))
+        return :($(doc)($(typeof)($(esc(ex)))))
+    end
+    binding = esc(bindingexpr(name))
     if isexpr(ex, :call) || isexpr(ex, :macrocall) || isexpr(ex, :where)
         sig = esc(signature(ex))
         :($(doc)($binding, $sig))
@@ -579,7 +585,6 @@ isregex(x) = isexpr(x, :macrocall, 3) && x.args[1] === Symbol("@r_str") && !isem
 repl(io::IO, ex::Expr; brief::Bool=true, mod::Module=Main, internal_accesses::Union{Nothing, Set{Pair{Module,Symbol}}}=nothing) = isregex(ex) ? :(apropos($io, $ex)) : _repl(ex, brief, mod, internal_accesses)
 repl(io::IO, str::AbstractString; brief::Bool=true, mod::Module=Main, internal_accesses::Union{Nothing, Set{Pair{Module,Symbol}}}=nothing) = :(apropos($io, $str))
 repl(io::IO, other; brief::Bool=true, mod::Module=Main, internal_accesses::Union{Nothing, Set{Pair{Module,Symbol}}}=nothing) = esc(:(@doc $other)) # TODO: track internal_accesses
-#repl(io::IO, other) = lookup_doc(other) # TODO
 
 repl(x; brief::Bool=true, mod::Module=Main) = repl(stdout, x; brief, mod)
 

test/docs.jl

@@ -1,6 +1,10 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-import Base.Docs: meta, @var, DocStr, parsedoc
+import Base.Docs: meta, DocStr, parsedoc, bindingexpr, namify
+
+macro var(x) # just for testing bindingexpr/nameify more conveniently
+    esc(bindingexpr(namify(x)))
+end
 
 # check that @doc can work before REPL is loaded
 @test !startswith(read(`$(Base.julia_cmd()) -E '@doc sin'`, String), "nothing")
@@ -58,6 +62,8 @@ macro macro_doctest() end
 @test (@eval @doc $(Meta.parse("``"))) == (@doc @cmd)
 @test (@eval @doc $(Meta.parse("123456789012345678901234567890"))) == (@doc @int128_str)
 @test (@eval @doc $(Meta.parse("1234567890123456789012345678901234567890"))) == (@doc @big_str)
+# Test that @doc doesn't crash on empty tuple expression (issue #XXXXX)
+@test (@doc :()) == (@doc Expr)
 
 # test that random stuff interpolated into docstrings doesn't break search or other methods here
 @doc doc"""
@@ -1239,7 +1245,7 @@ end
 
 # Bindings.
 
-import Base.Docs: @var, Binding, defined
+import Base.Docs: Binding, defined
 
 let x = Binding(Base, Symbol("@inline"))
     @test defined(x) == true
@@ -1565,8 +1571,7 @@ Base.@ccallable c51586_long()::Int = 3
 
 @testset "Docs docstrings" begin
     undoc = Docs.undocumented_names(Docs)
-    @test_broken isempty(undoc)
-    @test undoc == [Symbol("@var")]
+    @test isempty(undoc)
 end
 
 # Docing the macroception macro