Gemara

95a (Mishnah): If one finds Tefilin [on Shabbos in a field, where they are prone to be disgraced] he brings them in [to the city] one pair at a time.

95b (Rava): During the week one wears only one pair of Tefilin [at a time]. The same applies to saving on Shabbos.

99a (Mishnah): One may not stand in Reshus ha'Yachid and [stick out his mouth and] drink in Reshus ha'Rabim, or vice-versa.

The same applies to a winepress.

(Abaye): Likewise, one may not stand in Reshus ha'Yachid or Reshus ha'Rabim and drink in a Karmelis;

(Rava): The Isur of [transferring to or from a] Karmelis is only mid'Rabanan. Chachamim do not decree Gezeirah li'Gezeirah (a decree lest one come to transgress an Isur mid'Rabanan).

Support (Abaye, for himself - Mishnah): The same applies to a winepress.

The winepress must be a Karmelis, for the Mishnah already discussed a Reshus ha'Yachid and Reshus ha'Rabim!

Rejection (Rava and Rav Sheshes): No, the Mishnah teaches about a winepress regarding Ma'aser (whether one may drink in the winepress without tithing).

Support (for Abaye - Mishnah): A tailor may not go outside with his needle shortly before dark, lest he forget and carry it on Shabbos.

Suggestion: It is inserted in his garment. (It is not normal to carry this way. It is forbidden only mid'Rabanan.)

Rejection: No, it is in his hand.

Shabbos 12a (Beraisa - Tana d'Vei R. Yishmael): A person may walk outside with Tefilin on Erev Shabbos close to dark.

Question: Why is this permitted?

Answer: Rabah bar Rav Huna taught that one must constantly touch Tefilin. We learn from a Kal V'Chomer from the Tzitz (that the Kohen Gadol wears on his forehead):

The Tzitz has Hash-m's name on it only once, yet the Torah says "v'Hayah Al Mitzcho Tamid." The Kohen Gadol may not be Mesi'ach ha'Da'as from (cease to be aware of) it;

Hash-m's name is written 21 times in each Tefilah. All the more one may not be Mesi'ach ha'Da'as from them!

Therefore, he will not forget.

61a (Mishnah): If one went out (on Shabbos) with Tefilin, he is exempt.

(Rav Safra): This is even like the opinion that Tefilin does not apply on Shabbos, for this is considered wearing [and not carrying].

Rishonim

The Rif and Rosh (32b and 10:6) bring the Mishnah in Eruvin.

Rosh (ibid.): Abaye says that the same applies to [drinking in] a Karmelis. Rava says that we do not decree Gezeirah li'Gezeirah.

Rambam (Hilchos Shabbos 19:26): A tailor may not go outside with his needle shortly before dark on Erev Shabbos, lest he forget and take it out [on Shabbos]. One may go out with Tefilin just before dark. Since one must constantly touch Tefilin, he will not forget them. If one forgot and went out with Tefilin just before dark, and remembered that Tefilin are on his head, he covers them until he reaches his house or a Beis Midrash.

Poskim

Shulchan Aruch (OC 252:6): A tailor may not go outside with his needle in his hand shortly before dark, lest he forget and take it out [on Shabbos].

Beis Yosef (DH u'Mah): According to Rava, we do not decree lest someone transgress a mid'Rabanan law. Therefore, we forbid him to go out before dark with his needle in his hand, but not if it is inserted in his garment.

Beis Yosef (ibid., citing R. Yerucham): It is forbidden even if it is inserted in his garment. It seems that the Rif agrees, for he simply cited the Mishnah verbatim and the Heter for Tefilin. He did not bring Rava, who says that we do not decree for a decree. He holds that the Halachah does not follow Rava. Therefore, we forbid anyone, even if it is inserted in his garment. However, regarding standing in Reshus ha'Rabim and drinking in Reshus ha'Yachid, the Rif rules like Rava!

Rebuttal (Bach 11): If this (that we did not explain that Tefilin are permitted because it is Gezeirah li'Gezeirah) shows that the Halachah does not follow Rava, the Gemara should have challenged Rava from Tana d'vei R. Yishmael's Beraisa! Rather, it is not difficult for Rava, for it is common that one must remove Tefilin in order to eliminate. This is not called a decree, for everyone needs to eliminate! (People used to wear Tefilin the entire day - PF.) Also Rava would forbid Erev Shabbos, if not that people remember Tefilin. The Rif merely brought the Mishnah, which can be like Abaye.

