Changes after review:

- use source_files_iter helper, build single files and child dirs as well
- rename arg to paths, use 0.. pos arg

crates/cli/src/opts/build/core.rs

@@ -126,12 +126,10 @@ pub struct CoreBuildArgs {
     #[serde(skip)]
     pub skip: Option<Vec<SkipBuildFilter>>,
 
-    /// Build files within specific directories.
-    ///
-    /// Child directories are not accounted and should be explicitly added.
-    #[arg(long, num_args(1..))]
+    /// Build source files from specified paths.
+    #[arg(long, short, num_args(0..))]
     #[serde(skip)]
-    pub dirs: Option<Vec<PathBuf>>,
+    pub paths: Option<Vec<PathBuf>>,
 
     #[command(flatten)]
     #[serde(flatten)]

crates/forge/bin/cmd/build.rs

@@ -3,7 +3,7 @@ use clap::Parser;
 use eyre::Result;
 use foundry_cli::{opts::CoreBuildArgs, utils::LoadConfig};
 use foundry_common::compile::ProjectCompiler;
-use foundry_compilers::{Project, ProjectCompileOutput};
+use foundry_compilers::{utils::source_files_iter, Project, ProjectCompileOutput};
 use foundry_config::{
     figment::{
         self,
@@ -14,11 +14,13 @@ use foundry_config::{
     Config,
 };
 use serde::Serialize;
-use std::fs;
 use watchexec::config::{InitConfig, RuntimeConfig};
 
 foundry_config::merge_impl_figment_convert!(BuildArgs, args);
 
+// Extensions accepted by `forge build`
+const BUILD_EXTENSIONS: &[&str] = &["sol", "yul", "vy", "vyi"];
+
 /// CLI arguments for `forge build`.
 ///
 /// CLI arguments take the highest precedence in the Config/Figment hierarchy.
@@ -83,15 +85,9 @@ impl BuildArgs {
 
         // Collect sources to compile if build subdirectories specified.
         let mut files = vec![];
-        if let Some(dirs) = self.args.dirs {
+        if let Some(dirs) = self.args.paths {
             for dir in dirs {
-                for entry in fs::read_dir(dir)? {
-                    let entry = entry?;
-                    let path = entry.path();
-                    if path.is_file() {
-                        files.push(entry.path());
-                    }
-                }
+                files.extend(source_files_iter(dir, BUILD_EXTENSIONS));
             }
         }
 

crates/forge/tests/cli/cmd.rs

@@ -1667,7 +1667,7 @@ function test_run() external {}
     );
 });
 
-forgetest_init!(can_build_specific_dir, |prj, cmd| {
+forgetest_init!(can_build_specific_paths, |prj, cmd| {
     prj.wipe();
     prj.add_source(
         "Counter.sol",
@@ -1694,28 +1694,39 @@ function test_bar() external {}
     )
     .unwrap();
 
-    // Build only files within test dir
+    // Build 2 files within test dir
     prj.clear();
-    cmd.args(["build", "--dirs", "test", "--force"]);
+    cmd.args(["build", "--paths", "test", "--force"]);
     cmd.unchecked_output().stdout_matches_path(
-        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/can_build_test_dir.stdout"),
+        PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+            .join("tests/fixtures/can_build_path_with_two_files.stdout"),
+    );
+
+    // Build one file within src dir
+    prj.clear();
+    cmd.forge_fuse();
+    cmd.args(["build", "--paths", "src", "--force"]);
+    cmd.unchecked_output().stdout_matches_path(
+        PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+            .join("tests/fixtures/can_build_path_with_one_file.stdout"),
     );
 
-    // Build only files within src dir
+    // Build 3 files from test and src dirs
     prj.clear();
     cmd.forge_fuse();
-    cmd.args(["build", "--dirs", "src", "--force"]);
+    cmd.args(["build", "--paths", "src", "test", "--force"]);
     cmd.unchecked_output().stdout_matches_path(
-        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/can_build_src_dir.stdout"),
+        PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+            .join("tests/fixtures/can_build_path_with_three_files.stdout"),
     );
 
-    // Build multiple dirs
+    // Build single test file
     prj.clear();
     cmd.forge_fuse();
-    cmd.args(["build", "--dirs", "src", "--dirs", "test", "--force"]);
+    cmd.args(["build", "--paths", "test/Bar.sol", "--force"]);
     cmd.unchecked_output().stdout_matches_path(
         PathBuf::from(env!("CARGO_MANIFEST_DIR"))
-            .join("tests/fixtures/can_build_multiple_dirs.stdout"),
+            .join("tests/fixtures/can_build_path_with_one_file.stdout"),
     );
 });