*`terraform plan` and `terraform apply` will now warn when the `-target` option is used, to draw attention to the fact that the result of applying the plan is likely to be incomplete, and to remind to re-run `terraform plan` with no targets afterwards to ensure that the configuration has converged. [GH-22783]
* core: `ignore_changes` is now processed (in addition to existing behaviors) before the provider plan is run. This means that users may see fewer planned changes when using `ignore_changes`, as before this change, changes to ignored attributes were still being sent to CustomizeDiff in providers (which could mean cascading changes for some resources). This should be indicative that providers are no longer getting changes that were marked as ignored, but if unexpected plans are seen while using `ignore_changes`, investigate the settings in the `ignore_changes` block to ensure the appropriate attributes are set. ([#22520](https://github.com/hashicorp/terraform/issues/22520))
* provisioners/habitat: `accept_license` argument available to automate accepting the EULA, now required by this client ([#22745](https://github.com/hashicorp/terraform/issues/22745))
* config: add source addressing to unknown value errors in `for_each` ([#22760](https://github.com/hashicorp/terraform/issues/22760))
* command/console: support -var and -var-file flags ([#22145](https://github.com/hashicorp/terraform/issues/22145))
* command/show: Fixed bug with wrong errors being returned or swallowed. ([#22772](https://github.com/hashicorp/terraform/issues/22772))
* config: The `cidrhost`, `cidrsubnet`, and `cidrnetmask` functions now behave correctly with IPv6 prefixes that are short enough for the host portion to be greater than 64-bit or 32-bit (depending on the target architecture). ([#22505](https://github.com/hashicorp/terraform/issues/22505))
* config: Fixed bug on empty sets with `for_each` ([#22281](https://github.com/hashicorp/terraform/issues/22281))
* lang/funcs: New `fileset` function, for finding static local files that match a glob pattern. ([#22523](https://github.com/hashicorp/terraform/issues/22523))
* New functions `regex` and `regexall` allow applying a regular expression pattern to a string and retrieving any matching substring(s) ([#22353](https://github.com/hashicorp/terraform/issues/22353))
* config: reduce MinItems and MaxItems validation during decoding, to allow for use of dynamic blocks ([#22530](https://github.com/hashicorp/terraform/issues/22530))
* config: don't validate MinItems and MaxItems in CoerceValue, allowing providers to set incomplete values ([#22478](https://github.com/hashicorp/terraform/issues/22478))
* config: fix panic on tuples with `for_each` ([#22279](https://github.com/hashicorp/terraform/issues/22279))
* config: fix panic when using nested dynamic blocks ([#22314](https://github.com/hashicorp/terraform/issues/22314))
* config: ensure consistent evaluation when moving between single resources and `for_each` in addressing ([#22454](https://github.com/hashicorp/terraform/issues/22454))
* core: only start a single instance of each required provisioner ([#22553](https://github.com/hashicorp/terraform/issues/22553))
* command: fix issue where commands occasionally exited before the error message printed ([#22373](https://github.com/hashicorp/terraform/issues/22373))
* command/0.12upgrade: use user-supplied plugin-dir ([#22306](https://github.com/hashicorp/terraform/issues/22306))
* command/hook_ui: Truncate the ID considering multibyte characters ([#18823](https://github.com/hashicorp/terraform/issues/18823))
* command/fmt: Terraform fmt no longer inserts spaces after % ([#22356](https://github.com/hashicorp/terraform/issues/22356))
* command/state: Allow moving resources to modules not yet in state ([#22299](https://github.com/hashicorp/terraform/issues/22299))
* backend/google: Now using the OAuth2 token endpoint on `googleapis.com` instead of `google.com`. These endpoints are equivalent in functionality but `googleapis.com` hosts are resolvable from private Google Cloud Platform VPCs where other connectivity is restricted. ([#22451](https://github.com/hashicorp/terraform/issues/22451))
* backend/s3: After this update, the AWS Go SDK will prefer credentials found via the `AWS_PROFILE` environment variable when both the `AWS_PROFILE` environment variable and the `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` environment variables are statically defined. Previously the SDK would ignore the `AWS_PROFILE` environment variable, if static environment credentials were also specified. This is listed as a bug fix in the AWS Go SDK release notes. ([#22253](https://github.com/hashicorp/terraform/issues/22253))