Beis Yosef (ibid.): We can ask R. Yerucham's question also against the Rosh, who wrote like the Rif! Perhaps they rule like R. Yehudah, who says that one who goes out when the needle is inserted in his garment is Chayav Chatas, therefore, we forbid shortly before Shabbos. The Mishnah is like R. Yehudah. The Mishnah mentions a tailor. It connotes that the Isur is only for a tailor. Therefore, they brought the Mishnah verbatim. They rule like Rava, who does not decree for a decree.

Bach (11): The Rosh cited the opinions of Abaye and Rava. Surely, he holds that the Halachah follows Rava!

Shulchan Aruch (ibid.): One may go out wearing Tefilin shortly before dark, since he will not forget them.

Question: If we hold like Rava, why did the Gemara need to give a reason to permit going out with Tefilin before dark, because one is not Mesi'ach ha'Da'as from them? In any case it is permitted, for one does not carry them, rather, he wears them on his head. This is like a needle inserted in his garment. A Stam person may go out like this, for even if he would forget and go out like this on Shabbos, he is exempt! All the more so this applies to Tefilin, for even according to the opinion that there is no Mitzvah on Shabbos, one who wears them in Reshus ha'Rabim is exempt, for it is like wearing. Further, sometimes one may wear them in Reshus ha'Rabim l'Chatchilah (Eruvin 95a)!

Answer (Beis Yosef): Indeed, one may wear them on his head shortly before even without the reason that one may not be Mesi'ach ha'Da'as from them. This reason is needed to permit going with them in his hand or wrapped in a garment. If he would forget and go out like this on Shabbos, he would be liable.

Rebuttal (Taz 8): We learned from the Tzitz that one may not be Mesi'ach ha'Da'as from Tefilin. This is only when he wears them, but not when they are in his hand! Where did R. Yerucham see that the Rif rules like Rava in Eruvin? Perhaps it is a printing mistake; it should say 'the Rosh rules like Rava.' Perhaps in R. Yerucham's text, also the Rif brought the opinions of Abaye and Rava. The Beis Yosef said that the Rosh wrote exactly like the Rif. I say that the Rif rules unlike Rava, because R. Yishmael attributes the Heter to the fact that people remember Tefilin. Also, the Gemara held that the simple reading of the Mishnah connotes that the needle is in his garment. Also, the Gemara could have asked whether the Isur to go out with a needle is even in Karmelis, or only in Reshus ha'Rabim, like it asked about drinking in another Reshus. It did not ask, for the Mishnah connotes that the needle is on his garment (i.e. surely we forbid Gezeirah li'Gezeirah). This is why the Rif rules like Abaye.

Gra (DH Aval): The Gemara needed to say that one may not be Mesi'ach ha'Da'as from Tefilin, because Rava agrees that we decree Gezeirah li'Gezeirah when it is likely that one be negligent. Anything that one may not take to Reshus ha'Rabim, he may not take to a Chatzer, even if he is exempt in Reshus ha'Rabim (Shabbos 64a).

Beis Yosef (DH Aval): The Rambam and Tur hold that the Mishnah forbids going out with a needle in his hand, since if he did so on Shabbos, he would be Chayav Chatas. Since Tefilin was taught with them, this connotes that is in his hand. This is why the Heter depends on the Halachah that one must constantly touch Tefilin. When the Rambam discusses one who took Tefilin out on Shabbos, he says 'on his head.' This connotes that until now, he discusses (regarding Erev Shabbos) Tefilin in his hand. Also, the Beraisa taught that one may go out with Tefilin on Erev Shabbos. This implies that they are in his hand, for it did not specify 'on his head.' It seems that the Rif and Rosh hold like this. The Tur said 'since one may not be Mesi'ach ha'Da'as from them as long as they are on his head.' Perhaps he means that all the more so, one may not be Mesi'ach ha'Da'as from them when they are in his hand!'

Magen Avraham (24): We forbid when the Tefilin are in his hand, for he is Mesi'ach ha'Da'as from them.

Kaf ha'Chayim (80): The Mechaber connotes that Tefilin are permitted even in his hand. The Acharonim permit only if it is on his head.

Magen Avraham (25): Even though Hesech ha'Da'as from Tefilin is frivolity (but one need not think about them constantly), since this is forbidden, he remembers that he is wearing them and removes them before Shabbos.

Kaf ha'Chayim (79): One may not go out with a wallet just before dark, for it is unlike Tefilin [even though one always checks it - Bava Metzi'a 21b].

Bi'ur Halachah (DH b'Yado): The Gra and Pri Megadim hold that just before dark, all forbid even something for which one is exempt if he takes it out on Shabbos. Regarding Tefilin, it is permitted even just before dark.