Extend module docs with dynamic sources information

Author: dbanck

content/terraform/v1.15.x (beta)/docs/language/modules/sources.mdx

@@ -13,7 +13,11 @@ last_modified: 2025-11-19T19:11:19Z
 # Module Sources
 
 The `source` argument in [a `module` block](/terraform/language/modules/syntax)
-tells Terraform where to find the source code for the desired child module.
+is an expression that tells Terraform where to find the source code for the
+desired child module.
+
+The `source` expression can use references to input variables and local values.
+Any input variable referenced in `source` must declare `const = true`.
 
 Terraform uses this during the module installation step of `terraform init`
 to download the source code to a directory on local disk so that other Terraform commands can use it. 
@@ -485,3 +489,67 @@ Terraform will still extract the entire package to local disk, but will read
 the module from the subdirectory. As a result, it is safe for a module in
 a sub-directory of a package to use [a local path](#local-paths) to another
 module as long as it is in the _same_ package.
+
+## Source Expressions
+
+A module `source` can be an expression, which allows you to build a source address from constant input variables and local values.
+
+When a `source` expression refers to an input variable, that variable must set `const = true`. Terraform requires this so it can fully determine the module source during `terraform init`.
+
+For example, you can select a module source using a constant variable:
+
+```hcl
+module "consul" {
+  source = var.module_source
+}
+```
+
+```hcl
+variable "module_source" {
+  type  = string
+  const = true
+}
+```
+
+You can also compose a source from constant variables and local values:
+
+```hcl
+module "vpc" {
+  source = local.vpc_source
+}
+```
+
+```hcl
+variable "module_repo" {
+  type  = string
+  const = true
+}
+
+variable "module_ref" {
+  type  = string
+  const = true
+}
+
+locals {
+  vpc_source = "git::https://example.com/${var.module_repo}.git?ref=${var.module_ref}"
+}
+```
+
+The `version` argument also accepts expressions. If a `version` expression
+refers to an input variable, that variable must also set `const = true`.
+
+```hcl
+module "consul" {
+  source  = "hashicorp/consul/aws"
+  version = var.consul_version
+}
+```
+
+```hcl
+variable "consul_version" {
+  type  = string
+  const = true
+}
+```
+
+The `source` and `version` expressions can only reference things that are known during configuration loading. Terraform will show an error if they reference something that is only known after a plan.