* backend/s3: Add support for assuming role via web identity token via the `AWS_WEB_IDENTITY_TOKEN_FILE` and `AWS_ROLE_ARN` environment variables ([#22253](https://github.com/hashicorp/terraform/issues/22253))
* backend/s3: Support automatic region validation for `me-south-1`. For AWS operations to work in the new region, the region must be explicitly enabled as outlined in the [AWS Documentation](https://docs.aws.amazon.com/general/latest/gr/rande-manage.html#rande-manage-enable) ([#22253](https://github.com/hashicorp/terraform/issues/22253))
* backend/remote: remove misleading contents from error message ([#22148](https://github.com/hashicorp/terraform/issues/22148))
* backend/s3: Load credentials via the `AWS_PROFILE` environment variable (if available) when `AWS_PROFILE` is defined along with `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` ([#22253](https://github.com/hashicorp/terraform/issues/22253))
* config: Improve conditionals to returns the correct type when dynamic values are present but unevaluated ([#22137](https://github.com/hashicorp/terraform/issues/22137))
* config: Fix panic when mistakingly using `dynamic` on an attribute ([#22169](https://github.com/hashicorp/terraform/issues/22169))
* cli: Fix crash with reset connection during init ([#22146](https://github.com/hashicorp/terraform/issues/22146))
* cli: show all deposed instances and prevent crash in `show` command ([#22149](https://github.com/hashicorp/terraform/issues/22149))
* configs/configupgrade: Fix crash with nil hilNode ([#22181](https://github.com/hashicorp/terraform/issues/22181))
* command/fmt: now formats correctly in presence of here-docs ([#21434](https://github.com/hashicorp/terraform/issues/21434))
* helper/schema: don't skip deprecation check during validation when attribute value is unknown ([#22262](https://github.com/hashicorp/terraform/issues/22262))
* plugin/sdk: allow MinItems > 1 when dynamic blocks ([#22221](https://github.com/hashicorp/terraform/issues/22221))
* plugin/sdk: fix reflect panics in helper/schema validation ([#22236](https://github.com/hashicorp/terraform/issues/22236))
* command/format: No longer show no-ops in `terraform show`, since nothing will change ([#21907](https://github.com/hashicorp/terraform/issues/21907))
* backend/s3: Support for assuming role using credential process from the shared AWS configuration file (support profile containing both `credential_process` and `role_arn` configurations) ([#21908](https://github.com/hashicorp/terraform/issues/21908))
* connection/ssh: Abort ssh connections when the server is no longer responding ([#22037](https://github.com/hashicorp/terraform/issues/22037))
* connection/ssh: Support ssh diffie-hellman-group-exchange-sha256 key exchange ([#22037](https://github.com/hashicorp/terraform/issues/22037))
* backend/remote: fix conflict with normalized config dir and vcs root working directory ([#22096](https://github.com/hashicorp/terraform/issues/22096))
* backend/remote: be transparent about what filesystem prefix Terraform is uploading to the remote system, and why it's doing that ([#22121](https://github.com/hashicorp/terraform/issues/22121))
* configs: Ensure diagnostics are properly recorded from nested modules ([#22098](https://github.com/hashicorp/terraform/issues/22098))
* core: Prevent inconsistent final plan error when using dynamic in a set-type block ([#22057](https://github.com/hashicorp/terraform/issues/22057))
* lang/funcs: Allow null values in `compact` function ([#22044](https://github.com/hashicorp/terraform/issues/22044))
* core: Prevent crash when a resource has no current valid instance ([#21979](https://github.com/hashicorp/terraform/issues/21979))
* plugin/sdk: Prevent empty strings from being replaced with default values ([#21806](https://github.com/hashicorp/terraform/issues/21806))
* plugin/sdk: Ensure resource timeouts are not lost when there is an empty plan ([#21814](https://github.com/hashicorp/terraform/issues/21814))
* plugin/sdk: Don't add null elements to diagnostic paths when validating config ([#21884](https://github.com/hashicorp/terraform/issues/21884))
* lang/funcs: Add missing map of bool support for `lookup` ([#21863](https://github.com/hashicorp/terraform/issues/21863))
* config: Fix issue with downloading BitBucket modules from deprecated V1 API by updating go-getter dependency ([#21948](https://github.com/hashicorp/terraform/issues/21948))
