Nicer error when list/map assigned to string argument

[apparentlymart]

For example:

resource "aws_route_table_association" "foo" {
    route_table_id = "baz"
    subnet_id = ["a", "b"] // this is supposed to be just a string
}

Previous this would return the following sort of error:

expected type 'string', got unconvertible type '[]interface {}'

This is the raw error returned by the underlying mapstructure library. This is not a helpful error message for anyone who doesn't know Go's type system, and it exposes Terraform's internals to the UI.

Instead, catch these cases before we try to use mapstructure and return a more straightforward message.

This also fixes a crash caused when a computed list is assigned to a string attribute. Previously this was not caught at all due to the short-circuit for computed attributes, but now we check for lists and maps before we check for computed, and so we can fail in a nicer way in this case rather than crashing during the later Diff operation. This is safe because a computed list is materially different than a computed primitive: it's a []interface{} with some of the values being interpolated strings, instead of just a single interpolated string.

This issue was exposed by #2984.

[jszwedko]

👏

[josephholsten]

💃 @phinze, @catsby, @radeksimko how does this feel to your 👀?

[catsby]

This fixes #3577

[apparentlymart]

Given how long this has been sitting here, how simple this is, that there are now three separate bug reports for this, and that @catsby apparently tested it, I went ahead and merged this.

[catsby]

👍

[ghost]

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