glsl: fail when a shader's input var has not an equivalent out var in previous

GLSL ES 3.00 spec, 4.3.10 (Linking of Vertex Outputs and Fragment Inputs),
page 45 says the following:

"The type of vertex outputs and fragment input with the same name must match,
otherwise the link command will fail. The precision does not need to match.
Only those fragment inputs statically used (i.e. read) in the fragment shader
must be declared as outputs in the vertex shader; declaring superfluous vertex
shader outputs is permissible."
[...]
"The term static use means that after preprocessing the shader includes at
least one statement that accesses the input or output, even if that statement
is never actually executed."

And it includes a table with all the possibilities.

Similar table or content is present in other GLSL specs: GLSL 4.40, GLSL 1.50,
etc but for more stages (vertex and geometry shaders, etc).

This patch detects that case and returns a link error. It fixes the following
dEQP test:

  dEQP-GLES3.functional.shaders.linkage.varying.rules.illegal_usage_1

However, it adds a new regression in piglit because the test hasn't a
vertex shader and it checks the link status.

bin/glslparsertest \
tests/spec/glsl-1.50/compiler/gs-also-uses-smooth-flat-noperspective.geom pass \
1.50 --check-link

This piglit test is wrong according to the spec wording above, so if this patch
is merged it should be updated.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>

src/glsl/link_varyings.cpp

@@ -263,6 +263,19 @@ cross_validate_outputs_to_inputs(struct gl_shader_program *prog,
          if (output != NULL) {
             cross_validate_types_and_qualifiers(prog, input, output,
                                                 consumer->Stage, producer->Stage);
+         } else {
+            /* Check for input vars with unmatched output vars in prev stage
+             * taking into account that interface blocks could have a matching
+             * output but with different name, so we ignore them.
+             */
+            assert(!input->data.assigned);
+            if (input->data.used && !input->get_interface_type() &&
+                !input->data.explicit_location)
+               linker_error(prog,
+                            "%s shader input `%s' "
+                            "has no matching output in the previous stage\n",
+                            _mesa_shader_stage_to_string(consumer->Stage),
+                            input->name);
          }
       }
    }