Make repeated uv add operations simpler (#5922)

## Summary

Closes #5913.

crates/uv-workspace/src/pyproject.rs

@@ -30,7 +30,7 @@ pub struct PyProjectToml {
     pub tool: Option<Tool>,
     /// The raw unserialized document.
     #[serde(skip)]
-    pub(crate) raw: String,
+    pub raw: String,
 }
 
 impl PyProjectToml {

crates/uv-workspace/src/pyproject_mut.rs

@@ -55,11 +55,6 @@ impl PyProjectTomlMut {
         })
     }
 
-    /// Initialize a [`PyProjectToml`] from a [`PyProjectTomlMut`].
-    pub fn to_toml(&self) -> Result<PyProjectToml, Error> {
-        Ok(toml::from_str(&self.doc.to_string()).map_err(Box::new)?)
-    }
-
     /// Adds a project to the workspace.
     pub fn add_workspace(&mut self, path: impl AsRef<Path>) -> Result<(), Error> {
         // Get or create `tool.uv.workspace.members`.

crates/uv/src/commands/project/add.rs

@@ -2,9 +2,9 @@ use std::collections::hash_map::Entry;
 
 use anyhow::{Context, Result};
 use owo_colors::OwoColorize;
-use rustc_hash::{FxBuildHasher, FxHashMap};
-
 use pep508_rs::{ExtraName, Requirement, VersionOrUrl};
+use rustc_hash::{FxBuildHasher, FxHashMap};
+use tracing::debug;
 use uv_auth::store_credentials_from_url;
 use uv_cache::Cache;
 use uv_client::{BaseClientBuilder, Connectivity, FlatIndexClient, RegistryClientBuilder};
@@ -247,7 +247,14 @@ pub(crate) async fn add(
     }
 
     // Save the modified `pyproject.toml`.
-    fs_err::write(project.root().join("pyproject.toml"), pyproject.to_string())?;
+    let mut modified = false;
+    let content = pyproject.to_string();
+    if content == existing.raw {
+        debug!("No changes to `pyproject.toml`; skipping update");
+    } else {
+        fs_err::write(project.root().join("pyproject.toml"), &content)?;
+        modified = true;
+    }
 
     // If `--frozen`, exit early. There's no reason to lock and sync, and we don't need a `uv.lock`
     // to exist at all.
@@ -258,7 +265,7 @@ pub(crate) async fn add(
     // Update the `pypackage.toml` in-memory.
     let project = project
         .clone()
-        .with_pyproject_toml(pyproject.to_toml()?)
+        .with_pyproject_toml(toml::from_str(&content)?)
         .context("Failed to update `pyproject.toml`")?;
 
     // Lock and sync the environment, if necessary.
@@ -286,8 +293,10 @@ pub(crate) async fn add(
             let report = miette::Report::new(WithHelp { header, cause: err, help: Some("If this is intentional, run `uv add --frozen` to skip the lock and sync steps.") });
             anstream::eprint!("{report:?}");
 
-            // Revert the changes to the `pyproject.toml`.
+            // Revert the changes to the `pyproject.toml`, if necessary.
-            fs_err::write(project.root().join("pyproject.toml"), existing)?;
+            if modified {
+                fs_err::write(project.root().join("pyproject.toml"), existing)?;
+            }
 
             return Ok(ExitStatus::Failure);
         }
@@ -361,7 +370,9 @@ pub(crate) async fn add(
             modified = true;
         }
 
-        // Save the modified `pyproject.toml`.
+        // Save the modified `pyproject.toml`. No need to check for changes in the underlying
+        // string content, since the above loop _must_ change an empty specifier to a non-empty
+        // specifier.
         if modified {
             fs_err::write(project.root().join("pyproject.toml"), pyproject.to_string())?;
         }

crates/uv/tests/edit.rs

@@ -2829,3 +2829,86 @@ fn add_virtual() -> Result<()> {
 
     Ok(())
 }
+
+/// Add the same requirement multiple times.
+#[test]
+fn add_repeat() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        # ...
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add(&["anyio"]), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: `uv add` is experimental and may change without warning
+    Resolved 4 packages in [TIME]
+    Prepared 4 packages in [TIME]
+    Installed 4 packages in [TIME]
+     + anyio==4.3.0
+     + idna==3.6
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + sniffio==1.3.1
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(context.temp_dir.join("pyproject.toml"))?;
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        # ...
+        requires-python = ">=3.12"
+        dependencies = [
+            "anyio>=4.3.0",
+        ]
+        "###
+        );
+    });
+
+    uv_snapshot!(context.filters(), context.add(&["anyio"]), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: `uv add` is experimental and may change without warning
+    Resolved 4 packages in [TIME]
+    Audited 4 packages in [TIME]
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(context.temp_dir.join("pyproject.toml"))?;
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        # ...
+        requires-python = ">=3.12"
+        dependencies = [
+            "anyio>=4.3.0",
+        ]
+        "###
+        );
+    });
+
+    Ok(())
+}