* config: Fix conditionals to evaluate to the correct type when using null ([#21957](https://github.com/hashicorp/terraform/issues/21957))
* configs: Can now use references like `tags["foo"]` in `ignore_changes` to ignore in-place updates to specific keys in a map ([#21788](https://github.com/hashicorp/terraform/issues/21788))
* configs: Fix panic on missing value for `version` attribute in `provider` blocks. ([#21825](https://github.com/hashicorp/terraform/issues/21825))
* lang/funcs: Fix `merge` panic on null values. Now will give an error if null used ([#21695](https://github.com/hashicorp/terraform/issues/21695))
* backend/remote: Fix "Conflict" error if the first state snapshot written after a Terraform CLI upgrade has the same content as the prior state. ([#21811](https://github.com/hashicorp/terraform/issues/21811))
* backend/s3: Fix AWS shared configuration file credential source not assuming a role with environment and ECS credentials ([#21815](https://github.com/hashicorp/terraform/issues/21815))
* provisioners: new provisioner: `puppet` ([#18851](https://github.com/hashicorp/terraform/issues/18851))
*`range` function for generating a sequence of numbers as a list ([#21461](https://github.com/hashicorp/terraform/issues/21461))
*`yamldecode` and *experimental*`yamlencode` functions for working with YAML-serialized data ([#21459](https://github.com/hashicorp/terraform/issues/21459))
*`uuidv5` function for generating name-based (as opposed to pseudorandom) UUIDs ([#21244](https://github.com/hashicorp/terraform/issues/21244))
* backend/oss: Add support for Alibaba OSS remote state ([#16927](https://github.com/hashicorp/terraform/issues/16927))
* config: consider build metadata when interpreting module versions ([#21640](https://github.com/hashicorp/terraform/issues/21640))
* backend/http: implement retries for the http backend ([#19702](https://github.com/hashicorp/terraform/issues/19702))
* backend/swift: authentication mechanisms now more consistent with other OpenStack-compatible tools ([#18671](https://github.com/hashicorp/terraform/issues/18671))
* command/show: use the state snapshot included in the planfile when rendering a plan to json ([#21597](https://github.com/hashicorp/terraform/issues/21597))
* config: Fix issue with empty dynamic blocks failing when usign ConfigModeAttr ([#21549](https://github.com/hashicorp/terraform/issues/21549))
* core: Re-validate resource config during final plan ([#21555](https://github.com/hashicorp/terraform/issues/21555))
* core: Fix missing resource timeouts during destroy ([#21611](https://github.com/hashicorp/terraform/issues/21611))
* core: Don't panic when encountering an invalid `depends_on` ([#21590](https://github.com/hashicorp/terraform/issues/21590))
* backend: Fix panic when upgrading from a state with a hash value greater than MaxInt ([#21484](https://github.com/hashicorp/terraform/issues/21484))
* config: `distinct` function no longer panics when given an empty list ([#21538](https://github.com/hashicorp/terraform/issues/21538))
* config: Don't panic when a `version` constraint is added to a module that was previously initialized without one ([#21542](https://github.com/hashicorp/terraform/issues/21542))
* config: `matchkeys` function argument type checking will no longer fail incorrectly during validation ([#21576](https://github.com/hashicorp/terraform/issues/21576))
* backend/local: Don't panic if an instance in the state only has deposed instances, and no current instance ([#21575](https://github.com/hashicorp/terraform/issues/21575))
This is the aggregated summary of changes compared to v0.11.14. If you'd like to see the incremental changelog through each of the v0.12.0 prereleases, please refer to [the v0.12.0-rc1 changelog](https://github.com/hashicorp/terraform/blob/v0.12.0-rc1/CHANGELOG.md).
The focus of v0.12.0 was on improvements to the Terraform language made in response to all of the feedback and experience gathered on prior versions. We hope that these language improvements will help to make configurations for more complex situations more readable, and improve the usability of re-usable modules.
However, an overhaul of this kind inevitably means that 100% compatibility is not possible. The updated language is designed to be broadly compatible with the 0.11 language as documented, but some of the improvements required a slightly stricter parser and language model in order to resolve ambiguity or to give better feedback in error messages.
If you are upgrading to v0.12.0, we strongly recommend reading [the upgrade guide](https://www.terraform.io/upgrade-guides/0-12.html) to learn the recommended upgrade process, which includes a tool to automatically upgrade many improved language constructs and to indicate situations where human intuition is required to complete the upgrade.
* As noted above, the language overhaul means that several aspects of the language are now parsed or evaluated more strictly than before, so configurations that employ workarounds for prior version limitations or that followed conventions other than what was shown in documentation may require some updates. For more information, please refer to [the upgrade guide](https://www.terraform.io/upgrade-guides/0-12.html).
* In order to give better feedback about mistakes, Terraform now validates that all variable names set via `-var` and `-var-file` options correspond to declared variables, generating errors or warnings if not. In situations where automation is providing a fixed set of variables to all configurations (whether they are using them or not), use [`TF_VAR_` environment variables](https://www.terraform.io/docs/commands/environment-variables.html#tf_var_name) instead, which are ignored if they do not correspond to a declared variable.
* The wire protocol for provider and provisioner plugins has changed, so plugins built against prior versions of Terraform are not compatible with Terraform v0.12. The most commonly-downloaded providers already had v0.12-compatible releases at the time of v0.12.0 release, but some other providers (particularly those distributed independently of the `terraform init` installation mechanism) will need to make new releases before they can be used with Terraform v0.12 or later.
* The index API for automatic provider installation in `terraform init` is now provided by the Terraform Registry at `registry.terraform.io`, rather than the indexes directly on `releases.hashicorp.com`. The "releases" server is still currently the distribution source for the release archives themselves at the time of writing, but that may change over time.
* The serialization formats for persisted state snapshots and saved plans have changed. Third-party tools that parse these artifacts will need to be updated to support these new serialization formats.
For most use-cases, we recommend instead using [`terraform show -json`](https://www.terraform.io/docs/commands/show.html#json-output) to read the content of state or plan, in a form that is less likely to see significant breaking changes in future releases.
* [`terraform validate`](https://www.terraform.io/docs/commands/validate.html) now has a slightly smaller scope than before, focusing only on configuration syntax and type/value checking. This makes it safe to run in unattended scenarios, such as on save in a text editor.
* **First-class expressions:** Prior to v0.12, expressions could be used only via string interpolation, like `"${var.foo}"`. Expressions are now fully integrated into the language, allowing them to be used directly as argument values, like `ami = var.ami`.
* **`for` expressions:** This new expression construct allows the construction of a list or map by transforming and filtering elements from another list or map. For more information, refer to [the _`for` expressions_ documentation](https://www.terraform.io/docs/configuration/expressions.html#for-expressions).
* **Dynamic configuration blocks:** For nested configuration blocks accepted as part of a resource configuration, it is now possible to dynamically generate zero or more blocks corresponding to items in a list or map using the special new `dynamic` block construct. This is the official replacement for the common (but buggy) unofficial workaround of treating a block type name as if it were an attribute expecting a list of maps value, which worked sometimes before as a result of some unintended coincidences in the implementation.
* **Generalised "splat" operator:** The `aws_instance.foo.*.id` syntax was previously a special case only for resources with `count` set. It is now an operator within the expression language that can be applied to any list value. There is also an optional new splat variant that allows both index and attribute access operations on each item in the list. For more information, refer to [the _Splat Expressions_ documentation](https://www.terraform.io/docs/configuration/expressions.html#splat-expressions).
* **Nullable argument values:** It is now possible to use a conditional expression like `var.foo != "" ? var.foo : null` to conditionally leave an argument value unset, whereas before Terraform required the configuration author to provide a specific default value in this case. Assigning `null` to an argument is equivalent to omitting that argument entirely.
* **Rich types in module inputs variables and output values:** Terraform v0.7 added support for returning flat lists and maps of strings, but this is now generalized to allow returning arbitrary nested data structures with mixed types. Module authors can specify an expected [type constraint](https://www.terraform.io/docs/configuration/types.html) for each input variable to allow early type checking of arguments.
* **Resource and module object values:** An entire resource or module can now be treated as an object value within expressions, including passing them through input variables and output values to other modules, using an attribute-less reference syntax, like `aws_instance.foo`.
* **Extended template syntax:** The simple interpolation syntax from prior versions is extended to become a simple template language, with support for conditional interpolations and repeated interpolations through iteration. For more information, see [the _String Templates_ documentation](https://www.terraform.io/docs/configuration/expressions.html#string-templates).
* **`jsondecode` and `csvdecode` interpolation functions:** Due to the richer type system in the new configuration language implementation, we can now offer functions for decoding serialization formats. [`jsondecode`](https://www.terraform.io/docs/configuration/functions/jsondecode.html) is the opposite of [`jsonencode`](https://www.terraform.io/docs/configuration/functions/jsonencode.html), while [`csvdecode`](https://www.terraform.io/docs/configuration/functions/csvdecode.html) provides a way to load in lists of maps from a compact tabular representation.
* **Revamped error messages:** Error messages relating to configuration now always include information about where in the configuration the problem was found, along with other contextual information. We have also revisited many of the most common error messages to reword them for clarity, consistency, and actionability.
* **Structual plan output:** When Terraform renders the set of changes it plans to make, it will now use formatting designed to be similar to the input configuration language, including nested rendering of individual changes within multi-line strings, JSON strings, and nested collections.
*`terraform validate` now accepts an argument `-json` which produces machine-readable output. Please refer to the documentation for this command for details on the format and some caveats that consumers must consider when using this interface. ([#17539](https://github.com/hashicorp/terraform/issues/17539))
* The JSON-based variant of the Terraform language now has a more tightly-specified and reliable mapping to the native syntax variant. In prior versions, certain Terraform configuration features did not function as expected or were not usable via the JSON-based forms. For more information, see [the _JSON Configuration Syntax_ documentation](https://www.terraform.io/docs/configuration/syntax-json.html).
* The new built-in function [`templatefile`](https://www.terraform.io/docs/configuration/functions/templatefile.html) allows rendering a template from a file directly in the language, without installing the separate Template provider and using the `template_file` data source.
* The new built-in function [`formatdate`](https://www.terraform.io/docs/configuration/functions/formatdate.html), which is a specialized string formatting function for creating machine-oriented timestamp strings in various formats.
* The new built-in functions [`reverse`](https://www.terraform.io/docs/configuration/functions/reverse.html), which reverses the order of items in a list, and [`strrev`](https://www.terraform.io/docs/configuration/functions/strrev.html), which reverses the order of Unicode characters in a string.
* The `s3` state storage backend now supports `credential_source` in AWS configuration files, support for the new AWS regions `eu-north-1` and `ap-east-1`, and several other improvements previously made in the `aws` provider.
Quite a few bugs were fixed indirectly as a result of improvements to the underlying language engine, so a fully-comprehensive list of fixed bugs is not possible, but some of the more commonly-encountered bugs that are fixed in this release include:
* config: The conditional operator `... ? ... : ...` now works with result values of any type and only returns evaluation errors for the chosen result expression, as those familiar with this operator in other languages might expect.
* config: When using a splat expression like `aws_instance.foo.*.id`, the addition of a new instance to the set (whose `id` is therefore not known until after apply) will no longer cause all of the other ids in the resulting list to appear unknown.
* config: The `jsonencode` function now preserves the types of values passed to it, even inside nested structures, whereas before it had a tendency to convert primitive-typed values to string representations.
* config: The `format` and `formatlist` functions now attempt automatic type conversions when the given values do not match the "verbs" in the format string, rather than producing a result with error placeholders in it.
* config: Assigning a list containing one or more unknown values to an argument expecting a list no longer produces the incorrect error message "should be a list", because Terraform is now able to track the individual elements as being unknown rather than the list as a whole, and to track the type of each unknown value. (This also avoids any need to place seemingly-redundant list brackets around values that are already lists, which would now be interpreted as a list of lists.)
* cli: When `create_before_destroy` is enabled for a resource, replacement actions are reflected correctly in rendered plans as `+/-` rather than `-/+`, and described as such in the UI messages.
* core: Various root causes of the "diffs didn't match during apply" class of error are now checked at their source, allowing Terraform to either avoid the problem occurring altogether (ideally) or to provide a more actionable error message to help with reporting, finding, and fixing the bug.
